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Observed trends in ambient concentrations of C2–C8 hydrocarbons in the United Kingdom over the period from 1993 to 2004
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Atmospheric Environment 41 (2007) 2559–2569 www.elsevier.com/locate/atmosenv
Observed trends in ambient concentrations of C2–C8 hydrocarbons in the United Kingdom over the period from 1993 to 2004 G.J. Dollarda, P. Dumitreana, S. Tellinga, J. Dixonb, R.G. Derwentc, a
NETCEN, AEA Technology Environment, Harwell, Oxfordshire, UK b Department for Environment, Food and Rural Affairs, London, UK c rdscientific, Newbury, Berkshire, UK
Received 12 July 2006; received in revised form 14 November 2006; accepted 14 November 2006
Abstract Hourly measurements of up to 26 C2–C8 hydrocarbons have been made at eight urban background sites, three urbanindustrial sites, a kerbside and a rural site in the UK from 1993 onwards up until the end of December 2004. Average annual mean benzene and 1,3-butadiene concentrations at urban background locations have declined at about 20% per year and the observed declines have exactly mimicked the inferred declines in benzene and 1,3-butadiene emissions over the same period. Ninety-day rolling mean concentrations of ethylene, propylene, n- and i-butane, n- and i-pentane, isoprene and propane at urban and rural sites have also declined steadily by between 10% and 30% per year. Rolling mean concentrations of acetylene, 2- and 3-methylpentane, n-hexane, n-heptane, cis- and trans-but-2-ene, cis- and trans-pent-2ene, toluene, ethylbenzene and o-, m- and p-xylene at a roadside location in London have all declined at between 14% and 21% per year. These declines demonstrate that motor vehicle exhaust catalysts and evaporative canisters have effectively and efficiently controlled vehicular emissions of hydrocarbons in the UK. Urban ethane concentrations arising largely from natural gas leakage have remained largely unchanged over this same period. r 2006 Elsevier Ltd. All rights reserved. Keywords: C2–C8 hydrocarbons; VOCs; Trends; Urban measurements; Exhaust catalysts; Evaporative canisters
1. Introduction During summertime, anticyclonic conditions, elevated levels of ozone have been observed during most years and in most European countries (Larssen et al., 2002). These elevated levels sometimes exceed internationally accepted guidelines Corresponding author.
E-mail address: [email protected] (R.G. Derwent). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.11.020
(WHO, 1987) set to protect human health. Parties to the international convention on the long-range transboundary transport of air pollution have agreed the Geneva VOC Protocol (UN ECE, 1991) and the Gothenburg multi-pollutant Protocol (UN ECE, 1999) to reduce emissions of ozone precursors and hence photochemical ozone formation in Europe. Reductions in the emissions of volatile organic compounds (VOCs) and nitrogen oxides NOx within Europe have been achieved largely but not wholly by the implementation of
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petrol-engined motor vehicle emission controls mandated by the Commission of the European Communities (Commission of the European Communities CEC, 1991). These directives require the implementation of three-way catalysts to control exhaust emissions of carbon monoxide, nitrogen oxides and VOCs and canisters to control evaporative emissions of VOCs. There is an important air quality policy requirement to check the extent to which the exhaust catalysts and evaporative canisters have delivered the promised emission reductions and hence improvement in ozone air quality (Derwent et al., 2003). Of the main ozone precursors, VOCs and NOx, relatively little ambient VOC monitoring has been carried out across Europe with which to verify that the obligations to reduce ozone precursor emissions have actually been met. Here, we describe the results of an extensive monitoring programme carried out within the UK to establish the ambient levels of C2–C8 hydrocarbons and to monitor the impact of the exhaust catalysts and evaporative canisters on ambient concentrations. Hourly measurements of up to 26 C2–C8 hydrocarbons have been made at 11 urban background locations, a kerbside and a rural location, from 1993 onwards up until the end of December 2004. Because benzene and 1,3-butadiene (buta-1,3-diene) are toxic and carcinogenic, they are considered in somewhat more detail and a careful comparison of the trends with time is made with the results from emission inventories. For the remainder of the hydrocarbons monitored, their importance is largely as ozone precursors and so the focus of the analysis is on trends with time and the long-term performance of the exhaust catalysts and evaporative canisters. The study updates our previous analysis of the ambient levels of C2–C8 hydrocarbons (Derwent et al., 2000) with the addition of the kerbside site and extends the analysis from 1996 to the end of December 2004. 2. The network infrastructure and measurement methods The monitoring locations, the instrumentation used and the period of operation for each of the networkmonitoring stations is summarised in Table 1. 2.1. Instrumentation Throughout the operation of the network three different types of instrument have been used.
Initially, the equipment used to monitor hydrocarbons was the Chrompack VOCAIR analyser. This instrument was used at the 12 original monitoring stations. These analysers were phased out and replacements installed in the period 2000–01. The Chrompack VOCAIR allowed continuous gas chromatographic determinations at hourly intervals. The Chrompack system consisted of an automatic thermo-desorption/cryogenic trapping system (Auto TCT), connected to a Chrompack gas chromatograph (Dollard et al., 1995). From 2000 onwards the network has consisted of five sites, located at Cardiff, Glasgow, Harwell, Eltham and Marylebone Road (Table 1). Two types of instruments are employed at these sites. Three of the sites: Cardiff, Glasgow and Harwell, are fitted with Environment VOC71M analysers, configured to measure and report the concentrations of 1,3butadiene, benzene, toluene, ethylbenzene, (m+p)xylene and o-xylene. The two other sites: London Marylebone Road and London Eltham, are fitted with automatic Perkin Elmer Ozone Precursor Analysers OPA which are capable of measuring and reporting at least 29 hydrocarbons. Comparative studies suggest that the three analyzer types agree with each other to within 710% of the measured value. The benzene and the 1,3-butadiene data are used for comparison with the UK Air Quality Standard. The benzene data are reported to the European Commission. The data for up to 29 hydrocarbons monitored by the instruments installed at the London Eltham and London Marylebone Road sites are reported to the European Commission to satisfy one the monitoring requirements of the EU 3rd Daughter Directive.
2.2. Network management Since the Automatic Hydrocarbon Monitoring Network started, either AEA Technology or the National Physical Laboratory have undertaken the management and/or the quality assurance of the network. Currently, AEA Technology, based at Harwell, Oxfordshire, manages and quality assures the network with local operator support for routine site operations. The National Physical Laboratory, Teddington, supplies calibration gas mixtures for use at the sites. The data quality of the hourly archived data is characterized by an uncertainty at 95% confidence of 725% above 0.5 and 70.1 mg m3 below 0.5 mg m3.
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Table 1 Hydrocarbon network monitoring sites Site
Location
Start date
End date
Analyser
Belfast South UB Birmingham East UB Bristol East UB Cardiff East UB
IJ333726 SP115888 ST599729 ST193773 ST193773 ST184765 NT257730 NT257730 NS587652 SU468860 SU468860 SE307367 SJ439836 TQ440747 TQ440747 TQ281820 TQ299822 NZ505196 SU426123
01/08/1993 04/08/1993 01/05/1994 01/01/1994 01/01/2002 05/09/2002 13/05/1993 01/01/2002 01/08/2002 01/01/1995 01/01/2002 01/01/1995 24/11/1995 08/10/1993 17/10/2003 01/09/1997 08/02/1993 01/01/1993 14/09/1995
31/12/2000 31/12/2000 31/12/2000 31/12/2001 05/09/2002 Ongoing 31/12/2001 31/07/2002 Ongoing 31/12/2001 Ongoing 31/12/2000 31/12/2000 31/12/2000 Ongoing Ongoing 31/12/2000 31/12/2000 31/12/2000
VOCAIR VOCAIR VOCAIR VOCAIR VOC71M VOC71M VOCAIR VOC71M VOC71M VOCAIR VOC71M VOCAIR VOCAIR VOCAIR OPA OPA VOCAIR VOCAIR VOCAIR
Cardiff Centre UB Edinburgh Med. School UB Glasgow kerbsidea K Harwell R Leeds Potternewton UB Liverpool Speke UI London Eltham UB London Marylebone Road K London UCL UB Middlesbrough UI Southampton UI
Notes: Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic. a Data not analysed in this study because of the shortness of the record at this site.
2.3. Data management Each analyzer in the network has an associated PC operating several software packages: PC Anywhere, modem control software, and equipment specific chromatographic acquisition and analysis software. The GC software collects the raw chromatogram (binary file), integrates the peaks, and produces a report file. The site PC is polled daily via modem and all files are retrieved for storage on a workstation at Harwell in the network management centre. The sites are configured to complete a measurement for each VOC for each hour of the day. The routine operation of the network generates large numbers of hourly data (requiring processing of many raw data files). Each site can generate 8760 hourly values per year. An overall data capture of 80% has been achieved, taking into account quality assurance, data ratification and calibration (Derwent et al., 2000). The compounds routinely reported are shown in Table 2 and arise from a mixture of sources from natural gas, motor vehicle usage, and industrial activities. Identification of the compounds is carried out using pattern recognition software (AEA MatchFinder). Monthly batches of the files integrated using instrument specific software on the site PCs
are processed using the pattern recognition software on the workstation. The output data are checked using commercial spreadsheet software. Final quantification is through application of response factors derived from the injection of certified standards at each site. These standards, supplied by the National Physical Laboratory, are gravimetrically prepared primary or secondary gas mixtures containing trace concentrations of each of the hydrocarbon species in nitrogen. Final ratification of the data is performed at the QA/QC unit. Further details are given by Dollard et al. (1995) and Derwent et al. (2000). All of the data collected on the network have been archived and are available at the UK National Air Quality Archive: www.airquality.co.uk. Periods of instrumental breakdown were represented as blanks in the archive and did not contribute to 90day or annual averages. Because of the significant changes made to the network during the 1993–2004 period in terms of number of sampling sites, instrumentation and site location, care has to be taken to ensure that consistent data have been used in the evaluation of trends. The eight urban background sites in Table 1 using VOCAIR analysers had different start dates but the same end dates with no changes in site location. Instrumental performance was satisfactory
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Table 2 List of hydrocarbon compounds reported from the network Compound
Ethane Ethane Propane Propene Ethyne 2-methylpropane n-butane trans-2-butene 1-butene cis-2-butene 2-methylbutane n-pentane 1,3-butadiene trans-2-pentene 1-pentene cis-2-pentene 2-methylpentane 3-methylpentane n-hexane Isoprene Benzene 2,2,4trimethylpentaneb n-heptane n-octaneb Toluene Ethylbenzene (m+p)-xylenec o-xylene 1,3,5trimethylbenzeneb 1,2,4trimethylbenzeneb 1,2,3trimethylbenzeneb
Alternative name
Ethylene Propylene Acetylene i-butane
i-pentane
i-hexane
i-octaneb
Measured by (type of analyzer)a C, C, C, C, C, C, C, C, C, C, C, C, C, C, P C, C, C, C, C, C, P
P P P P P P P P P P P P P, V P
C, P C, C, C, C, P
P
P P P P P P, V
P, P, P, P,
V V V V
P P
a Refers to the type of analyzer these compounds were measured with, C ¼ Chrompack VOCAIR, P ¼ Perkin Elmer OPA, V ¼ Environnement VOC71M. b These VOCs do not have long enough data records for trend analysis in this study. c Refers to a mixture of two isomers that could not be separated with the chromatographic systems utilized in this study.
for the years 1995–2001 inclusive and so this was the chosen period for analysis for all the VOCs except benzene and 1,3-butadiene. This same period was chosen for trend analysis for all the VOCs except benzene and 1,3-butadiene at the rural site. At the kerbside site, measurements began during 1997 and are still ongoing without any changes in instrumentation and location. Trend analysis has been performed through to the end of 2004 for all VOCs.
