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Particle size and chemical composition of urban aerosols
The Science of the Total Environment 235 Ž1999. 15]24
Particle size and chemical composition of urban aerosols A.G. ClarkeU , G.A. Azadi-Boogar, G.E. Andrews Department of Fuel and Energy, Uni¨ ersity of Leeds, Leeds, LS2 9JT, UK
Abstract The size distribution of airborne particulates ŽPM10. has been measured by using Andersen Mark II cascade impactors. The measurements were done at four sites, three of which were in the Leeds area, including one roadside site and the fourth at a rural site. On average 10]20% of the mass of urban PM10 particles were found to be below 0.43 mm and 50% below l.5 mm. Extracted samples were analysed for sulphate, nitrate and chloride using ion chromatography, ion selective electrode to determine ammonium and Gran’s titration to determine acidity. The results show that both sulphate and nitrate peak on the 1.1-mm stage. Nitrate is spread over both coarse and fine modes and is depleted in the finest particles - 0.65 mm and enhanced in coarser particles ) 2.1 mm relative to sulphate. The chloride levels, dominated as they are by sea salt chloride, show a much coarser distribution with - 50% being in the fine fractions for either urban or rural areas. The ammonium particulates are totally in the fine particle mode in summer but there is a small amount in the coarse mode in winter. The cumulative size distribution confirms that ammonium is the component with the finest size distribution with 50% - 1.0 mm and 80% - 1.8 mm. The acidity size distribution is close to the sulphate distribution. The magnitudes of Hq for all sites were very low implying the aerosols are in general well neutralised but the fine particles are more acidic than coarse particles. Rural aerosols are less acidic than urban ones. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Particle size distribution; Aerosols; Cascade impactor; Sulphate; Nitrate; Chloride; Acidity
U
Corresponding author. Tel.: q44-113-2332510; fax: q44-113-2440572. E-mail address: [email protected] ŽA.G. Clarke.
0048-9697r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 9 9 . 0 0 1 8 6 - 2
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
1. Introduction
The health effects of urban aerosols and particularly those derived from vehicles are the focus of attention in many countries. This has been reflected in an increase in the amount of routine monitoring of atmospheric particles. In the UK the development of the Automated Urban Network ŽAUN. sites using TEOM PM10 instruments has provided the benefits of almost realtime particulate monitoring and an extensive data base which now stretches over several years. Local Authorities in connection with their responsibilities for local air quality management are also undertaking additional monitoring. However not all the questions can be answered by measurement of the total mass concentration of particles. Health effects may also be dependent on particle size and chemical composition. Identification of important sources with a view to control requires similar information. Detailed chemical analysis and statistically based source apportionment requires a major commitment of time and effort Že.g. Harrison et al., 1997.. Size resolved, chemical analysis work has not received a lot of attention in the UK although recent work has been undertaken in London ŽRickard and Ashmore, 1995.. Further afield there have been a number of studies using, most commonly, Andersen cascade impactors or occasionally MOUDI or low-pressure Berner impactors. These include background studies ŽOtley and Harrison, 1992; Francois et al., 1995., and continental sites ŽHorvath et al., 1996; Meszaros et al., 1997.. Results of some of these studies will be mentioned later. The objectives of this work were to characterise the chemical composition and size distribution of urban aerosols. In particular, the aim was to compare aerosol properties at several different types of location covering roadside and urban background situations. A rural site was also used to assess urban]rural differences. Measurements were made at four sites in and around the city of Leeds during the period March 1995]March 1996. Weekly samples were taken by Andersen Cascade
impactors which collect nine size ranges. In addition to particle mass concentration, analysis was undertaken for anions, ammonium, acidity and some information about volatile matter and carbon content was obtained. Not all analyses were attempted each week because the analytical requirements prevent this. Metals ŽPb, Fe, Zn, Ca. and PAH were also covered in the survey ŽClarke et al., 1996a. but details of those analyses will be presented elsewhere.
2. Experimental Cascade impactors were operated at four sites: 1. University of Leeds } Roadside. Ground floor window of the Houldsworth building, 5 m above busy road by traffic lights. Grid ref. SE292350. 2. University of Leeds } 5th floor. Almost vertically above Site 1 at a height of approximately 25 m. Grid ref. SE292350. 3. Leeds AUN Site } On the roof of the hut adjacent to the TEOM air intake. This site is approximately 1 km down the road from the University sites and 30 m from the A660 trunk route out of Leeds. Grid ref.SE299343. 4. Rural Site } Haverah Park. Twenty kilometres N of Leeds, 7 km W of Harrogate. Approximately 3 m above ground on the roof of a hut in open moorland, 0.5 km from minor road. Grid ref. SE235532. Monitoring commenced at the University sites at the end of February 1995, at the AUN site in June 1995 and at the rural site in August 1995, taking weekly samples. Final samples were taken in March 1996. A full set of mass data for all four sites was obtained for 17 weeks. Not all these were subjected to analysis. The number of samples analysed for the different components were: anions 10 weeks, ammonium and acidity 5 weeks. Ammonium and acidity were measured on the same samples as the anions. Four identical Andersen Mark 2 impactors were
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
used. These have eight impactor stages with 50% cut-off diameters ranging from 9 mm to 0.43 mm plus a back-up filter mounted in the base of the unit. They have a pre-separator which makes the units roughly equivalent to a PM10 sampler. Each run therefore produces nine samples for weighing and analysis. The impactors were mounted outside at the prevailing ambient temperature with 2]3-m lengths of tubing to the pumps which were mounted indoors. The air inlets were protected from rain ingress by inverted funnels with their base approximately 5 cm above the inlet level to avoid affecting the aerodynamics of particle collection. The impactors operated at ambient temperature and this means that the reported size distributions are for particles as they exist in the atmosphere } either solid particles or droplets depending on the relative humidity. Flow rate calibration is undertaken with a dry gas meter linked to the inlet side of the impactor. In general, flow stability was good and checks and adjustment were carried out every other week. The concentrations reported in the paper are based on gas flow at ambient temperature and are not corrected to a reference temperature. No impactor substrate is ideal for all analyses and it is therefore necessary to use different substrates according to the analytical requirements. We have used self-cut Whatman QM-A Quartz for all the analyses. These have the advantages of low SO 2 , NO x pickup and fairly low metals content. The same material is used for the backup filter as the substrate for upper stages and the use of a fibrous filter gives reduced errors of particle bounce on upper stages. The disadvantages include a rather high tare weight Ž0.4 g. and the potential for filter damage during handling. Slow moisture equilibration can lead to possible errors in total mass determination and the quartz has a tendency to collect volatile organic compounds enhancing the apparent mass. Although quartz is supposed to be free of anions we found nitrate on some of the blanks and therefore resorted to a preliminary water wash and oven drying. Substrates or back-up filters were equilibrated in a desiccator at constant
