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Diurnal cycles of short-lived tropospheric alkenes at a north Atlantic coastal site
Atmospheric Environment 33 (1999) 2417—2422
Short Communication Diurnal cycles of short-lived tropospheric alkenes at a north Atlantic coastal site A.C. Lewis *, J.B. McQuaid, N. Carslaw, M.J. Pilling The Environment Centre, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK Received 10 September 1998; received in revised form 30 November 1998; accepted 4 December 1998
Abstract Observation of diurnal cycles in atmospheric concentrations of reactive alkenes are reported from measurements performed at a North Atlantic coastal site (Mace Head, Eire 53°19 34 N; 9°54 14 W). Species seen to exhibit distinct cycles included isoprene, ethene, propene, 1-butene, iso-butene and a substituted C alkene. Five hundred and thirty air mass classified measurements were performed over a 4 week period at approximately hourly frequency and demonstrate that during periods when air flow resulted from unpolluted oceanic regions a clear daily cycle in concentrations existed, peaking at around solar noon for all species. These observations support the proposed mechanism of production via photochemical degradation of organic carbon in sea water. The observed concentrations showed strong correlation (propene R'0.75) with solar flux, with little relationship to other meteorological or chemical parameters. The species’ short atmospheric lifetimes indicate that the source of emission was from local coastal waters within close proximity of the sampling site. At solar noon concentrations of reactive alkenes from oceanic sources were responsible for up to 88% of non-methane hydrocarbon reaction with the hydroxyl radical at this coastal marine site. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Diurnal cycles; Reactive alkene; Air mass origin; Hydroxyl radical chemistry; Oceanic emissions
1. Introduction The important role played by reactive volatile organic compounds (VOCs) in tropospheric ozone chemistry is now well established. The presence of both biogenic and anthropogenic VOCs induces substantial perturbations from the CO/CH controlled chemistry of the remote unpolluted atmosphere. Hydroxyl radical oxidation of VOCs, effected through the initial photolysis of ozone at wavelengths below 320 nm, may result in the net forma-
tion of tropospheric ozone through the formation of peroxy radical intermediates and the subsequent conversion of NO to NO . VOC#OHPR#H O, R#O PRO , RO #NOPRO#NO , NO #hlPNO#O, O#O #MPO #M,
* Corresponding author.
Overall: VOC#NO #hlPO . V
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 9 - 4
2418
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
It is in the remotest areas of the troposphere that the influence of VOCs is least well understood. In the marine environment, for example, the flux of non-methane hydrocarbon species to the atmosphere may provide sufficient input to result in net photochemical gains in ozone given a sufficient concentration of NO . V Of the many organic species released to the atmosphere, alkenes exhibit some of the most rapid rates of reaction with the hydroxyl radical and further, reactions between alkenes and ozone have been reported to yield OH as a by-product (Atkinson, 1997). Therefore, any photochemical or biogenic mechanism resulting in a positive ocean/atmosphere flux of even small amounts of these compounds will have a significant effect on nonmethane hydrocarbon (NMHC) tropospheric chemistry in the oceanic environment. Hydroxyl radical loss processes due to NMHC chemistry in the clean marine troposphere have been estimated at around 10% of total loss (CO, 53%, CH , 21%, NO , 6%, O , 4%, H , 6%; V Carslaw et al., 1998). The mechanisms by which non-methane hydrocarbons are generated in sea water is still not clearly understood, however, a photochemical mechanism acting on dissolved organic matter has gained wide acceptance (Ratte et al., 1993). Very recent laboratory studies (Ratte et al., 1998) have shown that alkene formation is controlled by the solar photon flux at the sea surface, the penetration depth (euphotic layer) of light into the ocean, the wavelength (300—420 nm optimum) and dissolved organic carbon (DOC) dependent quantum yields. Spatial and seasonal variations in numerous NMHC species have been reported from the late 1960s initially by Swinnerton and Linnenbom (1967) and later by Linnenbom and Swinnerton (1970), Lamontagne et al. (1974) and Frank et al. (1970). In more recent years combined sea water and atmospheric measurements have been reported and estimates of global budgets made. A review of more than 1000 individual measurements has been made recently by Plass-Dulmer et al. (1995). Spatial and seasonal variations in non-methane hydrocarbons from sea water are currently poorly characterised, however, recent work by Broadgate et al. (1997) has shown a significant seasonal cycle in isoprene, correlated to chlorophyll content of water. The strong diurnal cycle reported in this study has not previously been observed in marine air, although no observations have previously been made with the high temporal resolution of data collection reported here.