Benzene and 1,3-butadiene trends at the rural and urban background sites have been extended forward from 2002 onwards using data from different instruments (VOCAIR vs. VOC71M) and with a change of site at Cardiff. Every effort has been made to ensure that these changes have not materially altered the evaluated trends. 3. Benzene and 1,3-butadiene Benzene and 1,3-butadiene are important hydrocarbons in their own right because in addition to taking part in photochemical ozone formation, they are toxic and carcinogenic. Air quality standards have been set at 5 ppb (16.3 mg m3) and 1 ppb (2.3 mg m3), respectively, for annual mean concentrations (EPAQS, 1994, 2002). At the start of the monitoring period, annual mean benzene and 1,3-butadiene levels were reported as 0.7–1.9 ppb (2.3–6.2 mg m3) and 0.11–0.38 ppb (0.25–0.86 mg m3), respectively, by Derwent et al. (2000) at urban background sites in the UK. These reported levels were well below the air quality standards set to protect human health. Both hydrocarbons arise principally from petrol-engined motor vehicle exhaust emissions with some contribution from evaporating petrol and so urban background concentrations should have been strongly influenced by the implementation of three-way catalysts and evaporative canisters. In view of their importance as toxic compounds for which annual mean air quality guidelines and standards are appropriate, trends in annual mean concentrations have been the main focus of our analysis. Table 3 summarises the annual mean air quality data for benzene and 1,3-butadiene monitored over the period from 1993 to 2004. For this purpose, the eleven urban background sites have been split into two groups. The first group contains eight sites: Belfast, Birmingham, Bristol, Cardiff, Edinburgh, Leeds, London UCL and London Eltham. These sites, see Table 1, are located away from the immediate and direct influence of traffic in towns and cities where motor vehicle traffic is the major source of hydrocarbons. The second group contains three sites: Liverpool, Middlesbrough and Southampton. These sites, see Table 1, are located in towns and cities where there are considerable industrial sources of hydrocarbons in addition to motor vehicle traffic. The entries in Table 3 for urban background locations represent the average
ARTICLE IN PRESS G.J. Dollard et al. / Atmospheric Environment 41 (2007) 2559–2569 Table 3 Annual mean benzene and 1,3-butadiene concentrations at rural, urban traffic-influenced, urban industrial-influenced and kerbside locations in the UK Year Benzene (mg m3)
1,3-butadiene (mg m3)
Rural UB UI Kerbside Rural UB 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
1.2 1.2 1.2 1.1 0.9 0.5 0.6 0.6 0.4 0.8
4.6 3.9 3.6 3.7 3.7 2.8 2.6 1.5 1.8 0.7 1.9 0.8
5.6 4.1 4.1 15.3 3.3 12.8 3.1 10.8 2.2 6.3 4.5 3.9 3.3 2.8
0.39 0.15 0.15 0.12 0.12 0.09 0.11 0.04 0.03 0.02
0.64 0.53 0.50 0.49 0.51 0.36 0.33 0.26 0.27 0.05 0.48 0.15
UI
0.85 0.56 0.52 0.41 0.38 0.35
Kerbside
3.3 2.4 1.9 1.6 1.1 0.95 0.64 0.57
Notes: Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic.
of the eight sites and for urban industrial, the average of the three sites. The average annual mean benzene at the urban background sites have declined steadily from 4.6 mg m3 (1.4 ppb) at the start of the monitoring period to 0.8 mg m3 (0.25 ppb) at the end. The benzene time series at the urban background sites showed a highly statistically significant downwards trend at the 99.9% level of significance using the non-parametric Mann–Kendall method (Salmi et al., 2002) with an estimated trend of 0.3470.04 mg m3 yr1 (0.1070.01 ppb yr1) using the non-parametric Sen’s method (Salmi et al., 2002) with 1s confidence limits on the slope. The observed downwards trend amounted to 23% per year when expressed relative to the mean concentration during the year 2000. Annual average benzene levels at the urbanindustrial sites were somewhat higher than those at the urban background sites, confirming that there were additional benzene sources over and above motor vehicle traffic in the vicinity of these sites. Despite these additional industrial sources, annual average benzene concentrations at the urban-industrial sites still showed the same downward trend, about 20% per year, as that shown by the urban background sites. Annual mean benzene concentrations at the kerbside site have fallen from 15.3 mg m3 (4.7 ppb) at the start of the measurement period to 2.8 mg m3
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(0.9 ppb) at the end. Levels have thus declined by about 30% per year from close to the air quality standard to well below it over the period from 1997 to 2004. At the kerbside site then, annual mean benzene levels have declined by 1.9 mg m3 yr1 (0.6 ppb yr1), a downwards trend of about 30% per year which is highly statistically significant at the 99.9% level of significance. The observed concentration trend at the rural site was found to be 0.170.02 mg m3 yr1 (0.0370.006 ppb yr1), a downwards trend of about 20% per year that is highly statistically significant at the 99% level of significance. Trends at single sites may be influenced by local factors such as changes in traffic and hence may not be indicative of countrywide trends. Annual mean 1,3-butadiene concentrations at the urban background sites have fallen from 0.64 mg m3 (0.28 ppb) at the start of the measurement period to 0.15 mg m3 (0.07 ppb) at the end. The 1,3-butadiene time series showed a highly statistically significant downwards trend at the 99% level of significance of 0.04370.007 mg m3 yr1 (0.01970.003 ppb yr1). This trend amounted to about 17% per year at the urban background sites. At the urban-industrial sites, annual mean 1,3-butadiene levels are somewhat higher than those at urban background sites but show a similar downwards trend of 18% per year. These differences in percentage trends are not considered significant. At the kerbside site, annual mean 1,3-butadiene concentrations exhibited a downwards trend of 0.34 mg m3 yr1 (0.15 ppb yr1) was observed which was significant at the 99.9% level of significance. The trend at the rural site was equally significant and was estimated to be 0.02070.007 mg m3 yr1 (0.0097 0.003 ppb yr1). Both trends were about 21% per year. Scatter plots of the hourly mean concentrations of benzene and 1,3-butadiene showed excellent correlation with correlation coefficients R in excess of 0.9 at most sites during 1996 in our previous study (Derwent et al., 2000). Least squares regression fits were used to define the 1,3-butadiene to benzene ratio at each site and an average ratio by mass of 0.1670.02 was reported in that study. Here, the same excellent level of correlation between the hourly mean concentrations of benzene and 1,3butadiene was found at all the urban background, kerbside and rural sites studied and in each year. Correlations were not always as good at the urbanindustrial sites where sporadic peaks of either
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benzene or 1,3-butadiene lead to significantly poorer correlations. Averaging over all sites and years when the correlation coefficients R were greater than 0.9, the mean 1,3-butadiene to benzene ratio was found to be 0.1770.03 by mass, in close agreement with the above ratio reported for 1996 previously. The 90-day rolling mean concentrations of benzene and 1,3-butadiene were calculated for each day of the available record at the kerbside site and were divided to obtain the ratio of the 90-day rolling mean concentrations. The mean value over the entire record was 0.21370.036, consistent with the above ratios by mass. Least-squares regression analysis showed that the ratio of the 90-day rolling means exhibited a small upwards trend of 1.370.14% per year. It is, therefore, likely that there has been little significant change in the 1,3butadiene to benzene ratio over the 8-year period at the kerbside site, despite the annual mean concentrations of both hydrocarbon concentrations declining by over a factor five. This would suggest that there has been no material change in the principal source of both hydrocarbons over the 8-year study period. This is consistent with this principal source being emissions from uncontrolled petrol-engined vehicles without catalysts and canisters, with the decline in ambient concentrations being driven by the reduction in vehicle-kilometres as these vehicles age and are scrapped. In view of the importance in air quality policy formulation of emission inventories, it is important to check these inventories against air quality data. Annual UK total benzene emissions have declined from 52,000 ton yr1 in 1993 to 18,000 ton yr1 (NAEI, 2005), a rate of decline of 15% per year. This decline takes into account the uptake of catalyst and canister technologies, deterioration of catalysts, cold-start emissions and emissions from diesel-engined vehicles. Over this same period, the average annual mean benzene concentrations at the urban background sites have declined by about 16% per year. The agreement in these rates of decline is most heartening. The ratio of average annual mean concentration at the urban background sites to UK total emissions have, therefore, remained largely unchanged at 0.0857 0.005 mg m3 per 1000 ton yr1. This shows that the observed urban background concentrations are exactly mimicking the inferred decline in the UK total emissions over the 1993–2000 period, provid-
ing strong evidence for the description of benzene emissions in the inventory. The observed urban background 1,3-butadiene trend over the 1993–2000 period agreed closely with the trend in UK total 1,3-butadiene emissions (NAEI, 2005), providing strong evidence for the description of 1,3-butadiene emissions in the inventory. Again, the ratio of the average annual mean concentration to the total UK emission remained largely unchanged at 0.06070.007 mg m3 per 1000 ton yr1, despite both changing by a large factor. However, there is no reason for this average ratio to differ significantly between benzene and 1,3-butadiene if both emission inventories were indeed dominated by motor vehicle emissions. The difference in the ratios implies that either the benzene emissions have been underestimated by about 30% or those of 1,3-butadiene have been overestimated by a similar amount. The presence of a northern hemisphere background benzene concentration could explain some of the difference in ratios. 4. Ethylene and propylene Ethylene (ethene) and propylene (propene) are two of the most important ozone precursors in north west Europe and contribute the second and sixth largest contributions to peak ozone formation in photochemical modelling studies (Derwent et al., 2003). Since photochemical ozone episodes may be observed at any time from the spring through to the autumn in north west Europe, the 90-day rolling mean concentration is a reasonable way to present air quality data for hydrocarbons that are ozone precursors but not air toxics or carcinogens for which annual means are the most appropriate. Motor vehicle exhaust emissions account for the majority of urban emissions of ethylene and propylene (NAEI, 2005). Table 4 contains a summary of the ethylene and propylene observations obtained from the available measurements, focusing on the 1995–2000 period when the majority of sites were operating reliably. Considering first the annual mean concentrations of ethylene reported in Table 4 for the year 2000. Concentrations were highest at the kerbside site and lowest at the rural site, with average concentrations spanning a factor 24. Urban levels were found midway between these extremes, with levels at urban-industrial sites somewhat elevated above urban background levels. Oil refineries appear to