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humidity overnight before weighing, and were treated similarly after exposure. Chloride, nitrate and sulphate were measured by ion chromatography after extraction in deionised water. To determine NHq an am4 monium selective ion electrode plus CORNING pHrIon meter Model 135 was used. Ten mils of extract were used to which 1 ml of 500 ppm CaŽNO 3 . 2 was added to improve speed of response and stability at low concentration. The accepted method for measuring particulate acid in air is Gran’s titration method in which strong acid is determined by titration with NaOH whilst monitoring the pH ŽLee and Brosset, 1978.. Previous measurements of aerosol acidity in this department had been made on 37-mm teflon membrane filters in conjunction with a dichotomous sampler ŽClarke and Karani, 1992; Clarke et al., 1989.. We opted to make measurements of acidity on the same solutions as were obtained for anion analysis by aqueous extraction of the quartz filters. The levels of acidity are low and the results should be regarded as indicative rather than highly accurate for two reasons. Firstly, the long sampling time of a week means that on-filter reactions of acid with ammonia may occur. What is measured on the filter may not reflect the state of the aerosol in the atmosphere. There also exists the possible neutralisation of some of the aerosol acidity by the quartz. Later, unpublished studies in this laboratory using Teflon substrates have given very similar acidity results, so we do not believe this factor distorts the results excessively. Quartz substrates have been used previously for acidity studies, as in the work by Morandi et al., 1983. Ten ml of sample extract were mixed with 2 ml of 0.02 M KCl solution and 1 ml of 0.0001 M perchloric acid Žs 0.1 meq. Hq. and titrated against 0.001 M NaOH. At very low acidity levels it is sometimes impossible to obtain results from the titration, hence the addition of a standard amount of acid which ensures a clear trend line in the Gran plot against which any sample acid Žor none. can be measured, 0.1 ml of NaOH corresponds to 250 neq. Hq on the filter and roughly 1 neq. Hqrm3 in the sampled air.
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
3. Results and discussion 3.1. Total mass concentrations Adding the masses collected on each stage and the back-up filter gives the total mass concentration collected by the sampler averaged over a week. Early results showed that the impactors appeared to collect up to a factor of two more mass than the Leeds AUN TEOM which prompted a re-examination of our experimental procedures. It was found that drawing moist air through a loaded impactor for 1]2 h followed by drying and weighing resulted in a mass collection, which spread over the usual sampling time of a week would appear as 7mgrm3 extra apparent mass concentration. The mass increases were spread evenly between all impaction stages. Data presented below have been corrected for this effect by subtraction of a standard amount from each stage. This is obviously an imprecise approach since the effect may vary with meteorological conditions. However it is considered that only the total mass data are affected and the chemical components measurements should be accurate. It is suggested that the differences between the TEOM and the impactor relate partly to moisture or organic vapour collection by the impactor substrates that are not compensated for in the normal dryingrweighing procedure. The differences also partly relate to volatile matter loss in the TEOM which operates at 508C and there will be significant mass loss relative to any sampler operating at ambient temperature } sometimes as low as 08C during the Leeds survey. The temperature Žand possibly flow rate. dependence of the TEOM results relating to collection of varying amounts of volatile matter is known both from results in the USA ŽMeyer et al., 1995; Allen et al., 1997. and more recently in the UK ŽSmith et al., 1997.. An examination of the scale of the instrumental differences relative to the absolute mass concentration or to meteorological conditions did not show any clear correlation. There was good agreement in the total mass collected at the three urban sites with the usual order being ground floor) fifth floor) AUN site.
The roadside enhancement relative to the urban background was 5]10 mgrm3 on a weekly average basis but previous work ŽClarke et al., 1996b. showed enhancements of 25]30 mgrm3 ŽTSP not PM10. during weekday daytimes associated with heavy traffic. The urban sites collected on average about 70% more mass than the rural site. The average mass concentration of the three urban sites was 41 mgrm3 for the period from 9 August 1995 to 20 March 1996 as against the rural average of 24 mgrm3 representing an urban enhancement of approximately 17 mgrm3. 3.2. Particle size distribution by mass Fig. 1 shows the 17 week average mass concentrations in each size range with the finest particles on the right-hand side of the diagram corresponding to the back-up filter. The axis values are the stage ECDs so, e.g. the value plotted at 0.65 mm represents particles in the range from 0.65 mm up to the next highest value, 1.1 mm. The mass concentration peaks at approximately 1 mm. Two features are immediately apparent. One is that the rural site has lower concentrations across the whole size range. Secondly the urban sites have an increase in concentration from the last stage to the back-up filter corresponding to freshly generated aerosols - 0.43 mm whereas the rural site does not show this feature. Fig. 2 shows the urban to rural mass concentration ratio. Urban aerosols have a higher proportion of vehicular Žand possibly industrial. emissions which are in the very fine size range compared to the rural site. The rural aerosol is also depleted in the larger particles which correspond to the effects of human activity including road dust raised by vehicular motion, building activities and industrial emissions. From the average cumulative frequency distributions for 17 weeks when data are available at all sites 10]20% of the urban mass concentration is in the finest particles - 0.43 mm, 50% below approximately 1.5 mm, and 80% of the PM10 mass is - 5 mm. Typically 60]70% of the PM10 mass is in the PM2.1 fraction and 30]40% in the coarse range with the larger proportions of coarse
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Fig. 1. Average mass concentration by impactor stage cut point.