In this work we report continuous atmospheric nonmethane hydrocarbon measurements performed at a fixed coastal site at Mace Head Eire 53°19 34 N; 9°54 14 W. The site is located along the north Atlantic cyclone track and benefits from clean unpolluted air flow from a wide range of oceanic regions.
2. Experimental Continuous hourly non-methane hydrocarbon measurements were performed using a method described previously by Lewis et al. (1996) based on programmed temperature vaporisation injection and capillary gas chromatography. Samples of ambient air were collected from a height of 10 m through a glass inlet manifold, pumped at high flow rates such that internal residence time was approximately 1 s. Inlet sample air was dried using a combination of a condensation trap held at !10°C and a magnesium perchlorate trap to remove remaining water and ozone. It has been suggested that there may be photochemical production mechanisms leading to alkene formation on the internal surfaces of glass manifolds. However, recent measurements, also conducted at the Mace Head site, acquired atmospheric samples via Teflon and stainless-steel manifolds and the same alkene cycles were observed. The determination of air mass origin is often performed using in situ meteorological observations. A superior diagnosis of air mass history may be obtained, from calculated back-trajectories based on wind fields analysis. In order to determine the air mass origin for samples collected during this study three-dimensional 5 day back-trajectories were calculated from the ECMWF T106 L31 (1.25°;1.25°;31 levels resolution) every 6 h of the measurement campaign. The arrival point for each trajectory was at 53°19 34 N; 9°54 14 W and 1000 mb (or the highest available pressure if the surface pressure was below 1000 mb). Emission rates from the ocean surface depend upon the transfer velocities, which are mainly a function of the wind velocity over the sea surface (Liss and Merlivat, 1986). During the study period the mean wind velocity for marine air was 8.4 m s\ (n"1132, s.d."3.18). Table 1 outlines the frequency distribution of the wind velocities. The wind velocity only fell below 5 m s\ for 13% of the study period.
Table 1 Frequency distribution of wind velocity for marine air Velocity (ms\)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Frequency
4
32
39
71
103
149
164
134
91
101
78
59
30
35
37
5
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
2419
Fig. 1. Site location and local topography.
Measurements were performed over the period April—May 1997 at the Mace Head Atmospheric Observatory as part of the Eastern Atlantic Spring Experiment (EASE97). The campaign formed the Oxidative Capacity of the Oceanic Atmosphere (OXICOA) component of the NERC funded Atmospheric Chemistry Studies of the Oceanic Environment (ACSOE) project. The station forms part of the Advanced Global Atmospheric Gases Experiment (AGAGE) experiment, as well as being a EUROTRAC/TOR (Tropospheric Ozone Research) station (Simmonds, 1996). A more detailed description of the site can be found in Cvitas and Kley (1994). Fig. 1 illustrates the sampling location in relation to the shoreline. A wide volatility range of alkene species was determined (along with alkane and aromatic species in the mass range C —C ). Air flow during the measurement period was relatively varied resulting in air mass arrival at Mace Head that was of USA, South Westerly, Polar, UK and European Continental origin. Over the course of the measurement period, it has therefore been necessary to determine very precisely periods when the air mass may have contained alkenes of anthropogenic origin and when the air could be considered to contain alkenes of exclusively oceanic origin. Table 2 demonstrates calculated alkene lifetimes assuming typical mean daytime (night time) radical concentrations [OH] 1.00;10 (8.00;10) molecules cm\, [NO ] 1.00;10 (1.20; 10) molecules cm\, [O ] 8.75;10 (1.25;10) mol ecules cm\, [Cl] 1.00;10 (1.00;10) molecules cm\. From these relatively short lifetimes it can be seen that if a minimum transit period of 72 h is imposed on any air
Table 2 Calculated alkene lifetimes in the summer North Atlantic oceanic environment Compound
Daytime lifetime (h)
Nighttime lifetime (h)
Total lifetime (h)
Ethene Propene 1-Butene iso-Butene Isoprene C Alkene
13.1 4.1 6.6 4.3 1.7 5.6
21.9 6.9 26.8 5.3 1.3 24.8
16.3 5.0 11.7 4.6 1.5 10.4
Average lifetime calculated assuming 14 h day/10 h night. Lifetime calculated for 1-hexene; the least reactive C alkene.
mass passing exclusively over the ocean, then it can be assumed that the arriving air mass will contain negligible alkenes of anthropogenic origin.