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act as large sources of ethylene and raise levels significantly above those anticipated from motor vehicle traffic alone. The average 90-day rolling mean concentrations of ethylene at the urban background sites over the 1995–2000 period shows a significant downwards trend of 0.28 mg m3 yr1 which is equivalent to 12% per year. Trends at the urban-industrial sites were found to be somewhat smaller, 5% per year, demonstrating the hydrocarbon emission reductions have been largely but not wholly confined to the motor vehicle traffic sector. In contrast, trends at the rural and kerbside sites are reported as 28% per year and 16% per year, respectively, confirming that motor vehicle emissions are dominant source of ethylene at both sites. As noted above, trends at single sites may not be indicative of UK-wide trends. A similar picture is found in Table 4 for propylene as for ethylene. Urban levels are again found to be midway between the kerbside and rural levels. The average annual mean concentrations for the urban-industrial sites are elevated over those found for the urban background sites, showing that propylene emissions from oil refineries are significant, as with ethylene. Trends at the urban background sites over the 1995–2000 period are given as 15% per year, 8% per year and 45% per year, respectively, at kerbside, urban background and
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rural sites. The controls implemented on the exhaust emissions of petrol-engined motor vehicles have therefore brought about a dramatic reduction in urban and rural levels of propylene during the period of this study. 5. n- and i-butane The isomers of butane, n-butane (butane) and ibutane (methylpropane), are important ozone precursors in north west Europe and account for the largest single contribution and the twelfth largest contributions, respectively, to photochemical ozone formation as reported in Derwent et al. (2003). Since they arise principally from the evaporation of motor spirit (NAEI, 2005), they provide an important indication of the performance of the canisters that have been fitted to petrol-engined motor vehicle to control evaporative emissions. Urban background levels of n-butane are midway between the kerbside and rural levels, with urbanindustrial sites showing only a small elevation in concentrations above urban background sites, see Table 4. Ninety-day rolling mean concentrations have shown consistent downwards trends at all sites. The trend in the average 90-day rolling mean concentrations at the urban background sites was found to be 0.93 mg m3 yr1, equivalent to 18%
Table 4 Annual mean concentrations in the year 2000 and trends in 90-day rolling mean concentrations at rural, urban traffic-influenced, urban industrial-influenced and kerbside locations for 10 hydrocarbons in the UK Species
Ethylene Propylene Toluene n-butane i-butane n-pentane i-pentane Ethaned Propanee Isoprene
Rurala
UBb
UIb
Kerbsidec
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
0.6 0.4 1.3 1.2 0.9 0.3 1.2 2.3 1.5 0.05
0.17 0.18 0.24 0.29 0.19 0.08 0.23 0.17 0.19 0.017
2.3 1.9 5.0 5.1 1.9 1.4 4.1 5.3 3.9 0.16
0.28 0.15 0.71 0.93 0.70 0.15 0.77 0.07 0.31 0.04
3.4 2.2 6.2 5.8 2.0 2.0 5.1 6.3 5.2 0.22
0.18 0.14 0.58 1.00 0.73 0.31 0.53 +0.23 0.65 0.05
14.6 7.2 28.8 19.9 9.7 5.3 23.7 9.6 5.5 0.95
2.4 1.1 3.8 2.2 1.7 0.61 3.6 +0.15 0.68 0.27
Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic. a Trends from 4/8/1995 to 31/12/2001. b Trends in 90-day rolling mean concentrations from 1/1/1995 to 31/12/2000. c Trends from 4/6/1997 to 31/12/2004 excepting n-butane which are for 1/1/1999 to 31/12/2004. d Cardiff is not included in analyses of ethane. e Belfast and Cardiff are not included in analyses of propane.
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per year. The respective trends at the rural and kerbside sites were 24% per year and 11% per year. The corresponding picture is similar for ibutane. The trend found for i-butane at the urban background sites was 0.70 mg m3 yr1, equivalent to 37% per year. Those found at the rural and kerbside sites were 21% per year and 18% per year, respectively. The magnitudes of the percentage downward trends observed for the evaporative hydrocarbons, n-butane and i-butane, are similar to those observed for the exhaust hydrocarbons, ethylene and propylene, at the different site types. This strongly suggests that the same factors are determining the influence of the emission controls, irrespective of whether the hydrocarbon emissions are from vehicle exhausts or fuel evaporation. This situation would apply if both technologies are highly efficient such that current emissions are heavily dominated by those from uncontrolled petrol-engined motor vehicles. In this case, the downward trends in both exhaust and evaporative hydrocarbons are determined by the decreasing vehicle-kilometres travelled by uncontrolled vehicles. In the future, when all of the uncontrolled vehicles have been scrapped, then emissions will be dictated by the remaining emissions from the controlled vehicles and the issue will then be the durability of the motor vehicle exhaust catalysts and evaporative canisters over the lifetime of the motor vehicles. Scatter plots of the hourly mean concentrations of n-butane and i-butane show excellent correlation with most sites and years exhibiting correlation coefficients R in excess of 0.9. The main exceptions to this pattern were the Birmingham and London Eltham sites where the scatter plots regularly showed huge deviations from general behaviour observed at the other urban background sites. Industrial sources contributed significantly to the observed levels of n-butane at the urban-industrial sites. Considering the sites and years when correlation coefficients R in excess of 0.9 were found, then the average regression slopes of n-butane vs. ibutane were found to be 1.8570.29 on a mass basis. This compares favourably with, though is somewhat lower than the ratio of 2.2270.3 reported in our previous study which considered only the 1996 observations. At the kerbside site, the ratio of the 90-day rolling means of n-butane to i-butane over the 1999–2004 period was 1.89670.021 and exhibited a small downwards trend of 0.12% per year.
6. n- and i-pentane The pentane isomers n-pentane (pentane) and ipentane (methylbutane) are important constituents of evaporating motor spirit (NAEI, 2005) and make the 7th and 5th largest contributions, respectively, to photochemical ozone formation in Europe (Derwent et al., 2003). Urban background concentrations of n-pentaneaveraged 1.4 mg m3 during the year 2000 and were found midway between the kerbside levels of 5.3 mg m3 and rural levels of 0.3 mg m3. Average concentrations at urban-industrial sites were elevated above those anticipated at urban background sites pointing to there being significant emission sources from oil refineries. Strong downward trends were found in the 90-day rolling mean concentrations of n-pentane over the 1995–2000 period. These were observed to be 0.15 mg m3 yr1 or 11% per year for the urban background sites, 11% per year and 26% per year at the kerbside and rural sites, respectively. Downward trends of these magnitudes are similar to those found for the exhaust and evaporative hydrocarbons discussed above and point to the efficient control of motor vehicle emissions of n-pentane by the evaporative canisters. The behaviour found for i-pentane was exactly analogous to that found for n-pentane, see Table 4. Levels of i-pentane spanned the range of a factor of 20 from kerbside sites to rural sites compared with the range of 18 for n-pentane. Downwards trends in the 90-day rolling mean concentrations were found to be 19% per year, 15% per year and 19% per year at the urban background, kerbside and rural sites, respectively. Scatter plots of n-pentane vs. i-pentane showed excellent correlation with most sites and years generating correlation coefficients R in excess of 0.9. Averaging those sites and years that showed excellent correlation, the linear regression slopes defined an average ratio by mass of 0.29770.060. This average ratio by mass is somewhat higher than that of 0.24 that we reported in our study of the 1996 data (Derwent et al., 2000). The sites at Cardiff and Edinburgh showed evidence of sporadic sources with enhanced n-pentane to i-pentane ratios, which destroyed the close correlation generally found at urban background sites. The urban-industrial sites showed evidence of significant sources of n-pentane and i-pentane with a ratio by mass of close to 1:1, presumably from refinery sources. Over the 1997– 2004 period, the average n-pentane to i-pentane
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ratio observed at the kerbside site was 0.23770.032 and exhibited an upwards trend of +5.0% per year. This would point to there being a small additional source of n-pentane and i-pentane other than that from motor vehicle evaporative emissions with a characteristically different ratio by mass of the pentane isomers. 7. Isoprene The hydrocarbon isoprene (2-methylbuta-1,3diene) is commonly regarded as being derived from natural biogenic sources associated with plants and trees and is a potent source of photochemical ozone. Our previous study found evidence for an urban source of isoprene which was tentatively identified as motor vehicular, based on its close correlation with benzene (Derwent et al., 2000). Table 4 summarises the results found here for isoprene. Highest concentrations during 2000, of all the sites monitored were reported at the kerbside site with a mean concentration of 0.95 mg m3. Urban background concentrations were about 0.16 mg m3, a factor of about six lower than those at the kerbside site. Rural concentrations averaged 0.05 mg m3, about a factor of 19 below kerbside levels. Urbanindustrial sites showed somewhat higher concentrations compared with urban background sites, pointing to there possibly being oil refinery sources of isoprene in addition to the motor traffic sources found in most urban areas. Downwards trends have been observed in the 90day rolling mean concentrations of isoprene monitored at all the sites over the period from 1995 to 2000. A strong downward trend of 0.27 mg m3 yr1 or 28% per year has been reported for the kerbside site, see Table 4. Averaged over the urban background sites, trends of 0.04 mg m3 yr1 or 25% per year were found compared with 0.017 mg m3 yr1 or 34% per year at the rural site. It is concluded that, certainly at the start of the observations, isoprene emissions and urban concentrations were dominated by emissions from motor vehicles. As a result of motor vehicle emission controls, urban isoprene concentrations have declined by close to an order of magnitude over the period of the study. Scatter plots of hourly mean observations of isoprene and benzene show evidence of good correlation with correlation coefficients in excess of 0.8 at a small number of sites and years. Leastsquares fits were used to derive the ratio of isoprene