Fig. 2. Average urban to rural ratio of particle mass concentration as a function of particle size.
material in the summer months. This is consistent with the work of Harrison et al. Ž1997. in Birmingham who found approximately 60% of PM10 was PM2.5 overall but they observed a
Fig. 3. Size distribution of total particulate mass based on percentage of mass in each size range at four sites.
sharper summer to winter variation Žsummer approx. 80% and winter approx. 45%. ŽFig. 3.. Two plots of D MrDlog D Ž D Ms incremental mass concentration, Dlog D s increment in log
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
20
particle diameter. for the averaged data at the AUN and rural sites are shown in Fig. 3. These figures are based on the percentages of mass in each size range and since Dlog D is 2 for the full range the average value on the vertical axis is 50 in each case. Since there is no lower size limit to the particles collected on the back-up filter an arbitrary value of 0.1 mm has been assumed. The first band on the plot covers the range 0.1]0.43 mm. A peak at 9]10 mm often appears on these plots which is considered to be an artefact of the sampling system. The data presentation is based on the assumption that the preseparator removes all material ) 10 mm and the top stage of the impactor removes material ) 9 mm. In practice, the top stage probably collects some particles ) 10 mm which have found their way through the pre-separator which is a rather crude device compared to an EPA approved PM10 head D M is increased while Dlog D is very small for this stage resulting in the anomalous peak on the graph. Sometimes the distributions show a clear bimodal pattern as at the AUN site, with one peak approximately 1 mm and another approximately 4]6 mm. Sometimes the two modes are merged. This is consistent with observations in other cities, e.g. Seoul } bimodal peaks at 0.8 mm and approximately 5.0 mm ŽBaik et al., 1996.; Jakarta } bimodal peaks at 0.6 and 6.3 mm ŽZou and Hooper, 1997.; Vienna } bimodal peaks for most elements 0.2]0.8 mm and ) 3 mm ŽHorvath et al., 1996.
3.3. Mass concentrations of sulphate, nitrate and chloride Table 1 shows the average total sulphate, nitrate and chloride levels for the three urban sites collected by the impactors during 8 separate weeks in Leeds during 1995r1996 together with the corresponding rural values for 4 weeks. The averages over all 8 weeks were 4.04 of NOy 3 and 5.47 mgrm3 of SO42y and 1.26 mgrm3 of chloride. If all the nitrate and sulphate were ammonium salts the contribution to the total mass concentration of these components is 12.7 mgrm3. The annual mean secondary PM10 contribution of nitrate and sulphate for the area of the UK covering Leeds derived from modelling is approximately 8]10 mgrm3 ŽMetcalfe et al., 1995; QUARG, 1996.. Secondary PM10 for various sites across the UK are approximately half the total measured PM10 ŽStedman, 1998.. The Leeds data are consistent with this picture. In all of the weeks where comparative data are available, the urban sulphate and nitrate levels are higher that the rural levels. However for sulphate the differences are very small and the rural sulphate is not less than 0.85 of the urban value. The differences for nitrate are somewhat larger with the rural to urban ratios being 0.46]0.84. This suggests an additional urban source of nitrate. The main sources of chloride in the atmosphere are sea salt and coal combustion. Table 1
Table 1 Weekly average mass concentrations of aerosol components at Leeds 1995r1996 Week beginning
Urban NO3 y
Rural NO3 y
Urban SO42y
Rural SO4 2y
Urban Cly
Rural Cly
Urban NH4q
Rural NH4 q
Urban Hq
Rural Hq
Total Mass
15.5.95 21.6.95 5.7.95 26.7.95 9.8.95 18.10.95 15.11.95 17.1.96 Average
1.16 4.13 4.53 7.2 2.95 4.35 2.77 5.26 4.04
] ] ] ] 2.42 2.31 1.27 4.43 ]
2.32 3.73 5.48 6.07 6.68 4.34 3.96 11.18 5.47
] ] ] ] 5.87 4.12 2.80 9.57 ]
0.52 0.53 1.17 0.5 0.62 1.27 2.6 2.89 1.26
] ] ] ] 0.5 0.99 1.94 1.26 ]
] ] ] ] 1.18 0.67 1.22 4.89 ]
] ] ] ] 0.83 0.36 0.94 2.35 ]
] ] ] ] 32 16 11 20 ]
] ] ] ] 27 3 4 8 ]
18 26 54 55 42 42 44 69 44
Units: Acidity Hq neq.rm3. All other parameters mgrm3.