3. Results and discussion From the continuous measurement of NHMC species over the period April and May 1997, 530 individual data points were screened for analytical integrity, and clear trajectory classification. The differing synoptic conditions prevailing during the measurement period are summarised in Fig. 2 which shows typical 5 day back-trajectory analysis and ambient concentrations of the anthropogenic
2420
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
Fig. 2. Typical back-trajectories and alkene time series showing strong diurnal cycles.
Table 3 Summary of alkene concentrations observed at Mace Head Polar/tropical/westerly
Anticyclonic
Compound
Average pptv
Maximum pptv
Minimum pptv
Peak [HC] hours from solar noon
Average pptv
Maximum pptv
Minimum pptv
Ethene Propene 1-Butene iso-Butene Isoprene C Alkene
19.3 23.8 5.2 12.8 2.6 31.1
67.5 95.7 46.3 138.9 37.1 94.0
0.0 0.0 0.0 0.0 0.0 0.0
#0.2 #0.1 0.0 #0.1 !0.1 !0.2
51.7 51.6 13.4 26.1 15.5 40.4
146.9 136.0 30.4 52.4 97.7 70.5
30.1 25.7 5.0 18.0 0.0 22.1
marker compound acetylene plus propene and iso-butene (offset in the plot by 50 pptv). The time series is annotated with generalised air mass classifications. When the air mass was of oceanic origin (defined previously as having spent at least the previous 72 h over the ocean) a very clear diurnal cycle in propene and isobutene concentrations can be observed. The maximum of each cycle is at around solar noon for all species. These
observations are similar to measurements of isoprene performed at the same site in 1996 (Lewis et al., 1996). During periods of direct transport of air masses from the UK/Europe under anticyclonic conditions little discernible diurnal cycle in these alkenes is visible, however, their ubiquitous presence in anthropogenic pollution results in notably elevated concentrations. During periods when the air mass contains significant pollution but
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
has arrived at Mace Head via an oceanic route, the diurnal cycle is similarly superimposed with highly variable concentrations of anthropogenic emissions. Under polluted conditions the concentrations of alkene species are closely correlated to other alkane and aromatic nonmethane hydrocarbon species. Several other alkene species, namely ethene, 1-butene, isoprene and the C alkene, were also seen to exhibit very similar diurnal cycles when the air mass was classified as being of oceanic origin.
Fig. 3. Relationship between alkenes and solar flux in oceanic air.
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Table 3 shows a summary of all species observed to exhibit a diurnal cycle in concentration in air of oceanic origin, along with the concentrations of these species observed under polluted anticyclonic conditions. Table 3 illustrates that the concentrations observed during polluted anticyclonic air flow are of a similar magnitude when compared to alkenes present in ‘clean’ air of exclusively oceanic origin. The presence of elevated concentrations of propene, iso-butene and ethene is most significant since these species have been identified (PORG 4, 1997) to have amongst the highest photochemical ozone creation potentials (POCP) of any NMHCs, and are calculated to be capable of producing ozone at rates of up to 0.8 ppb h\ under high NO conditions. V The observed concentrations demonstrated little dependence on wind speed, although only a relatively limited range of wind speeds was encountered over the measurement period (mean 8.4 m s\). A far greater dependence of alkene concentration in oceanic air with solar flux was observed, with a positive relationship and significant R values for all of the reported alkenes, e.g. propene, y"0.0283x#15.015 (R"0.7656), isobutene y"0.0223x#5.8966 (R"0.7653). The scatter plot of the relationship between observed propene and iso-butene concentrations and solar flux in air of oceanic origin is presented in Fig. 3. The role of the observed alkenes on localised tropospheric hydroxyl radical chemistry is most significant. The alkenes observed to follow a diurnal cycle may be responsible for over 75% of NMHC reactions responsible for OH radical removal. Table 4 shows the fractional contribution to OH removal of alkenes displaying a diurnal cycle, compared with total alkane/remaining alkenes (C —C ) reaction. The trajectories of oceanic origin have been sub-classified into polar and tropical/westerly groupings, since the differences in air mass chemical composition are such that significantly different fractional removal of OH by oceanic source alkenes occurs. The reduced significance of alkenes in polar air results from the elevated levels of other non-methane hydrocarbons observed to be present in this classification of air mass.