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to benzene during those years when good correlation was observed. Averaged over all these occasions, a mean isoprene to benzene ratio by mass of 0.12870.030 was found in close agreement to 0.11370.034 reported for 1996 by Derwent et al. (2000). The good correlations observed at the Harwell (rural), Liverpool (urban-industrial) and London Marylebone Road (kerbside) sites were lost at the end of the study period. This was found to be due to the presence at the end of the study period, of an additional isoprene source other than from motor vehicles. The order of magnitude reduction in isoprene and benzene levels due to motor vehicle emission controls has exposed the natural source of isoprene and destroyed the close correlation between isoprene and benzene at the end of the study period. Monthly mean isoprene concentrations show clear-cut seasonal cycles with maxima during the summer months at most urban background sites. Taking the London Eltham site as typical then, over the entire 1993–2004 period, monthly mean isoprene levels during the winter months were generally in the range 0.2–0.3 mg m3 and rose to about 0.6 mg m3 during summertime. Splitting these data into two periods: 1993–1996 and 2000–2004, we find that levels in winter, spring and autumn have fallen substantially between the start and the end of the study period by factors of two to three. In contrast, levels have fallen only by about one-thirds during the summer period. The implication is that the motor vehicle source of isoprene has been reduced dramatically particularly during wintertime whilst the summertime natural source has remained much the same. At the London Marylebone Road site, the seasonal shifts in monthly mean isoprene concentrations have been even more dramatic. Levels have fallen by factors of between three and five throughout the year and the seasonal cycle has changed from showing wintertime peaks at the start to showing summertime peaks by the end of the study period. 8. Ethane and propane Ethane and propane in urban areas are generally derived from natural gas leakage (NAEI, 2005) and consequentially show markedly different diurnal cycles to the other hydrocarbons that are derived from motor vehicle traffic (Derwent et al., 2000). Because of local gross contamination, the data from the Cardiff and Belfast sites have not been included in the analyses in Table 4. Of all the hydrocarbons
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monitored in this study, the range in annual mean concentrations across the different site types, see Table 4, for the year 2000 is the narrowest for ethane and propane. The ratios of the most to least polluted sites span factors of 4.2 and 3.7, respectively, for ethane and propane whereas such factors may approach 20 for the other hydrocarbons studied. There are appreciable northern hemisphere background concentrations of these hydrocarbons (Derwent et al., 2000) and these account for the relatively high rural concentrations which are observed. Indeed, ethane and propane are the highest concentration hydrocarbons reported in Table 4 for the rural site with mean concentrations of 2.3 and 1.5 mg m3, respectively. Concentrations at the urban-industrial sites are considerably enhanced over those for urban background sites for both hydrocarbons demonstrating that oil refineries are significant sources. Trends in 90-day rolling mean ethane concentrations do not present a uniform picture by any means, compared with the other hydrocarbons studied. Ethane concentrations appear to rising slightly at the kerbside and urban-industrial sites by +2% per year and +4% per year, respectively. At the urban background and rural sites, ethane concentrations are falling by 1% per year and by 7% per year, respectively. Within the range of errors and uncertainties, such trends are considered small and it is likely that ethane levels are showing no trends of any significance. In contrast, 90-day rolling mean propane concentrations are showing strong but variable downwards trends: 13% per year at the rural site, 8% per year at the urban background sites, 13% per year at the urban-industrial sites and 12% per year at the kerbside site. This points to there having been a significant motor vehicle contribution to propane levels at all sites that is being steadily reduced by motor vehicle emission controls. There appears to have been a negligible motor vehicle contribution to ethane levels, with levels being dominated by natural gas leakage. This will soon be the situation for propane as the motor vehicle contribution eventually becomes vanishingly small. Scatter plots of hourly mean ethane and propane concentrations show close correlations at some sites during some years. Considering only those sites and years that gave correlation coefficients R greater than 0.8, then the least-squares regression slopes defined average ethane to propane ratios of 1.3670.30 by mass. The corresponding ratio in our previous study (Derwent et al., 2000) was 1.24 which is close to the above value.
9. Other hydrocarbons The 90-day rolling mean concentrations of 14 additional hydrocarbon species observed at the London Marylebone Road kerbside location, over the 1997–2004 period were calculated and the trends estimated, see Table 5. The 14 additional hydrocarbons included one alkyne, 5 alkenes, 4 alkanes and 4 aromatic species, with one of the latter species being an unresolved mixture of two isomers, mxylene and p-xylene. All these species showed highly significant downward trends that encompasses the range from 14 to 21% per year. It is, therefore, highly likely that all of these species are derived largely from motor vehicle exhaust or evaporative emissions. Their declines in concentrations at the kerbside location reflect the decreasing usage of uncontrolled petrol-engined motor vehicles without catalysts and canisters, as noted above.
10. Discussion and conclusions This study reports results from a comprehensive high time resolution (hourly) monitoring programme for 26 hydrocarbons carried out from 1993 to the end of December 2004 at 11 urban background, a kerbside and a rural location in the Table 5 Trends in 90-day rolling mean concentrations from 1998 to 2004 at a kerbside location in London, together with their whole period mean concentrations and percentage trends, for 14 hydrocarbons Species
Mean concentration (mg m3)
Trend (mg m3 per year)
Percentage trend (% per year)
Acetylene trans but-2-ene cis but-2-ene but-1-ene trans pent-2-ene cis pent-2-ene 2-methylpentane 3-methylpentane n-hexane n-heptane Toluene Ethylbenzene m7p-xylene o-xylene
6.5 1.2 0.95 1.3 1.3 0.72 6.3 3.6 2.2 1.4 21.7 4.0 13.9 5.1
0.95 0.22 0.18 0.24 0.22 0.12 1.09 0.62 0.38 0.29 3.74 0.70 2.23 0.89
15 18 19 19 17 17 17 17 18 21 17 18 16 17
Notes: m7p-xylene refers to a mixture of the two isomers that could not be separated with the chromatographic systems utilized in this study.
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UK. Benzene concentrations have declined at 20 to 30% per year and 1,3-butadiene concentrations at 17 to 21% per year, across all the locations and site types. Similar significant downward trends have been reported for 22 additional hydrocarbon species. Ethane and propane have shown distinctly different behaviour, exhibiting no significant trends and small downwards trends, respectively. The trends in benzene and isoprene at the kerbside location appear to be significantly greater that those reported for the other VOC species at this and other sites. This may not necessarily reflect real differences in behaviour across the UK because of the difficulty in interpreting trends at single sites because of local site influences. The observed dramatic reduction in hydrocarbon concentrations across the UK is attributed to the implementation of exhaust gas catalyst and evaporative canister control technologies on petrolengined motor vehicles. The results are consistent with the almost complete elimination of hydrocarbon emissions from the newer controlled motor vehicles. The principal source of the observed hydrocarbon concentrations appears to be the older uncontrolled motor vehicles. The scrappage and reduced number of vehicle kilometres driven by the older uncontrolled motor vehicles has brought about a dramatic decline in hydrocarbon emissions and hence hydrocarbon concentrations. This will, in turn, have produced a marked reduction in episodic peak ozone concentrations.
Acknowledgements This study and the monitoring network that underpinned it, was supported by the Air and Environmental Quality Division of the Department for Environment, Food and Rural Affairs under contract RMP 2423. The assistance of Wei-Wei Qu and Kevin Clemitshaw of Imperial College London with the analysis of the monitoring data is gratefully acknowledged. The help and assistance provided by Trevor Davies, John Stedman and Tim Murrells of NETCEN is much appreciated.
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References Commission of the European Communities CEC, 1991. Council directive amending directive 70/220/EEC on the approximation of the laws of member states relating to the measures to be taken against air pollution by emissions from motor vehicles. 91/441/EEC. Journal of the European Communities, L242/1-L242/106. Derwent, R.G., Davies, T.J., Delaney, M., Dollard, G.J., Field, R.A., Dumitrean, P., Nason, P.D., Jones, B.M.R., Pepler, S.A., 2000. Analysis and interpretation of the continuous monitoring data for 26 C2–C8 hydrocarbons at 12 United Kingdom sites during 1996. Atmospheric Environment 34, 297–312. Derwent, R.G., Jenkin, M.E., Saunders, S.M., Pilling, M.J., Simmonds, P.G., Passant, N.R., Dollard, G.J., Dumitrean, P., Kent, A., 2003. Photochemical ozone formation in north west Europe and its control. Atmospheric Environment 37, 1983–1991. Dollard, G.J., Davies, T.J., Jones, B.M.R., Nason, P.D., Chandler, J., Dumitrean, P., Delaney, M., Watkins, D., Field, R.A., 1995. The UK hydrocarbon monitoring network. In: Lester, R.E., Harrison, R.M. (Eds.), Volatile Organic Compounds in the Atmosphere, Issues in Environmental Science and Technology, Number 4. The Royal Society of Chemistry, Cambridge, UK. EPAQS, 1994. Expert Panel on Air Quality Standards. Benzene. HMSO, London. EPAQS, 2002. Expert Panel on Air Quality Standards. Second Report on 1,3-butadiene. DEFRA, London. Larssen, S., Barrett, K.J., Fiala, J., Goodwin, J., Hagen, L.O., Henriksen, J.F., de Leeuw, F., Tarrason, L., 2002. Air Quality in Europe. State and Trends 1990–1999. European Environment Agency, Copenhagen, Denmark. NAEI, 2005. National Atmospheric Emission Inventory. /http:// airquality.co.ukS. Salmi, T., Maatta, A., Anttila, P., Ruoho-Airola, T., Amnell, T., 2002. Detecting trends of annual values of atmospheric pollutants by the Mann–Kendall test and Sen’s slope estimates—the Excel template application MAKESENS. Publications on Air Quality No. 31, Finnish Meteorological Institute, Helsinki, Finland. UN ECE, 1991. Protocol to the 1979 Convention on long-range transboundary air pollution concerning the control of emissions of volatile organic compounds or their transboundary fluxes. ECE/EB.AIR/30. United Nations Economic Commission for Europe, Geneva, Switzerland. UN ECE, 1999. Protocol to the 1979 Convention on Long-range Transboundary Air Pollution to Abate Acidification, Eutrophication and Ground-level Ozone. United Nations Economic Commission for Europe, Geneva, Switzerland. WHO, 1987. Air Quality Guidelines for Europe. World Health Organisation Regional Publications, European Series No. 23, Copenhagen, Denmark.