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Table 2 Cumulative size distributions for aerosol components
PM10 Mass Sulphate Nitrate Chloride Ammonium Acidity
Fig. 4. Size distribution of chloride, based on percentage mass in each size range at the ground floor site. Fig. 5. Size distribution of nitrate, based on percentage mass in each size range at the ground floor site. Fig. 6. Size distribution of sulphate, based on percentage mass in each size range at the ground floor site. Fig. 7. Size distribution of ammonium, based on percentage mass in each size range at the ground floor site.
of of of of
shows results for 8 weekly samples at the urban sites and 4 at the rural site. For 3 of the weeks the rural chloride is 75]80% of the urban value which is consistent with the uniform distribution expected for sea salt. The slight urban excess of chloride may relate to local HCl emissions or possibly the resuspension of sea salt deposited on roads or other paved areas. For the 4th week in January 1996 the difference is much larger, indicating an additional road salt contribution of the order of 1 mg Clrm3. 3.4. Size distribution of ions The plots of DMrDlogD for the three anions and ammonium at the ground floor site are shown
20% below
50% below
80% below
0.5 mm 0.5 0.9 1.2 0.6 0.5
1.5 mm 1.1 1.6 3.2 1.0 1.3
5.0 mm 3.0 4.5 6.0 1.8 4.0
in Figs. 4]7, which are the averages for 8 sampling weeks. Sulphate ŽFig. 6. appears bimodal with one main peak in the region of 1 mm and another smaller one between 3 and 6 mm. These can tentatively be assigned to ammonium sulphate or bisulphate and metal sulphates such as CaSO4 2H 2 O. Nitrate is more uniformly spread over the whole range up to 6 mm. Chloride aerosols are relatively coarse with a peak in the size distribution in the 4]6-mm range. The cumulative size distribution curves for the urban sites have the features indicated in Table 2. Note that the recorded sizes could relate to either dry particles or droplets } in whichever state the aerosols exist at the prevailing relative humidity during sampling. The mass of both sulphate and nitrate peak on the 1.1-mm stage of the impactor but, relative to sulphate, nitrate is depleted in the finest particles - 0.65 mm and enhanced in coarser particles G 2.1 mm. This is a well known phenomenon and relates to the atmospheric chemistry of the sulphur and nitrogen species. Žsee Willison et al., 1985.. SO 2 is converted in the atmosphere either in the vapour phase or in the droplet phase to H 2 SO4 . The vapour pressure of sulphuric acid is very small and it reacts rapidly and non-reversibly to form non-volatile sulphates Žmainly ammonium and some calcium.. NO x is converted to nitric acid vapour which is present at significant levels in the atmosphere. It may react to form ammonium nitrate but the reaction is reversible especially at higher summer-time temperatures. A much higher proportion has a chance to react with other minerals such as sea salt Žproducing NaNO 3 . or calcium carbonate Žproducing CaŽNO 3 . 2 .. Since these tend to be fairly coarse a higher proportion of nitrate ends up associated
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Žwrb 9 August 1995. and one winter week Žwrb 17 January 1996. are shown in Table 3. Significant features include:
with coarser particles. Similar findings have been reported from Hungary where the ammonium and sulphate ions have peaks in the 0.5]1.0-mm range and the nitrate, in summer, has a peak in the coarse mode ŽMeszaros et al., 1997.. The data obtained for ammonium are summarised in Tables 1 and 2 above. The ammonium size distribution has a very strong peak near to 1 mm but very little mass at larger sizes ŽFig. 7.. This is reasonable because it will be primarily associated with sulphate and nitrate. The only large ammonium particles are expected to be those containing fine ammonium salts which have grown by coagulation with other particles. On the other hand there may be large sulphate and nitrate particles in compounds which are not ammonium salts as discussed in the previous section. Table 2 indicates that ammonium is the component with the finest size distribution. An attempt has been made to apportion the sulphate and nitrate to the categories of fine and coarse ammonium salt, fine and coarse metallic salt and Žin the case of sulphate. fine and coarse acidic species Ži.e. H 2 SO4 , NH 4 HSO4 .. Fine in this context refers to impactor stages collecting particles - 2.1 mm and coarse particles 2.1]10 mm. To do this it is necessary to assign the ammonium in each impactor size range to the sulphate and nitrate present. The ammonium is assigned pro-rata to the number of equivalents of SO42y and NOy 3 . This approach is not necessarily correct but is the best that can be done. The fraction of metallic salts is obtained by difference. The results of this analysis for one summer week
v v
v
v
v
an absence of coarse ammonium in summer; a much higher proportion of ammonium salts in winter; the dominance of metallic nitrates in summer changing to a balance between ammonium and metallic nitrates in winter; free sulphuric acid and bisulphate are a small proportion of the total sulphate; and rural aerosols are better neutralised than urban ones.
3.5. Acidity The absolute amounts of acidity are shown in Table 1 and the cumulative size distribution is indicated in Table 2 above. The acidity distribution is close to the sulphate distribution. There is a steadily increasing trend to higher values at smaller sizes although there is more acidity in the medium and coarse particles than one might expect. The rural aerosols have the lowest acid concentrations. There appears to be an enhancement of acidity at the roadside relative to the fifth floor and AUN site but only in the finest particle size ranges - 0.65 mm Žlast stage and back-up filter. and the excess is only 1]2 neq.rm3. The most acid particles seem to be the smallest ones ŽF 0.65 mm. and those in the mid-range Ž2]5 mm.. The existence of freshly generated fine sulphuric aerosols from vehicles and possibly oil
Table 3 Sulphate and Nitrate apportionment for one summer and one winter week Sulphate %
Nitrate %
Summer
Coarse NH4 Coarse metal Coarse acid Fine NH4 Fine metal Fine acid
Winter
Summer
Winter
Urban
Rural
Urban
Rural
Urban
Rural
Urban
Rural
0 7 10 43 27 13
0 9 9 34 33 15
12 4 3 75 1 5
24 20 2 45 6 3
0 74
0 60
25 11
29 28
12 14
16 24
59 5
37 6
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
combustion in urban areas is to be expected. Homogeneous nucleation of very fine sulphuric acid droplets may also occur. The apparent peak in the mid-range is unusual but is consistently shown at all sites. As discussed in the experimental section, determination of acidity on quartz filters is not ideal and we would consider the values approximate. On the other hand the combination of the ammonium and acidity measurements confirm earlier work by the Leeds group ŽClarke et al., 1984. that the aerosols are generally well neutralised. For example, 28 samples taken by dichotomous samplers onto teflon filters in winter 1988]1989 gave average acidity of 9 neq.rm3 in the presence of 122 neq.rm3 of sulphate ŽClarke et al., 1989. while 21 samples in winter 1990 gave average acidity of 4]5 neq.rm3 in the presence of 129 neq.rm3 of sulphate ŽClarke and Karani, 1992..