Table 4 Percentage contribution of grouped non-methane hydrocarbons to OH removal Compound
Alkenes showing diurnal cycle Alkanes/remaining alkenes (range C —C )
Polar
Tropical/westerly
Mean %
Min %
Max %
Mean %
Min %
Max %
76 20
58 33
82 15
88 10
76 19
88 10
Total OH loss due to NMHC calculated to be 10—12% (other processes; CO &70%, CH &20%) (from Carslaw et al., 1998).
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A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
The residual 2—8% of loss reactions may be apportioned to trace concentrations of aromatic and alkyne species.
4. Conclusions A significant diurnal cycle in tropospheric reactive alkene species has been observed over a 1 month period of continuous measurements at a north Atlantic coastal site. The cycle was observed in marine air irrespective of the long-range air mass characterisation, suggesting that the emission source area is relatively close to the monitoring location. The short atmospheric lifetimes of the observed species confirm that the emission was occurring within coastal waters, in close proximity to the measurement site. The peak concentrations of all reported alkene species were observed at around solar noon. The alkenes displaying diurnal cycles may play a most significant role in localised hydroxyl radical chemistry, and have been calculated to constitute up to 88% of NMHC initiated OH reactions. The observed atmospheric concentrations of the six reported alkenes were demonstrated to show a close correlation to solar flux, supporting a mechanism of alkene production via photochemical degradation of organic carbon (except in the case of isoprene which is biologically produced) in sea water.
Acknowledgements This work was performed as part of the ACSOE EASE97 experiment, supported by the U.K. Natural Environment Research Council. The authors would like to thank BADC, ECMWF, UKMO, and Paul Berrisford and John Methven of University of Reading for access to, and calculation of the trajectories mentioned in this paper. This paper is ACSOE publication number APC045.
References Atkinson, R., 1997. Gas phase tropospheric chemistry of volatile organic compounds: 1 alkanes and alkenes. Journal of Physical Chemistry Reference Data 26, 215—290. Broadgate, W.J., Liss, P.S., Penkett, S.A., 1997. Seasonal emissions of isoprene and other reactive hydrocarbon gases from the ocean. Geophysical Research Letters 24, 2675—2678. Carslaw, N., Creasey, D.J., Heard, D.E., Lewis, A.C., McQuaid, J.B., Pilling, M.J., Monks, P.S., Bandy, B.J., Penkett, S.A.,
1998. Modelling OH, HO and RO radicals in the marine boundary layer: 1. Model construction and comparison with measurements. Journal of Geophysical Research, (submitted). Cvitas, T., Kley, D., 1994. The TOR Network. A Description of TOR Measurement Stations. EUROTRAC special publications, ISS, Garmisch-Partenkirchen, Germany. ECMWF, 1994. The Description of ECMWF/WCRP Level III-A, Global Atmospheric Data Archive. Frank, D.J., Sackett, W.M., Hall, R., Fredericks, A., 1970. Methane, ethane and propane concentrations in the Gulf of Mexico. Journal of Geophysical Research 78, 5248—5258. Lamontagne, R.A., Swinnerton, J.W., Linnenbom, V.J., 1974. C —C hydrocarbons in the North and South Pacific. Tellus 26, 71—77. Lewis, A.C., McQuaid, J.B., Seakins, P.W., Pilling, M.J., Bartle, K.D., Ridgeon, P., 1996. Atmospheric monitoring of volatile organic compounds using programmed temperature vaporisation injection. Journal of High Resolution Chromatography 19, 686—690. Lewis, A.C., Bartle, K.D., McQuaid, J.B., Heard, D.E., Pilling, M.J., Seakins, P.W., 1997. In situ diurnal GC measurements of NMHC and DMS at a remote coastal location (Mace Head, Eire) July/August 1996. Journal of the Chemical Society, Faraday Transactions 93, 2921—2927. Linnenbom, V.J., Swinnerton, J.W., 1970. Low molecular weight hydrocarbons and carbon monoxide in sea water. In: Hood (Ed.), Organic Matter in Natural Waters. University of Alaska Press, Fairbanks, pp. 455—467. Liss, P.S., Merlivat, L., 1986. Air-sea exchange rates: introduction and synthesis. In: Buat-Menard, P. (Ed.), The Role of Air-sea Exchange in Geochemical Cycling. Reidel Publishing Company, Dordrecht, pp. 113—127. Plass-Dulmer, C., Koppmann, R., Ratte, M., Rudolph, J., 1995. Light non-methane hydrocarbons in seawater. Global Biogeochemical Cycles 9, 79—100. PORG 4, Ozone in the United Kingdom, 1997. Fourth Report of the Photochemical Oxidants Review Group. Department of the Environment, Transport and the Regions. ISBN 1870393-30—9. Ratte, M., Plass-Dulmer, C., Koppmann, R., Rudolph, J., Denga, J., 1993. Production mechanism of C —C hydro carbons in seawater: field measurements and experiments. Global Biogeochemical Cycles 7, 369—378. Ratte, M., Bujok, O., Spitzy, A., Rudolph, J., 1998. Photochemical alkene formation in seawater from dissolved organic carbon: results from laboratory experiments. Journal of Geophysical Research 103, 5707—5717. Simmonds, P.G., Derwent, R.G., McCulloch, A., O’Doherty, S., Gaudry, A., 1996. Long-term trends in concentrations of halocarbons and radiatively active trace gases in Atlantic and European air masses monitored at Mace Head, Ireland from 1987—1994. Atmospheric Environment, 30, 4041—4063. Swinnerton, J.W., Linnenbom, V.J., 1967. Determination of C to C hydrocarbons in seawater by gas chromatography. Journal of Gas Chromatography 5, 570—573.
Short Communication Diurnal cycles of short-lived tropospheric alkenes at a north Atlantic coastal site A.C. Lewis *, J.B. McQuaid, N. Carslaw, M.J. Pilling The Environment Centre, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK Received 10 September 1998; received in revised form 30 November 1998; accepted 4 December 1998
Abstract Observation of diurnal cycles in atmospheric concentrations of reactive alkenes are reported from measurements performed at a North Atlantic coastal site (Mace Head, Eire 53°19 34 N; 9°54 14 W). Species seen to exhibit distinct cycles included isoprene, ethene, propene, 1-butene, iso-butene and a substituted C alkene. Five hundred and thirty air mass classified measurements were performed over a 4 week period at approximately hourly frequency and demonstrate that during periods when air flow resulted from unpolluted oceanic regions a clear daily cycle in concentrations existed, peaking at around solar noon for all species. These observations support the proposed mechanism of production via photochemical degradation of organic carbon in sea water. The observed concentrations showed strong correlation (propene R'0.75) with solar flux, with little relationship to other meteorological or chemical parameters. The species’ short atmospheric lifetimes indicate that the source of emission was from local coastal waters within close proximity of the sampling site. At solar noon concentrations of reactive alkenes from oceanic sources were responsible for up to 88% of non-methane hydrocarbon reaction with the hydroxyl radical at this coastal marine site. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Diurnal cycles; Reactive alkene; Air mass origin; Hydroxyl radical chemistry; Oceanic emissions
1. Introduction The important role played by reactive volatile organic compounds (VOCs) in tropospheric ozone chemistry is now well established. The presence of both biogenic and anthropogenic VOCs induces substantial perturbations from the CO/CH controlled chemistry of the remote unpolluted atmosphere. Hydroxyl radical oxidation of VOCs, effected through the initial photolysis of ozone at wavelengths below 320 nm, may result in the net forma-
tion of tropospheric ozone through the formation of peroxy radical intermediates and the subsequent conversion of NO to NO . VOC#OHPR#H O, R#O PRO , RO #NOPRO#NO , NO #hlPNO#O, O#O #MPO #M,
* Corresponding author.