Atmospheric Environment 41 (2007) 2559–2569 www.elsevier.com/locate/atmosenv
Observed trends in ambient concentrations of C2–C8 hydrocarbons in the United Kingdom over the period from 1993 to 2004 G.J. Dollarda, P. Dumitreana, S. Tellinga, J. Dixonb, R.G. Derwentc, a
NETCEN, AEA Technology Environment, Harwell, Oxfordshire, UK b Department for Environment, Food and Rural Affairs, London, UK c rdscientific, Newbury, Berkshire, UK
Received 12 July 2006; received in revised form 14 November 2006; accepted 14 November 2006
Abstract Hourly measurements of up to 26 C2–C8 hydrocarbons have been made at eight urban background sites, three urbanindustrial sites, a kerbside and a rural site in the UK from 1993 onwards up until the end of December 2004. Average annual mean benzene and 1,3-butadiene concentrations at urban background locations have declined at about 20% per year and the observed declines have exactly mimicked the inferred declines in benzene and 1,3-butadiene emissions over the same period. Ninety-day rolling mean concentrations of ethylene, propylene, n- and i-butane, n- and i-pentane, isoprene and propane at urban and rural sites have also declined steadily by between 10% and 30% per year. Rolling mean concentrations of acetylene, 2- and 3-methylpentane, n-hexane, n-heptane, cis- and trans-but-2-ene, cis- and trans-pent-2ene, toluene, ethylbenzene and o-, m- and p-xylene at a roadside location in London have all declined at between 14% and 21% per year. These declines demonstrate that motor vehicle exhaust catalysts and evaporative canisters have effectively and efficiently controlled vehicular emissions of hydrocarbons in the UK. Urban ethane concentrations arising largely from natural gas leakage have remained largely unchanged over this same period. r 2006 Elsevier Ltd. All rights reserved. Keywords: C2–C8 hydrocarbons; VOCs; Trends; Urban measurements; Exhaust catalysts; Evaporative canisters
1. Introduction During summertime, anticyclonic conditions, elevated levels of ozone have been observed during most years and in most European countries (Larssen et al., 2002). These elevated levels sometimes exceed internationally accepted guidelines Corresponding author.
E-mail address: [email protected] (R.G. Derwent). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.11.020
(WHO, 1987) set to protect human health. Parties to the international convention on the long-range transboundary transport of air pollution have agreed the Geneva VOC Protocol (UN ECE, 1991) and the Gothenburg multi-pollutant Protocol (UN ECE, 1999) to reduce emissions of ozone precursors and hence photochemical ozone formation in Europe. Reductions in the emissions of volatile organic compounds (VOCs) and nitrogen oxides NOx within Europe have been achieved largely but not wholly by the implementation of
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petrol-engined motor vehicle emission controls mandated by the Commission of the European Communities (Commission of the European Communities CEC, 1991). These directives require the implementation of three-way catalysts to control exhaust emissions of carbon monoxide, nitrogen oxides and VOCs and canisters to control evaporative emissions of VOCs. There is an important air quality policy requirement to check the extent to which the exhaust catalysts and evaporative canisters have delivered the promised emission reductions and hence improvement in ozone air quality (Derwent et al., 2003). Of the main ozone precursors, VOCs and NOx, relatively little ambient VOC monitoring has been carried out across Europe with which to verify that the obligations to reduce ozone precursor emissions have actually been met. Here, we describe the results of an extensive monitoring programme carried out within the UK to establish the ambient levels of C2–C8 hydrocarbons and to monitor the impact of the exhaust catalysts and evaporative canisters on ambient concentrations. Hourly measurements of up to 26 C2–C8 hydrocarbons have been made at 11 urban background locations, a kerbside and a rural location, from 1993 onwards up until the end of December 2004. Because benzene and 1,3-butadiene (buta-1,3-diene) are toxic and carcinogenic, they are considered in somewhat more detail and a careful comparison of the trends with time is made with the results from emission inventories. For the remainder of the hydrocarbons monitored, their importance is largely as ozone precursors and so the focus of the analysis is on trends with time and the long-term performance of the exhaust catalysts and evaporative canisters. The study updates our previous analysis of the ambient levels of C2–C8 hydrocarbons (Derwent et al., 2000) with the addition of the kerbside site and extends the analysis from 1996 to the end of December 2004. 2. The network infrastructure and measurement methods The monitoring locations, the instrumentation used and the period of operation for each of the networkmonitoring stations is summarised in Table 1. 2.1. Instrumentation Throughout the operation of the network three different types of instrument have been used.
Initially, the equipment used to monitor hydrocarbons was the Chrompack VOCAIR analyser. This instrument was used at the 12 original monitoring stations. These analysers were phased out and replacements installed in the period 2000–01. The Chrompack VOCAIR allowed continuous gas chromatographic determinations at hourly intervals. The Chrompack system consisted of an automatic thermo-desorption/cryogenic trapping system (Auto TCT), connected to a Chrompack gas chromatograph (Dollard et al., 1995). From 2000 onwards the network has consisted of five sites, located at Cardiff, Glasgow, Harwell, Eltham and Marylebone Road (Table 1). Two types of instruments are employed at these sites. Three of the sites: Cardiff, Glasgow and Harwell, are fitted with Environment VOC71M analysers, configured to measure and report the concentrations of 1,3butadiene, benzene, toluene, ethylbenzene, (m+p)xylene and o-xylene. The two other sites: London Marylebone Road and London Eltham, are fitted with automatic Perkin Elmer Ozone Precursor Analysers OPA which are capable of measuring and reporting at least 29 hydrocarbons. Comparative studies suggest that the three analyzer types agree with each other to within 710% of the measured value. The benzene and the 1,3-butadiene data are used for comparison with the UK Air Quality Standard. The benzene data are reported to the European Commission. The data for up to 29 hydrocarbons monitored by the instruments installed at the London Eltham and London Marylebone Road sites are reported to the European Commission to satisfy one the monitoring requirements of the EU 3rd Daughter Directive.
2.2. Network management Since the Automatic Hydrocarbon Monitoring Network started, either AEA Technology or the National Physical Laboratory have undertaken the management and/or the quality assurance of the network. Currently, AEA Technology, based at Harwell, Oxfordshire, manages and quality assures the network with local operator support for routine site operations. The National Physical Laboratory, Teddington, supplies calibration gas mixtures for use at the sites. The data quality of the hourly archived data is characterized by an uncertainty at 95% confidence of 725% above 0.5 and 70.1 mg m3 below 0.5 mg m3.
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Table 1 Hydrocarbon network monitoring sites Site
Location
Start date
End date
Analyser
Belfast South UB Birmingham East UB Bristol East UB Cardiff East UB
IJ333726 SP115888 ST599729 ST193773 ST193773 ST184765 NT257730 NT257730 NS587652 SU468860 SU468860 SE307367 SJ439836 TQ440747 TQ440747 TQ281820 TQ299822 NZ505196 SU426123
01/08/1993 04/08/1993 01/05/1994 01/01/1994 01/01/2002 05/09/2002 13/05/1993 01/01/2002 01/08/2002 01/01/1995 01/01/2002 01/01/1995 24/11/1995 08/10/1993 17/10/2003 01/09/1997 08/02/1993 01/01/1993 14/09/1995
31/12/2000 31/12/2000 31/12/2000 31/12/2001 05/09/2002 Ongoing 31/12/2001 31/07/2002 Ongoing 31/12/2001 Ongoing 31/12/2000 31/12/2000 31/12/2000 Ongoing Ongoing 31/12/2000 31/12/2000 31/12/2000
VOCAIR VOCAIR VOCAIR VOCAIR VOC71M VOC71M VOCAIR VOC71M VOC71M VOCAIR VOC71M VOCAIR VOCAIR VOCAIR OPA OPA VOCAIR VOCAIR VOCAIR
Cardiff Centre UB Edinburgh Med. School UB Glasgow kerbsidea K Harwell R Leeds Potternewton UB Liverpool Speke UI London Eltham UB London Marylebone Road K London UCL UB Middlesbrough UI Southampton UI
Notes: Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic. a Data not analysed in this study because of the shortness of the record at this site.
2.3. Data management Each analyzer in the network has an associated PC operating several software packages: PC Anywhere, modem control software, and equipment specific chromatographic acquisition and analysis software. The GC software collects the raw chromatogram (binary file), integrates the peaks, and produces a report file. The site PC is polled daily via modem and all files are retrieved for storage on a workstation at Harwell in the network management centre. The sites are configured to complete a measurement for each VOC for each hour of the day. The routine operation of the network generates large numbers of hourly data (requiring processing of many raw data files). Each site can generate 8760 hourly values per year. An overall data capture of 80% has been achieved, taking into account quality assurance, data ratification and calibration (Derwent et al., 2000). The compounds routinely reported are shown in Table 2 and arise from a mixture of sources from natural gas, motor vehicle usage, and industrial activities. Identification of the compounds is carried out using pattern recognition software (AEA MatchFinder). Monthly batches of the files integrated using instrument specific software on the site PCs
are processed using the pattern recognition software on the workstation. The output data are checked using commercial spreadsheet software. Final quantification is through application of response factors derived from the injection of certified standards at each site. These standards, supplied by the National Physical Laboratory, are gravimetrically prepared primary or secondary gas mixtures containing trace concentrations of each of the hydrocarbon species in nitrogen. Final ratification of the data is performed at the QA/QC unit. Further details are given by Dollard et al. (1995) and Derwent et al. (2000). All of the data collected on the network have been archived and are available at the UK National Air Quality Archive: www.airquality.co.uk. Periods of instrumental breakdown were represented as blanks in the archive and did not contribute to 90day or annual averages. Because of the significant changes made to the network during the 1993–2004 period in terms of number of sampling sites, instrumentation and site location, care has to be taken to ensure that consistent data have been used in the evaluation of trends. The eight urban background sites in Table 1 using VOCAIR analysers had different start dates but the same end dates with no changes in site location. Instrumental performance was satisfactory
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Table 2 List of hydrocarbon compounds reported from the network Compound
Ethane Ethane Propane Propene Ethyne 2-methylpropane n-butane trans-2-butene 1-butene cis-2-butene 2-methylbutane n-pentane 1,3-butadiene trans-2-pentene 1-pentene cis-2-pentene 2-methylpentane 3-methylpentane n-hexane Isoprene Benzene 2,2,4trimethylpentaneb n-heptane n-octaneb Toluene Ethylbenzene (m+p)-xylenec o-xylene 1,3,5trimethylbenzeneb 1,2,4trimethylbenzeneb 1,2,3trimethylbenzeneb
Alternative name
Ethylene Propylene Acetylene i-butane
i-pentane
i-hexane
i-octaneb
Measured by (type of analyzer)a C, C, C, C, C, C, C, C, C, C, C, C, C, C, P C, C, C, C, C, C, P
P P P P P P P P P P P P P, V P
C, P C, C, C, C, P
P
P P P P P P, V
P, P, P, P,
V V V V
P P
a Refers to the type of analyzer these compounds were measured with, C ¼ Chrompack VOCAIR, P ¼ Perkin Elmer OPA, V ¼ Environnement VOC71M. b These VOCs do not have long enough data records for trend analysis in this study. c Refers to a mixture of two isomers that could not be separated with the chromatographic systems utilized in this study.
for the years 1995–2001 inclusive and so this was the chosen period for analysis for all the VOCs except benzene and 1,3-butadiene. This same period was chosen for trend analysis for all the VOCs except benzene and 1,3-butadiene at the rural site. At the kerbside site, measurements began during 1997 and are still ongoing without any changes in instrumentation and location. Trend analysis has been performed through to the end of 2004 for all VOCs.