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reactions of nitric acid with atmospheric particles while in winter there was more ammonium nitrate. There is a significant urban enhancement of nitrate. 5. Chloride is generally in the coarse mode and derives from seasalt. The occasional influence of road salt is seen in winter. 6. The aerosols are well neutralised. Levels of strong acidity are low.
Acknowledgements The financial support of the UK Department of the Environment Žnow the Department of the Environment, Transport and the Regions. for this work is gratefully acknowledged. References
4. Conclusions
1. The roadside site gave small particle mass concentration enhancements of 5]10 mgrm3 relative to other urban sites on a long term average basis. The average urban level was 15]20 mgrm3 above the rural site. On average 10]20% of the urban mass concentration is in the finest particles is - 0.43 mm, 50% below approximately 1.5 mm, and 80% of the PM10 mass is - 5 mm. 2. Rural aerosols are depleted in the coarsest and finest size ranges relative to urban situations. On the one hand this relates to the elevation of coarse dust levels by human activity and on the other to the presence of sources of fine particles in the city such as traffic. 3. Sulphate is mainly in the fine particles although there is a significant amount of coarse sulphate which is presumed to be metal salts. There is little urban to rural difference. 4. Nitrate is in general coarser than sulphate. In summer there appeared to be a preponderance of metal salt arising from secondary
Allen G, Sioutas C et al. Evaluation of the TEOM method for measurement of ambient particulate mass in urban areas. J Air Waste Manage Ass 1997;47:682]689. Baik NJ, Kim YP, Moon KC. Visibility study in Seoul, 1993. Atmos Environ 1996;30:2319]2328. Clarke AG, Karani GN. Characterisation of the carbonate content of atmospheric aerosols. J Atmos Chem 1992;14:119]128. Clarke AG, Willison MJ, Zeki EM. Aerosol neutralisation by atmospheric ammonia. Physico-chemical behaviour of atmospheric pollutants. In: Versino B, Angeletti G, editors. Proceedings of the Third European Symposium ŽCOST 61A bis.. Dordrecht: D.Reidel Pub. Co., 1984:331]338. Clarke AG, Karani GN, Lambert DR. Acid]base relationships of atmospheric particles and their influence on rainwater composition. In: Man and his ecosystem. Proceedings of the Eighth World Clean Air Congress. The Hague, vol. 3. Amsterdam: Elsevier, 1989:639]644. Clarke AG, Chen J-M, Pipitsangchand S, Azadi-Bougar GA. Vehicular particular emissions and roadside air pollution. Sci Total Environ 1996a;189r90:417]422. Clarke AG et al. Final report to DoE on the sources and chemistry of atmospheric particles. Research Contract EPGr1r5r44, November 1996. Francois F, Maenhaut W et al. Intercomparison of elemental concentrations in total and size-fractionated aerosol samples collected during the Mace Head experiment. Atmos Environ 1995;29:837]849. Harrison RM, Deacon AR, Jones MR, Appleby RS. Sources and processes affecting concentrations of PM10 and PM2.5 particulate matter in Birmingham ŽUK.. Atmos Environ 1997;31:4103]4117.
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
Horvath H, Kasahara M, Pesava P. The size distribution and composition of the atmospheric aerosol at a rural and urban location. J Aerosol Sci 1996;27:417]435. Lee YH, Brosset C. The slope of Gran’s plot: a useful function in the examination of precipitation, the water-soluble part of airborne particles, and lake water. Water Air Soil Pollut 1978;10:457]469. Metcalfe SE, Whyatt D, Derwent RG. A comparison of model and observed network estimates of sulphur deposition across Great Britain for 1990 and its likely source attribution. Q J Royal Met Soc 1995;121:1387]1412. Meyer MB, Rupprecht E, Patashnick H. Considerations for the sampling and measurement of ambient particulate mass. Presented at Particulate matter: health and regulatory issues. Pittsburg, PA, 4]6 April, 1995. Meszaros E, Barcza T et al. Size distributions of inorganic and organic species in the atmospheric aerosol in Hungary. J Aerosol Sci 1997;28:1163]1175. Morandi MT, Kenip TJ, Cobourn WG, Husar RB, Lioy PJ. The measurement of H2SO4 and other sulphate species at Tuxedo, New York with a thermal analysis flame photometric detector and simultaneously collected quartz filter samples. Atmos Environ 1983;17:843]848.
Otley CJ, Harrison RM. The spatial distribution and particlesize of some inorganic nitrogen, sulphur and chlorine species over the North Sea. Atmos Environ Part A 1992;9:1689]1699. QUARG. Quality of urban air review group, third report. Airborne Particulate Matter in the United Kingdom, 1996. Rickard A, Ashmore MR. The size fractionation and ionic composition of airborne particulates in the London Borough of Greenwich. Clean Air 1995;26:37]42. Smith S, Stribley T, Barratt B, Perryman C. Determination of PM10 by TEOM, ACCU, and cascade impactor instruments in the London Borough of Greenwich. Clean Air 1997;27:70]73. Stedman J. The secondary particle contribution to elevated PM10 concentrations in the UK. Clean Air 1998;28:87]93. Willison MJ, Clarke AG, Zeki EM. Seasonal variation in atmospheric aerosol concentration and composition at urban and rural sites in Northern England. Atmos Environ 1985;19:1081]1089. Zou LY, Hooper MA. Size-resolved airborne particles and their morphology in central Jakarta. Atmos Environ 1997;8:1167]1172.