Overall: VOC#NO #hlPO . V
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 9 - 4
2418
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
It is in the remotest areas of the troposphere that the influence of VOCs is least well understood. In the marine environment, for example, the flux of non-methane hydrocarbon species to the atmosphere may provide sufficient input to result in net photochemical gains in ozone given a sufficient concentration of NO . V Of the many organic species released to the atmosphere, alkenes exhibit some of the most rapid rates of reaction with the hydroxyl radical and further, reactions between alkenes and ozone have been reported to yield OH as a by-product (Atkinson, 1997). Therefore, any photochemical or biogenic mechanism resulting in a positive ocean/atmosphere flux of even small amounts of these compounds will have a significant effect on nonmethane hydrocarbon (NMHC) tropospheric chemistry in the oceanic environment. Hydroxyl radical loss processes due to NMHC chemistry in the clean marine troposphere have been estimated at around 10% of total loss (CO, 53%, CH , 21%, NO , 6%, O , 4%, H , 6%; V Carslaw et al., 1998). The mechanisms by which non-methane hydrocarbons are generated in sea water is still not clearly understood, however, a photochemical mechanism acting on dissolved organic matter has gained wide acceptance (Ratte et al., 1993). Very recent laboratory studies (Ratte et al., 1998) have shown that alkene formation is controlled by the solar photon flux at the sea surface, the penetration depth (euphotic layer) of light into the ocean, the wavelength (300—420 nm optimum) and dissolved organic carbon (DOC) dependent quantum yields. Spatial and seasonal variations in numerous NMHC species have been reported from the late 1960s initially by Swinnerton and Linnenbom (1967) and later by Linnenbom and Swinnerton (1970), Lamontagne et al. (1974) and Frank et al. (1970). In more recent years combined sea water and atmospheric measurements have been reported and estimates of global budgets made. A review of more than 1000 individual measurements has been made recently by Plass-Dulmer et al. (1995). Spatial and seasonal variations in non-methane hydrocarbons from sea water are currently poorly characterised, however, recent work by Broadgate et al. (1997) has shown a significant seasonal cycle in isoprene, correlated to chlorophyll content of water. The strong diurnal cycle reported in this study has not previously been observed in marine air, although no observations have previously been made with the high temporal resolution of data collection reported here.
In this work we report continuous atmospheric nonmethane hydrocarbon measurements performed at a fixed coastal site at Mace Head Eire 53°19 34 N; 9°54 14 W. The site is located along the north Atlantic cyclone track and benefits from clean unpolluted air flow from a wide range of oceanic regions.
2. Experimental Continuous hourly non-methane hydrocarbon measurements were performed using a method described previously by Lewis et al. (1996) based on programmed temperature vaporisation injection and capillary gas chromatography. Samples of ambient air were collected from a height of 10 m through a glass inlet manifold, pumped at high flow rates such that internal residence time was approximately 1 s. Inlet sample air was dried using a combination of a condensation trap held at !10°C and a magnesium perchlorate trap to remove remaining water and ozone. It has been suggested that there may be photochemical production mechanisms leading to alkene formation on the internal surfaces of glass manifolds. However, recent measurements, also conducted at the Mace Head site, acquired atmospheric samples via Teflon and stainless-steel manifolds and the same alkene cycles were observed. The determination of air mass origin is often performed using in situ meteorological observations. A superior diagnosis of air mass history may be obtained, from calculated back-trajectories based on wind fields analysis. In order to determine the air mass origin for samples collected during this study three-dimensional 5 day back-trajectories were calculated from the ECMWF T106 L31 (1.25°;1.25°;31 levels resolution) every 6 h of the measurement campaign. The arrival point for each trajectory was at 53°19 34 N; 9°54 14 W and 1000 mb (or the highest available pressure if the surface pressure was below 1000 mb). Emission rates from the ocean surface depend upon the transfer velocities, which are mainly a function of the wind velocity over the sea surface (Liss and Merlivat, 1986). During the study period the mean wind velocity for marine air was 8.4 m s\ (n"1132, s.d."3.18). Table 1 outlines the frequency distribution of the wind velocities. The wind velocity only fell below 5 m s\ for 13% of the study period.
Table 1 Frequency distribution of wind velocity for marine air Velocity (ms\)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Frequency
4
32
39
71
103
149
164
134
91
101
78
59
30
35
37
5
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
2419
Fig. 1. Site location and local topography.