Benzene and 1,3-butadiene trends at the rural and urban background sites have been extended forward from 2002 onwards using data from different instruments (VOCAIR vs. VOC71M) and with a change of site at Cardiff. Every effort has been made to ensure that these changes have not materially altered the evaluated trends. 3. Benzene and 1,3-butadiene Benzene and 1,3-butadiene are important hydrocarbons in their own right because in addition to taking part in photochemical ozone formation, they are toxic and carcinogenic. Air quality standards have been set at 5 ppb (16.3 mg m3) and 1 ppb (2.3 mg m3), respectively, for annual mean concentrations (EPAQS, 1994, 2002). At the start of the monitoring period, annual mean benzene and 1,3-butadiene levels were reported as 0.7–1.9 ppb (2.3–6.2 mg m3) and 0.11–0.38 ppb (0.25–0.86 mg m3), respectively, by Derwent et al. (2000) at urban background sites in the UK. These reported levels were well below the air quality standards set to protect human health. Both hydrocarbons arise principally from petrol-engined motor vehicle exhaust emissions with some contribution from evaporating petrol and so urban background concentrations should have been strongly influenced by the implementation of three-way catalysts and evaporative canisters. In view of their importance as toxic compounds for which annual mean air quality guidelines and standards are appropriate, trends in annual mean concentrations have been the main focus of our analysis. Table 3 summarises the annual mean air quality data for benzene and 1,3-butadiene monitored over the period from 1993 to 2004. For this purpose, the eleven urban background sites have been split into two groups. The first group contains eight sites: Belfast, Birmingham, Bristol, Cardiff, Edinburgh, Leeds, London UCL and London Eltham. These sites, see Table 1, are located away from the immediate and direct influence of traffic in towns and cities where motor vehicle traffic is the major source of hydrocarbons. The second group contains three sites: Liverpool, Middlesbrough and Southampton. These sites, see Table 1, are located in towns and cities where there are considerable industrial sources of hydrocarbons in addition to motor vehicle traffic. The entries in Table 3 for urban background locations represent the average
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1,3-butadiene (mg m3)
Rural UB UI Kerbside Rural UB 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
1.2 1.2 1.2 1.1 0.9 0.5 0.6 0.6 0.4 0.8
4.6 3.9 3.6 3.7 3.7 2.8 2.6 1.5 1.8 0.7 1.9 0.8
5.6 4.1 4.1 15.3 3.3 12.8 3.1 10.8 2.2 6.3 4.5 3.9 3.3 2.8
0.39 0.15 0.15 0.12 0.12 0.09 0.11 0.04 0.03 0.02
0.64 0.53 0.50 0.49 0.51 0.36 0.33 0.26 0.27 0.05 0.48 0.15
UI
0.85 0.56 0.52 0.41 0.38 0.35
Kerbside
3.3 2.4 1.9 1.6 1.1 0.95 0.64 0.57
Notes: Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic.
of the eight sites and for urban industrial, the average of the three sites. The average annual mean benzene at the urban background sites have declined steadily from 4.6 mg m3 (1.4 ppb) at the start of the monitoring period to 0.8 mg m3 (0.25 ppb) at the end. The benzene time series at the urban background sites showed a highly statistically significant downwards trend at the 99.9% level of significance using the non-parametric Mann–Kendall method (Salmi et al., 2002) with an estimated trend of 0.3470.04 mg m3 yr1 (0.1070.01 ppb yr1) using the non-parametric Sen’s method (Salmi et al., 2002) with 1s confidence limits on the slope. The observed downwards trend amounted to 23% per year when expressed relative to the mean concentration during the year 2000. Annual average benzene levels at the urbanindustrial sites were somewhat higher than those at the urban background sites, confirming that there were additional benzene sources over and above motor vehicle traffic in the vicinity of these sites. Despite these additional industrial sources, annual average benzene concentrations at the urban-industrial sites still showed the same downward trend, about 20% per year, as that shown by the urban background sites. Annual mean benzene concentrations at the kerbside site have fallen from 15.3 mg m3 (4.7 ppb) at the start of the measurement period to 2.8 mg m3
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(0.9 ppb) at the end. Levels have thus declined by about 30% per year from close to the air quality standard to well below it over the period from 1997 to 2004. At the kerbside site then, annual mean benzene levels have declined by 1.9 mg m3 yr1 (0.6 ppb yr1), a downwards trend of about 30% per year which is highly statistically significant at the 99.9% level of significance. The observed concentration trend at the rural site was found to be 0.170.02 mg m3 yr1 (0.0370.006 ppb yr1), a downwards trend of about 20% per year that is highly statistically significant at the 99% level of significance. Trends at single sites may be influenced by local factors such as changes in traffic and hence may not be indicative of countrywide trends. Annual mean 1,3-butadiene concentrations at the urban background sites have fallen from 0.64 mg m3 (0.28 ppb) at the start of the measurement period to 0.15 mg m3 (0.07 ppb) at the end. The 1,3-butadiene time series showed a highly statistically significant downwards trend at the 99% level of significance of 0.04370.007 mg m3 yr1 (0.01970.003 ppb yr1). This trend amounted to about 17% per year at the urban background sites. At the urban-industrial sites, annual mean 1,3-butadiene levels are somewhat higher than those at urban background sites but show a similar downwards trend of 18% per year. These differences in percentage trends are not considered significant. At the kerbside site, annual mean 1,3-butadiene concentrations exhibited a downwards trend of 0.34 mg m3 yr1 (0.15 ppb yr1) was observed which was significant at the 99.9% level of significance. The trend at the rural site was equally significant and was estimated to be 0.02070.007 mg m3 yr1 (0.0097 0.003 ppb yr1). Both trends were about 21% per year. Scatter plots of the hourly mean concentrations of benzene and 1,3-butadiene showed excellent correlation with correlation coefficients R in excess of 0.9 at most sites during 1996 in our previous study (Derwent et al., 2000). Least squares regression fits were used to define the 1,3-butadiene to benzene ratio at each site and an average ratio by mass of 0.1670.02 was reported in that study. Here, the same excellent level of correlation between the hourly mean concentrations of benzene and 1,3butadiene was found at all the urban background, kerbside and rural sites studied and in each year. Correlations were not always as good at the urbanindustrial sites where sporadic peaks of either
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benzene or 1,3-butadiene lead to significantly poorer correlations. Averaging over all sites and years when the correlation coefficients R were greater than 0.9, the mean 1,3-butadiene to benzene ratio was found to be 0.1770.03 by mass, in close agreement with the above ratio reported for 1996 previously. The 90-day rolling mean concentrations of benzene and 1,3-butadiene were calculated for each day of the available record at the kerbside site and were divided to obtain the ratio of the 90-day rolling mean concentrations. The mean value over the entire record was 0.21370.036, consistent with the above ratios by mass. Least-squares regression analysis showed that the ratio of the 90-day rolling means exhibited a small upwards trend of 1.370.14% per year. It is, therefore, likely that there has been little significant change in the 1,3butadiene to benzene ratio over the 8-year period at the kerbside site, despite the annual mean concentrations of both hydrocarbon concentrations declining by over a factor five. This would suggest that there has been no material change in the principal source of both hydrocarbons over the 8-year study period. This is consistent with this principal source being emissions from uncontrolled petrol-engined vehicles without catalysts and canisters, with the decline in ambient concentrations being driven by the reduction in vehicle-kilometres as these vehicles age and are scrapped. In view of the importance in air quality policy formulation of emission inventories, it is important to check these inventories against air quality data. Annual UK total benzene emissions have declined from 52,000 ton yr1 in 1993 to 18,000 ton yr1 (NAEI, 2005), a rate of decline of 15% per year. This decline takes into account the uptake of catalyst and canister technologies, deterioration of catalysts, cold-start emissions and emissions from diesel-engined vehicles. Over this same period, the average annual mean benzene concentrations at the urban background sites have declined by about 16% per year. The agreement in these rates of decline is most heartening. The ratio of average annual mean concentration at the urban background sites to UK total emissions have, therefore, remained largely unchanged at 0.0857 0.005 mg m3 per 1000 ton yr1. This shows that the observed urban background concentrations are exactly mimicking the inferred decline in the UK total emissions over the 1993–2000 period, provid-
ing strong evidence for the description of benzene emissions in the inventory. The observed urban background 1,3-butadiene trend over the 1993–2000 period agreed closely with the trend in UK total 1,3-butadiene emissions (NAEI, 2005), providing strong evidence for the description of 1,3-butadiene emissions in the inventory. Again, the ratio of the average annual mean concentration to the total UK emission remained largely unchanged at 0.06070.007 mg m3 per 1000 ton yr1, despite both changing by a large factor. However, there is no reason for this average ratio to differ significantly between benzene and 1,3-butadiene if both emission inventories were indeed dominated by motor vehicle emissions. The difference in the ratios implies that either the benzene emissions have been underestimated by about 30% or those of 1,3-butadiene have been overestimated by a similar amount. The presence of a northern hemisphere background benzene concentration could explain some of the difference in ratios. 4. Ethylene and propylene Ethylene (ethene) and propylene (propene) are two of the most important ozone precursors in north west Europe and contribute the second and sixth largest contributions to peak ozone formation in photochemical modelling studies (Derwent et al., 2003). Since photochemical ozone episodes may be observed at any time from the spring through to the autumn in north west Europe, the 90-day rolling mean concentration is a reasonable way to present air quality data for hydrocarbons that are ozone precursors but not air toxics or carcinogens for which annual means are the most appropriate. Motor vehicle exhaust emissions account for the majority of urban emissions of ethylene and propylene (NAEI, 2005). Table 4 contains a summary of the ethylene and propylene observations obtained from the available measurements, focusing on the 1995–2000 period when the majority of sites were operating reliably. Considering first the annual mean concentrations of ethylene reported in Table 4 for the year 2000. Concentrations were highest at the kerbside site and lowest at the rural site, with average concentrations spanning a factor 24. Urban levels were found midway between these extremes, with levels at urban-industrial sites somewhat elevated above urban background levels. Oil refineries appear to