Particle size and chemical composition of urban aerosols A.G. ClarkeU , G.A. Azadi-Boogar, G.E. Andrews Department of Fuel and Energy, Uni¨ ersity of Leeds, Leeds, LS2 9JT, UK
Abstract The size distribution of airborne particulates ŽPM10. has been measured by using Andersen Mark II cascade impactors. The measurements were done at four sites, three of which were in the Leeds area, including one roadside site and the fourth at a rural site. On average 10]20% of the mass of urban PM10 particles were found to be below 0.43 mm and 50% below l.5 mm. Extracted samples were analysed for sulphate, nitrate and chloride using ion chromatography, ion selective electrode to determine ammonium and Gran’s titration to determine acidity. The results show that both sulphate and nitrate peak on the 1.1-mm stage. Nitrate is spread over both coarse and fine modes and is depleted in the finest particles - 0.65 mm and enhanced in coarser particles ) 2.1 mm relative to sulphate. The chloride levels, dominated as they are by sea salt chloride, show a much coarser distribution with - 50% being in the fine fractions for either urban or rural areas. The ammonium particulates are totally in the fine particle mode in summer but there is a small amount in the coarse mode in winter. The cumulative size distribution confirms that ammonium is the component with the finest size distribution with 50% - 1.0 mm and 80% - 1.8 mm. The acidity size distribution is close to the sulphate distribution. The magnitudes of Hq for all sites were very low implying the aerosols are in general well neutralised but the fine particles are more acidic than coarse particles. Rural aerosols are less acidic than urban ones. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Particle size distribution; Aerosols; Cascade impactor; Sulphate; Nitrate; Chloride; Acidity
U
Corresponding author. Tel.: q44-113-2332510; fax: q44-113-2440572. E-mail address: [email protected] ŽA.G. Clarke.
0048-9697r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 9 9 . 0 0 1 8 6 - 2
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
1. Introduction
The health effects of urban aerosols and particularly those derived from vehicles are the focus of attention in many countries. This has been reflected in an increase in the amount of routine monitoring of atmospheric particles. In the UK the development of the Automated Urban Network ŽAUN. sites using TEOM PM10 instruments has provided the benefits of almost realtime particulate monitoring and an extensive data base which now stretches over several years. Local Authorities in connection with their responsibilities for local air quality management are also undertaking additional monitoring. However not all the questions can be answered by measurement of the total mass concentration of particles. Health effects may also be dependent on particle size and chemical composition. Identification of important sources with a view to control requires similar information. Detailed chemical analysis and statistically based source apportionment requires a major commitment of time and effort Že.g. Harrison et al., 1997.. Size resolved, chemical analysis work has not received a lot of attention in the UK although recent work has been undertaken in London ŽRickard and Ashmore, 1995.. Further afield there have been a number of studies using, most commonly, Andersen cascade impactors or occasionally MOUDI or low-pressure Berner impactors. These include background studies ŽOtley and Harrison, 1992; Francois et al., 1995., and continental sites ŽHorvath et al., 1996; Meszaros et al., 1997.. Results of some of these studies will be mentioned later. The objectives of this work were to characterise the chemical composition and size distribution of urban aerosols. In particular, the aim was to compare aerosol properties at several different types of location covering roadside and urban background situations. A rural site was also used to assess urban]rural differences. Measurements were made at four sites in and around the city of Leeds during the period March 1995]March 1996. Weekly samples were taken by Andersen Cascade
impactors which collect nine size ranges. In addition to particle mass concentration, analysis was undertaken for anions, ammonium, acidity and some information about volatile matter and carbon content was obtained. Not all analyses were attempted each week because the analytical requirements prevent this. Metals ŽPb, Fe, Zn, Ca. and PAH were also covered in the survey ŽClarke et al., 1996a. but details of those analyses will be presented elsewhere.