Measurements were performed over the period April—May 1997 at the Mace Head Atmospheric Observatory as part of the Eastern Atlantic Spring Experiment (EASE97). The campaign formed the Oxidative Capacity of the Oceanic Atmosphere (OXICOA) component of the NERC funded Atmospheric Chemistry Studies of the Oceanic Environment (ACSOE) project. The station forms part of the Advanced Global Atmospheric Gases Experiment (AGAGE) experiment, as well as being a EUROTRAC/TOR (Tropospheric Ozone Research) station (Simmonds, 1996). A more detailed description of the site can be found in Cvitas and Kley (1994). Fig. 1 illustrates the sampling location in relation to the shoreline. A wide volatility range of alkene species was determined (along with alkane and aromatic species in the mass range C —C ). Air flow during the measurement period was relatively varied resulting in air mass arrival at Mace Head that was of USA, South Westerly, Polar, UK and European Continental origin. Over the course of the measurement period, it has therefore been necessary to determine very precisely periods when the air mass may have contained alkenes of anthropogenic origin and when the air could be considered to contain alkenes of exclusively oceanic origin. Table 2 demonstrates calculated alkene lifetimes assuming typical mean daytime (night time) radical concentrations [OH] 1.00;10 (8.00;10) molecules cm\, [NO ] 1.00;10 (1.20; 10) molecules cm\, [O ] 8.75;10 (1.25;10) mol ecules cm\, [Cl] 1.00;10 (1.00;10) molecules cm\. From these relatively short lifetimes it can be seen that if a minimum transit period of 72 h is imposed on any air
Table 2 Calculated alkene lifetimes in the summer North Atlantic oceanic environment Compound
Daytime lifetime (h)
Nighttime lifetime (h)
Total lifetime (h)
Ethene Propene 1-Butene iso-Butene Isoprene C Alkene
13.1 4.1 6.6 4.3 1.7 5.6
21.9 6.9 26.8 5.3 1.3 24.8
16.3 5.0 11.7 4.6 1.5 10.4
Average lifetime calculated assuming 14 h day/10 h night. Lifetime calculated for 1-hexene; the least reactive C alkene.
mass passing exclusively over the ocean, then it can be assumed that the arriving air mass will contain negligible alkenes of anthropogenic origin.
3. Results and discussion From the continuous measurement of NHMC species over the period April and May 1997, 530 individual data points were screened for analytical integrity, and clear trajectory classification. The differing synoptic conditions prevailing during the measurement period are summarised in Fig. 2 which shows typical 5 day back-trajectory analysis and ambient concentrations of the anthropogenic
2420
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
Fig. 2. Typical back-trajectories and alkene time series showing strong diurnal cycles.
Table 3 Summary of alkene concentrations observed at Mace Head Polar/tropical/westerly
Anticyclonic
Compound
Average pptv
Maximum pptv
Minimum pptv
Peak [HC] hours from solar noon
Average pptv
Maximum pptv
Minimum pptv
Ethene Propene 1-Butene iso-Butene Isoprene C Alkene
19.3 23.8 5.2 12.8 2.6 31.1
67.5 95.7 46.3 138.9 37.1 94.0
0.0 0.0 0.0 0.0 0.0 0.0
#0.2 #0.1 0.0 #0.1 !0.1 !0.2
51.7 51.6 13.4 26.1 15.5 40.4
146.9 136.0 30.4 52.4 97.7 70.5
30.1 25.7 5.0 18.0 0.0 22.1
marker compound acetylene plus propene and iso-butene (offset in the plot by 50 pptv). The time series is annotated with generalised air mass classifications. When the air mass was of oceanic origin (defined previously as having spent at least the previous 72 h over the ocean) a very clear diurnal cycle in propene and isobutene concentrations can be observed. The maximum of each cycle is at around solar noon for all species. These
observations are similar to measurements of isoprene performed at the same site in 1996 (Lewis et al., 1996). During periods of direct transport of air masses from the UK/Europe under anticyclonic conditions little discernible diurnal cycle in these alkenes is visible, however, their ubiquitous presence in anthropogenic pollution results in notably elevated concentrations. During periods when the air mass contains significant pollution but
A.C. Lewis et al. / Atmospheric Environment 33 (1999) 2417—2422
has arrived at Mace Head via an oceanic route, the diurnal cycle is similarly superimposed with highly variable concentrations of anthropogenic emissions. Under polluted conditions the concentrations of alkene species are closely correlated to other alkane and aromatic nonmethane hydrocarbon species. Several other alkene species, namely ethene, 1-butene, isoprene and the C alkene, were also seen to exhibit very similar diurnal cycles when the air mass was classified as being of oceanic origin.