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act as large sources of ethylene and raise levels significantly above those anticipated from motor vehicle traffic alone. The average 90-day rolling mean concentrations of ethylene at the urban background sites over the 1995–2000 period shows a significant downwards trend of 0.28 mg m3 yr1 which is equivalent to 12% per year. Trends at the urban-industrial sites were found to be somewhat smaller, 5% per year, demonstrating the hydrocarbon emission reductions have been largely but not wholly confined to the motor vehicle traffic sector. In contrast, trends at the rural and kerbside sites are reported as 28% per year and 16% per year, respectively, confirming that motor vehicle emissions are dominant source of ethylene at both sites. As noted above, trends at single sites may not be indicative of UK-wide trends. A similar picture is found in Table 4 for propylene as for ethylene. Urban levels are again found to be midway between the kerbside and rural levels. The average annual mean concentrations for the urban-industrial sites are elevated over those found for the urban background sites, showing that propylene emissions from oil refineries are significant, as with ethylene. Trends at the urban background sites over the 1995–2000 period are given as 15% per year, 8% per year and 45% per year, respectively, at kerbside, urban background and
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rural sites. The controls implemented on the exhaust emissions of petrol-engined motor vehicles have therefore brought about a dramatic reduction in urban and rural levels of propylene during the period of this study. 5. n- and i-butane The isomers of butane, n-butane (butane) and ibutane (methylpropane), are important ozone precursors in north west Europe and account for the largest single contribution and the twelfth largest contributions, respectively, to photochemical ozone formation as reported in Derwent et al. (2003). Since they arise principally from the evaporation of motor spirit (NAEI, 2005), they provide an important indication of the performance of the canisters that have been fitted to petrol-engined motor vehicle to control evaporative emissions. Urban background levels of n-butane are midway between the kerbside and rural levels, with urbanindustrial sites showing only a small elevation in concentrations above urban background sites, see Table 4. Ninety-day rolling mean concentrations have shown consistent downwards trends at all sites. The trend in the average 90-day rolling mean concentrations at the urban background sites was found to be 0.93 mg m3 yr1, equivalent to 18%
Table 4 Annual mean concentrations in the year 2000 and trends in 90-day rolling mean concentrations at rural, urban traffic-influenced, urban industrial-influenced and kerbside locations for 10 hydrocarbons in the UK Species
Ethylene Propylene Toluene n-butane i-butane n-pentane i-pentane Ethaned Propanee Isoprene
Rurala
UBb
UIb
Kerbsidec
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
2000 mean (mg m3)
Trend (mg m3 yr1)
0.6 0.4 1.3 1.2 0.9 0.3 1.2 2.3 1.5 0.05
0.17 0.18 0.24 0.29 0.19 0.08 0.23 0.17 0.19 0.017
2.3 1.9 5.0 5.1 1.9 1.4 4.1 5.3 3.9 0.16
0.28 0.15 0.71 0.93 0.70 0.15 0.77 0.07 0.31 0.04
3.4 2.2 6.2 5.8 2.0 2.0 5.1 6.3 5.2 0.22
0.18 0.14 0.58 1.00 0.73 0.31 0.53 +0.23 0.65 0.05
14.6 7.2 28.8 19.9 9.7 5.3 23.7 9.6 5.5 0.95
2.4 1.1 3.8 2.2 1.7 0.61 3.6 +0.15 0.68 0.27
Site classification: UB, urban background; UI, urban with industrial influence; R, rural; K, kerbside, urban and under the immediate influence of road traffic. a Trends from 4/8/1995 to 31/12/2001. b Trends in 90-day rolling mean concentrations from 1/1/1995 to 31/12/2000. c Trends from 4/6/1997 to 31/12/2004 excepting n-butane which are for 1/1/1999 to 31/12/2004. d Cardiff is not included in analyses of ethane. e Belfast and Cardiff are not included in analyses of propane.
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per year. The respective trends at the rural and kerbside sites were 24% per year and 11% per year. The corresponding picture is similar for ibutane. The trend found for i-butane at the urban background sites was 0.70 mg m3 yr1, equivalent to 37% per year. Those found at the rural and kerbside sites were 21% per year and 18% per year, respectively. The magnitudes of the percentage downward trends observed for the evaporative hydrocarbons, n-butane and i-butane, are similar to those observed for the exhaust hydrocarbons, ethylene and propylene, at the different site types. This strongly suggests that the same factors are determining the influence of the emission controls, irrespective of whether the hydrocarbon emissions are from vehicle exhausts or fuel evaporation. This situation would apply if both technologies are highly efficient such that current emissions are heavily dominated by those from uncontrolled petrol-engined motor vehicles. In this case, the downward trends in both exhaust and evaporative hydrocarbons are determined by the decreasing vehicle-kilometres travelled by uncontrolled vehicles. In the future, when all of the uncontrolled vehicles have been scrapped, then emissions will be dictated by the remaining emissions from the controlled vehicles and the issue will then be the durability of the motor vehicle exhaust catalysts and evaporative canisters over the lifetime of the motor vehicles. Scatter plots of the hourly mean concentrations of n-butane and i-butane show excellent correlation with most sites and years exhibiting correlation coefficients R in excess of 0.9. The main exceptions to this pattern were the Birmingham and London Eltham sites where the scatter plots regularly showed huge deviations from general behaviour observed at the other urban background sites. Industrial sources contributed significantly to the observed levels of n-butane at the urban-industrial sites. Considering the sites and years when correlation coefficients R in excess of 0.9 were found, then the average regression slopes of n-butane vs. ibutane were found to be 1.8570.29 on a mass basis. This compares favourably with, though is somewhat lower than the ratio of 2.2270.3 reported in our previous study which considered only the 1996 observations. At the kerbside site, the ratio of the 90-day rolling means of n-butane to i-butane over the 1999–2004 period was 1.89670.021 and exhibited a small downwards trend of 0.12% per year.
6. n- and i-pentane The pentane isomers n-pentane (pentane) and ipentane (methylbutane) are important constituents of evaporating motor spirit (NAEI, 2005) and make the 7th and 5th largest contributions, respectively, to photochemical ozone formation in Europe (Derwent et al., 2003). Urban background concentrations of n-pentaneaveraged 1.4 mg m3 during the year 2000 and were found midway between the kerbside levels of 5.3 mg m3 and rural levels of 0.3 mg m3. Average concentrations at urban-industrial sites were elevated above those anticipated at urban background sites pointing to there being significant emission sources from oil refineries. Strong downward trends were found in the 90-day rolling mean concentrations of n-pentane over the 1995–2000 period. These were observed to be 0.15 mg m3 yr1 or 11% per year for the urban background sites, 11% per year and 26% per year at the kerbside and rural sites, respectively. Downward trends of these magnitudes are similar to those found for the exhaust and evaporative hydrocarbons discussed above and point to the efficient control of motor vehicle emissions of n-pentane by the evaporative canisters. The behaviour found for i-pentane was exactly analogous to that found for n-pentane, see Table 4. Levels of i-pentane spanned the range of a factor of 20 from kerbside sites to rural sites compared with the range of 18 for n-pentane. Downwards trends in the 90-day rolling mean concentrations were found to be 19% per year, 15% per year and 19% per year at the urban background, kerbside and rural sites, respectively. Scatter plots of n-pentane vs. i-pentane showed excellent correlation with most sites and years generating correlation coefficients R in excess of 0.9. Averaging those sites and years that showed excellent correlation, the linear regression slopes defined an average ratio by mass of 0.29770.060. This average ratio by mass is somewhat higher than that of 0.24 that we reported in our study of the 1996 data (Derwent et al., 2000). The sites at Cardiff and Edinburgh showed evidence of sporadic sources with enhanced n-pentane to i-pentane ratios, which destroyed the close correlation generally found at urban background sites. The urban-industrial sites showed evidence of significant sources of n-pentane and i-pentane with a ratio by mass of close to 1:1, presumably from refinery sources. Over the 1997– 2004 period, the average n-pentane to i-pentane
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ratio observed at the kerbside site was 0.23770.032 and exhibited an upwards trend of +5.0% per year. This would point to there being a small additional source of n-pentane and i-pentane other than that from motor vehicle evaporative emissions with a characteristically different ratio by mass of the pentane isomers. 7. Isoprene The hydrocarbon isoprene (2-methylbuta-1,3diene) is commonly regarded as being derived from natural biogenic sources associated with plants and trees and is a potent source of photochemical ozone. Our previous study found evidence for an urban source of isoprene which was tentatively identified as motor vehicular, based on its close correlation with benzene (Derwent et al., 2000). Table 4 summarises the results found here for isoprene. Highest concentrations during 2000, of all the sites monitored were reported at the kerbside site with a mean concentration of 0.95 mg m3. Urban background concentrations were about 0.16 mg m3, a factor of about six lower than those at the kerbside site. Rural concentrations averaged 0.05 mg m3, about a factor of 19 below kerbside levels. Urbanindustrial sites showed somewhat higher concentrations compared with urban background sites, pointing to there possibly being oil refinery sources of isoprene in addition to the motor traffic sources found in most urban areas. Downwards trends have been observed in the 90day rolling mean concentrations of isoprene monitored at all the sites over the period from 1995 to 2000. A strong downward trend of 0.27 mg m3 yr1 or 28% per year has been reported for the kerbside site, see Table 4. Averaged over the urban background sites, trends of 0.04 mg m3 yr1 or 25% per year were found compared with 0.017 mg m3 yr1 or 34% per year at the rural site. It is concluded that, certainly at the start of the observations, isoprene emissions and urban concentrations were dominated by emissions from motor vehicles. As a result of motor vehicle emission controls, urban isoprene concentrations have declined by close to an order of magnitude over the period of the study. Scatter plots of hourly mean observations of isoprene and benzene show evidence of good correlation with correlation coefficients in excess of 0.8 at a small number of sites and years. Leastsquares fits were used to derive the ratio of isoprene