2. Experimental Cascade impactors were operated at four sites: 1. University of Leeds } Roadside. Ground floor window of the Houldsworth building, 5 m above busy road by traffic lights. Grid ref. SE292350. 2. University of Leeds } 5th floor. Almost vertically above Site 1 at a height of approximately 25 m. Grid ref. SE292350. 3. Leeds AUN Site } On the roof of the hut adjacent to the TEOM air intake. This site is approximately 1 km down the road from the University sites and 30 m from the A660 trunk route out of Leeds. Grid ref.SE299343. 4. Rural Site } Haverah Park. Twenty kilometres N of Leeds, 7 km W of Harrogate. Approximately 3 m above ground on the roof of a hut in open moorland, 0.5 km from minor road. Grid ref. SE235532. Monitoring commenced at the University sites at the end of February 1995, at the AUN site in June 1995 and at the rural site in August 1995, taking weekly samples. Final samples were taken in March 1996. A full set of mass data for all four sites was obtained for 17 weeks. Not all these were subjected to analysis. The number of samples analysed for the different components were: anions 10 weeks, ammonium and acidity 5 weeks. Ammonium and acidity were measured on the same samples as the anions. Four identical Andersen Mark 2 impactors were
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
used. These have eight impactor stages with 50% cut-off diameters ranging from 9 mm to 0.43 mm plus a back-up filter mounted in the base of the unit. They have a pre-separator which makes the units roughly equivalent to a PM10 sampler. Each run therefore produces nine samples for weighing and analysis. The impactors were mounted outside at the prevailing ambient temperature with 2]3-m lengths of tubing to the pumps which were mounted indoors. The air inlets were protected from rain ingress by inverted funnels with their base approximately 5 cm above the inlet level to avoid affecting the aerodynamics of particle collection. The impactors operated at ambient temperature and this means that the reported size distributions are for particles as they exist in the atmosphere } either solid particles or droplets depending on the relative humidity. Flow rate calibration is undertaken with a dry gas meter linked to the inlet side of the impactor. In general, flow stability was good and checks and adjustment were carried out every other week. The concentrations reported in the paper are based on gas flow at ambient temperature and are not corrected to a reference temperature. No impactor substrate is ideal for all analyses and it is therefore necessary to use different substrates according to the analytical requirements. We have used self-cut Whatman QM-A Quartz for all the analyses. These have the advantages of low SO 2 , NO x pickup and fairly low metals content. The same material is used for the backup filter as the substrate for upper stages and the use of a fibrous filter gives reduced errors of particle bounce on upper stages. The disadvantages include a rather high tare weight Ž0.4 g. and the potential for filter damage during handling. Slow moisture equilibration can lead to possible errors in total mass determination and the quartz has a tendency to collect volatile organic compounds enhancing the apparent mass. Although quartz is supposed to be free of anions we found nitrate on some of the blanks and therefore resorted to a preliminary water wash and oven drying. Substrates or back-up filters were equilibrated in a desiccator at constant
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humidity overnight before weighing, and were treated similarly after exposure. Chloride, nitrate and sulphate were measured by ion chromatography after extraction in deionised water. To determine NHq an am4 monium selective ion electrode plus CORNING pHrIon meter Model 135 was used. Ten mils of extract were used to which 1 ml of 500 ppm CaŽNO 3 . 2 was added to improve speed of response and stability at low concentration. The accepted method for measuring particulate acid in air is Gran’s titration method in which strong acid is determined by titration with NaOH whilst monitoring the pH ŽLee and Brosset, 1978.. Previous measurements of aerosol acidity in this department had been made on 37-mm teflon membrane filters in conjunction with a dichotomous sampler ŽClarke and Karani, 1992; Clarke et al., 1989.. We opted to make measurements of acidity on the same solutions as were obtained for anion analysis by aqueous extraction of the quartz filters. The levels of acidity are low and the results should be regarded as indicative rather than highly accurate for two reasons. Firstly, the long sampling time of a week means that on-filter reactions of acid with ammonia may occur. What is measured on the filter may not reflect the state of the aerosol in the atmosphere. There also exists the possible neutralisation of some of the aerosol acidity by the quartz. Later, unpublished studies in this laboratory using Teflon substrates have given very similar acidity results, so we do not believe this factor distorts the results excessively. Quartz substrates have been used previously for acidity studies, as in the work by Morandi et al., 1983. Ten ml of sample extract were mixed with 2 ml of 0.02 M KCl solution and 1 ml of 0.0001 M perchloric acid Žs 0.1 meq. Hq. and titrated against 0.001 M NaOH. At very low acidity levels it is sometimes impossible to obtain results from the titration, hence the addition of a standard amount of acid which ensures a clear trend line in the Gran plot against which any sample acid Žor none. can be measured, 0.1 ml of NaOH corresponds to 250 neq. Hq on the filter and roughly 1 neq. Hqrm3 in the sampled air.
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
3. Results and discussion 3.1. Total mass concentrations Adding the masses collected on each stage and the back-up filter gives the total mass concentration collected by the sampler averaged over a week. Early results showed that the impactors appeared to collect up to a factor of two more mass than the Leeds AUN TEOM which prompted a re-examination of our experimental procedures. It was found that drawing moist air through a loaded impactor for 1]2 h followed by drying and weighing resulted in a mass collection, which spread over the usual sampling time of a week would appear as 7mgrm3 extra apparent mass concentration. The mass increases were spread evenly between all impaction stages. Data presented below have been corrected for this effect by subtraction of a standard amount from each stage. This is obviously an imprecise approach since the effect may vary with meteorological conditions. However it is considered that only the total mass data are affected and the chemical components measurements should be accurate. It is suggested that the differences between the TEOM and the impactor relate partly to moisture or organic vapour collection by the impactor substrates that are not compensated for in the normal dryingrweighing procedure. The differences also partly relate to volatile matter loss in the TEOM which operates at 508C and there will be significant mass loss relative to any sampler operating at ambient temperature } sometimes as low as 08C during the Leeds survey. The temperature Žand possibly flow rate. dependence of the TEOM results relating to collection of varying amounts of volatile matter is known both from results in the USA ŽMeyer et al., 1995; Allen et al., 1997. and more recently in the UK ŽSmith et al., 1997.. An examination of the scale of the instrumental differences relative to the absolute mass concentration or to meteorological conditions did not show any clear correlation. There was good agreement in the total mass collected at the three urban sites with the usual order being ground floor) fifth floor) AUN site.