Fig. 3. Relationship between alkenes and solar flux in oceanic air.
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Table 3 shows a summary of all species observed to exhibit a diurnal cycle in concentration in air of oceanic origin, along with the concentrations of these species observed under polluted anticyclonic conditions. Table 3 illustrates that the concentrations observed during polluted anticyclonic air flow are of a similar magnitude when compared to alkenes present in ‘clean’ air of exclusively oceanic origin. The presence of elevated concentrations of propene, iso-butene and ethene is most significant since these species have been identified (PORG 4, 1997) to have amongst the highest photochemical ozone creation potentials (POCP) of any NMHCs, and are calculated to be capable of producing ozone at rates of up to 0.8 ppb h\ under high NO conditions. V The observed concentrations demonstrated little dependence on wind speed, although only a relatively limited range of wind speeds was encountered over the measurement period (mean 8.4 m s\). A far greater dependence of alkene concentration in oceanic air with solar flux was observed, with a positive relationship and significant R values for all of the reported alkenes, e.g. propene, y"0.0283x#15.015 (R"0.7656), isobutene y"0.0223x#5.8966 (R"0.7653). The scatter plot of the relationship between observed propene and iso-butene concentrations and solar flux in air of oceanic origin is presented in Fig. 3. The role of the observed alkenes on localised tropospheric hydroxyl radical chemistry is most significant. The alkenes observed to follow a diurnal cycle may be responsible for over 75% of NMHC reactions responsible for OH radical removal. Table 4 shows the fractional contribution to OH removal of alkenes displaying a diurnal cycle, compared with total alkane/remaining alkenes (C —C ) reaction. The trajectories of oceanic origin have been sub-classified into polar and tropical/westerly groupings, since the differences in air mass chemical composition are such that significantly different fractional removal of OH by oceanic source alkenes occurs. The reduced significance of alkenes in polar air results from the elevated levels of other non-methane hydrocarbons observed to be present in this classification of air mass.
Table 4 Percentage contribution of grouped non-methane hydrocarbons to OH removal Compound
Alkenes showing diurnal cycle Alkanes/remaining alkenes (range C —C )
Polar
Tropical/westerly
Mean %
Min %
Max %
Mean %
Min %
Max %
76 20
58 33
82 15
88 10
76 19
88 10
Total OH loss due to NMHC calculated to be 10—12% (other processes; CO &70%, CH &20%) (from Carslaw et al., 1998).
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The residual 2—8% of loss reactions may be apportioned to trace concentrations of aromatic and alkyne species.
4. Conclusions A significant diurnal cycle in tropospheric reactive alkene species has been observed over a 1 month period of continuous measurements at a north Atlantic coastal site. The cycle was observed in marine air irrespective of the long-range air mass characterisation, suggesting that the emission source area is relatively close to the monitoring location. The short atmospheric lifetimes of the observed species confirm that the emission was occurring within coastal waters, in close proximity to the measurement site. The peak concentrations of all reported alkene species were observed at around solar noon. The alkenes displaying diurnal cycles may play a most significant role in localised hydroxyl radical chemistry, and have been calculated to constitute up to 88% of NMHC initiated OH reactions. The observed atmospheric concentrations of the six reported alkenes were demonstrated to show a close correlation to solar flux, supporting a mechanism of alkene production via photochemical degradation of organic carbon (except in the case of isoprene which is biologically produced) in sea water.
Acknowledgements This work was performed as part of the ACSOE EASE97 experiment, supported by the U.K. Natural Environment Research Council. The authors would like to thank BADC, ECMWF, UKMO, and Paul Berrisford and John Methven of University of Reading for access to, and calculation of the trajectories mentioned in this paper. This paper is ACSOE publication number APC045.
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