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to benzene during those years when good correlation was observed. Averaged over all these occasions, a mean isoprene to benzene ratio by mass of 0.12870.030 was found in close agreement to 0.11370.034 reported for 1996 by Derwent et al. (2000). The good correlations observed at the Harwell (rural), Liverpool (urban-industrial) and London Marylebone Road (kerbside) sites were lost at the end of the study period. This was found to be due to the presence at the end of the study period, of an additional isoprene source other than from motor vehicles. The order of magnitude reduction in isoprene and benzene levels due to motor vehicle emission controls has exposed the natural source of isoprene and destroyed the close correlation between isoprene and benzene at the end of the study period. Monthly mean isoprene concentrations show clear-cut seasonal cycles with maxima during the summer months at most urban background sites. Taking the London Eltham site as typical then, over the entire 1993–2004 period, monthly mean isoprene levels during the winter months were generally in the range 0.2–0.3 mg m3 and rose to about 0.6 mg m3 during summertime. Splitting these data into two periods: 1993–1996 and 2000–2004, we find that levels in winter, spring and autumn have fallen substantially between the start and the end of the study period by factors of two to three. In contrast, levels have fallen only by about one-thirds during the summer period. The implication is that the motor vehicle source of isoprene has been reduced dramatically particularly during wintertime whilst the summertime natural source has remained much the same. At the London Marylebone Road site, the seasonal shifts in monthly mean isoprene concentrations have been even more dramatic. Levels have fallen by factors of between three and five throughout the year and the seasonal cycle has changed from showing wintertime peaks at the start to showing summertime peaks by the end of the study period. 8. Ethane and propane Ethane and propane in urban areas are generally derived from natural gas leakage (NAEI, 2005) and consequentially show markedly different diurnal cycles to the other hydrocarbons that are derived from motor vehicle traffic (Derwent et al., 2000). Because of local gross contamination, the data from the Cardiff and Belfast sites have not been included in the analyses in Table 4. Of all the hydrocarbons
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monitored in this study, the range in annual mean concentrations across the different site types, see Table 4, for the year 2000 is the narrowest for ethane and propane. The ratios of the most to least polluted sites span factors of 4.2 and 3.7, respectively, for ethane and propane whereas such factors may approach 20 for the other hydrocarbons studied. There are appreciable northern hemisphere background concentrations of these hydrocarbons (Derwent et al., 2000) and these account for the relatively high rural concentrations which are observed. Indeed, ethane and propane are the highest concentration hydrocarbons reported in Table 4 for the rural site with mean concentrations of 2.3 and 1.5 mg m3, respectively. Concentrations at the urban-industrial sites are considerably enhanced over those for urban background sites for both hydrocarbons demonstrating that oil refineries are significant sources. Trends in 90-day rolling mean ethane concentrations do not present a uniform picture by any means, compared with the other hydrocarbons studied. Ethane concentrations appear to rising slightly at the kerbside and urban-industrial sites by +2% per year and +4% per year, respectively. At the urban background and rural sites, ethane concentrations are falling by 1% per year and by 7% per year, respectively. Within the range of errors and uncertainties, such trends are considered small and it is likely that ethane levels are showing no trends of any significance. In contrast, 90-day rolling mean propane concentrations are showing strong but variable downwards trends: 13% per year at the rural site, 8% per year at the urban background sites, 13% per year at the urban-industrial sites and 12% per year at the kerbside site. This points to there having been a significant motor vehicle contribution to propane levels at all sites that is being steadily reduced by motor vehicle emission controls. There appears to have been a negligible motor vehicle contribution to ethane levels, with levels being dominated by natural gas leakage. This will soon be the situation for propane as the motor vehicle contribution eventually becomes vanishingly small. Scatter plots of hourly mean ethane and propane concentrations show close correlations at some sites during some years. Considering only those sites and years that gave correlation coefficients R greater than 0.8, then the least-squares regression slopes defined average ethane to propane ratios of 1.3670.30 by mass. The corresponding ratio in our previous study (Derwent et al., 2000) was 1.24 which is close to the above value.
9. Other hydrocarbons The 90-day rolling mean concentrations of 14 additional hydrocarbon species observed at the London Marylebone Road kerbside location, over the 1997–2004 period were calculated and the trends estimated, see Table 5. The 14 additional hydrocarbons included one alkyne, 5 alkenes, 4 alkanes and 4 aromatic species, with one of the latter species being an unresolved mixture of two isomers, mxylene and p-xylene. All these species showed highly significant downward trends that encompasses the range from 14 to 21% per year. It is, therefore, highly likely that all of these species are derived largely from motor vehicle exhaust or evaporative emissions. Their declines in concentrations at the kerbside location reflect the decreasing usage of uncontrolled petrol-engined motor vehicles without catalysts and canisters, as noted above.
10. Discussion and conclusions This study reports results from a comprehensive high time resolution (hourly) monitoring programme for 26 hydrocarbons carried out from 1993 to the end of December 2004 at 11 urban background, a kerbside and a rural location in the Table 5 Trends in 90-day rolling mean concentrations from 1998 to 2004 at a kerbside location in London, together with their whole period mean concentrations and percentage trends, for 14 hydrocarbons Species
Mean concentration (mg m3)
Trend (mg m3 per year)
Percentage trend (% per year)
Acetylene trans but-2-ene cis but-2-ene but-1-ene trans pent-2-ene cis pent-2-ene 2-methylpentane 3-methylpentane n-hexane n-heptane Toluene Ethylbenzene m7p-xylene o-xylene
6.5 1.2 0.95 1.3 1.3 0.72 6.3 3.6 2.2 1.4 21.7 4.0 13.9 5.1
0.95 0.22 0.18 0.24 0.22 0.12 1.09 0.62 0.38 0.29 3.74 0.70 2.23 0.89
15 18 19 19 17 17 17 17 18 21 17 18 16 17
Notes: m7p-xylene refers to a mixture of the two isomers that could not be separated with the chromatographic systems utilized in this study.
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UK. Benzene concentrations have declined at 20 to 30% per year and 1,3-butadiene concentrations at 17 to 21% per year, across all the locations and site types. Similar significant downward trends have been reported for 22 additional hydrocarbon species. Ethane and propane have shown distinctly different behaviour, exhibiting no significant trends and small downwards trends, respectively. The trends in benzene and isoprene at the kerbside location appear to be significantly greater that those reported for the other VOC species at this and other sites. This may not necessarily reflect real differences in behaviour across the UK because of the difficulty in interpreting trends at single sites because of local site influences. The observed dramatic reduction in hydrocarbon concentrations across the UK is attributed to the implementation of exhaust gas catalyst and evaporative canister control technologies on petrolengined motor vehicles. The results are consistent with the almost complete elimination of hydrocarbon emissions from the newer controlled motor vehicles. The principal source of the observed hydrocarbon concentrations appears to be the older uncontrolled motor vehicles. The scrappage and reduced number of vehicle kilometres driven by the older uncontrolled motor vehicles has brought about a dramatic decline in hydrocarbon emissions and hence hydrocarbon concentrations. This will, in turn, have produced a marked reduction in episodic peak ozone concentrations.
Acknowledgements This study and the monitoring network that underpinned it, was supported by the Air and Environmental Quality Division of the Department for Environment, Food and Rural Affairs under contract RMP 2423. The assistance of Wei-Wei Qu and Kevin Clemitshaw of Imperial College London with the analysis of the monitoring data is gratefully acknowledged. The help and assistance provided by Trevor Davies, John Stedman and Tim Murrells of NETCEN is much appreciated.
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References Commission of the European Communities CEC, 1991. Council directive amending directive 70/220/EEC on the approximation of the laws of member states relating to the measures to be taken against air pollution by emissions from motor vehicles. 91/441/EEC. Journal of the European Communities, L242/1-L242/106. Derwent, R.G., Davies, T.J., Delaney, M., Dollard, G.J., Field, R.A., Dumitrean, P., Nason, P.D., Jones, B.M.R., Pepler, S.A., 2000. Analysis and interpretation of the continuous monitoring data for 26 C2–C8 hydrocarbons at 12 United Kingdom sites during 1996. Atmospheric Environment 34, 297–312. Derwent, R.G., Jenkin, M.E., Saunders, S.M., Pilling, M.J., Simmonds, P.G., Passant, N.R., Dollard, G.J., Dumitrean, P., Kent, A., 2003. Photochemical ozone formation in north west Europe and its control. Atmospheric Environment 37, 1983–1991. Dollard, G.J., Davies, T.J., Jones, B.M.R., Nason, P.D., Chandler, J., Dumitrean, P., Delaney, M., Watkins, D., Field, R.A., 1995. The UK hydrocarbon monitoring network. In: Lester, R.E., Harrison, R.M. (Eds.), Volatile Organic Compounds in the Atmosphere, Issues in Environmental Science and Technology, Number 4. The Royal Society of Chemistry, Cambridge, UK. EPAQS, 1994. Expert Panel on Air Quality Standards. Benzene. HMSO, London. EPAQS, 2002. Expert Panel on Air Quality Standards. Second Report on 1,3-butadiene. DEFRA, London. Larssen, S., Barrett, K.J., Fiala, J., Goodwin, J., Hagen, L.O., Henriksen, J.F., de Leeuw, F., Tarrason, L., 2002. Air Quality in Europe. State and Trends 1990–1999. European Environment Agency, Copenhagen, Denmark. NAEI, 2005. National Atmospheric Emission Inventory. /http:// airquality.co.ukS. Salmi, T., Maatta, A., Anttila, P., Ruoho-Airola, T., Amnell, T., 2002. Detecting trends of annual values of atmospheric pollutants by the Mann–Kendall test and Sen’s slope estimates—the Excel template application MAKESENS. Publications on Air Quality No. 31, Finnish Meteorological Institute, Helsinki, Finland. UN ECE, 1991. Protocol to the 1979 Convention on long-range transboundary air pollution concerning the control of emissions of volatile organic compounds or their transboundary fluxes. ECE/EB.AIR/30. United Nations Economic Commission for Europe, Geneva, Switzerland. UN ECE, 1999. Protocol to the 1979 Convention on Long-range Transboundary Air Pollution to Abate Acidification, Eutrophication and Ground-level Ozone. United Nations Economic Commission for Europe, Geneva, Switzerland. WHO, 1987. Air Quality Guidelines for Europe. World Health Organisation Regional Publications, European Series No. 23, Copenhagen, Denmark.