The roadside enhancement relative to the urban background was 5]10 mgrm3 on a weekly average basis but previous work ŽClarke et al., 1996b. showed enhancements of 25]30 mgrm3 ŽTSP not PM10. during weekday daytimes associated with heavy traffic. The urban sites collected on average about 70% more mass than the rural site. The average mass concentration of the three urban sites was 41 mgrm3 for the period from 9 August 1995 to 20 March 1996 as against the rural average of 24 mgrm3 representing an urban enhancement of approximately 17 mgrm3. 3.2. Particle size distribution by mass Fig. 1 shows the 17 week average mass concentrations in each size range with the finest particles on the right-hand side of the diagram corresponding to the back-up filter. The axis values are the stage ECDs so, e.g. the value plotted at 0.65 mm represents particles in the range from 0.65 mm up to the next highest value, 1.1 mm. The mass concentration peaks at approximately 1 mm. Two features are immediately apparent. One is that the rural site has lower concentrations across the whole size range. Secondly the urban sites have an increase in concentration from the last stage to the back-up filter corresponding to freshly generated aerosols - 0.43 mm whereas the rural site does not show this feature. Fig. 2 shows the urban to rural mass concentration ratio. Urban aerosols have a higher proportion of vehicular Žand possibly industrial. emissions which are in the very fine size range compared to the rural site. The rural aerosol is also depleted in the larger particles which correspond to the effects of human activity including road dust raised by vehicular motion, building activities and industrial emissions. From the average cumulative frequency distributions for 17 weeks when data are available at all sites 10]20% of the urban mass concentration is in the finest particles - 0.43 mm, 50% below approximately 1.5 mm, and 80% of the PM10 mass is - 5 mm. Typically 60]70% of the PM10 mass is in the PM2.1 fraction and 30]40% in the coarse range with the larger proportions of coarse
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Fig. 1. Average mass concentration by impactor stage cut point.
Fig. 2. Average urban to rural ratio of particle mass concentration as a function of particle size.
material in the summer months. This is consistent with the work of Harrison et al. Ž1997. in Birmingham who found approximately 60% of PM10 was PM2.5 overall but they observed a
Fig. 3. Size distribution of total particulate mass based on percentage of mass in each size range at four sites.
sharper summer to winter variation Žsummer approx. 80% and winter approx. 45%. ŽFig. 3.. Two plots of D MrDlog D Ž D Ms incremental mass concentration, Dlog D s increment in log
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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particle diameter. for the averaged data at the AUN and rural sites are shown in Fig. 3. These figures are based on the percentages of mass in each size range and since Dlog D is 2 for the full range the average value on the vertical axis is 50 in each case. Since there is no lower size limit to the particles collected on the back-up filter an arbitrary value of 0.1 mm has been assumed. The first band on the plot covers the range 0.1]0.43 mm. A peak at 9]10 mm often appears on these plots which is considered to be an artefact of the sampling system. The data presentation is based on the assumption that the preseparator removes all material ) 10 mm and the top stage of the impactor removes material ) 9 mm. In practice, the top stage probably collects some particles ) 10 mm which have found their way through the pre-separator which is a rather crude device compared to an EPA approved PM10 head D M is increased while Dlog D is very small for this stage resulting in the anomalous peak on the graph. Sometimes the distributions show a clear bimodal pattern as at the AUN site, with one peak approximately 1 mm and another approximately 4]6 mm. Sometimes the two modes are merged. This is consistent with observations in other cities, e.g. Seoul } bimodal peaks at 0.8 mm and approximately 5.0 mm ŽBaik et al., 1996.; Jakarta } bimodal peaks at 0.6 and 6.3 mm ŽZou and Hooper, 1997.; Vienna } bimodal peaks for most elements 0.2]0.8 mm and ) 3 mm ŽHorvath et al., 1996.
3.3. Mass concentrations of sulphate, nitrate and chloride Table 1 shows the average total sulphate, nitrate and chloride levels for the three urban sites collected by the impactors during 8 separate weeks in Leeds during 1995r1996 together with the corresponding rural values for 4 weeks. The averages over all 8 weeks were 4.04 of NOy 3 and 5.47 mgrm3 of SO42y and 1.26 mgrm3 of chloride. If all the nitrate and sulphate were ammonium salts the contribution to the total mass concentration of these components is 12.7 mgrm3. The annual mean secondary PM10 contribution of nitrate and sulphate for the area of the UK covering Leeds derived from modelling is approximately 8]10 mgrm3 ŽMetcalfe et al., 1995; QUARG, 1996.. Secondary PM10 for various sites across the UK are approximately half the total measured PM10 ŽStedman, 1998.. The Leeds data are consistent with this picture. In all of the weeks where comparative data are available, the urban sulphate and nitrate levels are higher that the rural levels. However for sulphate the differences are very small and the rural sulphate is not less than 0.85 of the urban value. The differences for nitrate are somewhat larger with the rural to urban ratios being 0.46]0.84. This suggests an additional urban source of nitrate. The main sources of chloride in the atmosphere are sea salt and coal combustion. Table 1
Table 1 Weekly average mass concentrations of aerosol components at Leeds 1995r1996 Week beginning
Urban NO3 y
Rural NO3 y
Urban SO42y
Rural SO4 2y
Urban Cly
Rural Cly
Urban NH4q
Rural NH4 q
Urban Hq
Rural Hq
Total Mass
15.5.95 21.6.95 5.7.95 26.7.95 9.8.95 18.10.95 15.11.95 17.1.96 Average
1.16 4.13 4.53 7.2 2.95 4.35 2.77 5.26 4.04
] ] ] ] 2.42 2.31 1.27 4.43 ]
2.32 3.73 5.48 6.07 6.68 4.34 3.96 11.18 5.47
] ] ] ] 5.87 4.12 2.80 9.57 ]
0.52 0.53 1.17 0.5 0.62 1.27 2.6 2.89 1.26
] ] ] ] 0.5 0.99 1.94 1.26 ]
] ] ] ] 1.18 0.67 1.22 4.89 ]
] ] ] ] 0.83 0.36 0.94 2.35 ]
] ] ] ] 32 16 11 20 ]
] ] ] ] 27 3 4 8 ]
18 26 54 55 42 42 44 69 44
Units: Acidity Hq neq.rm3. All other parameters mgrm3.
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Table 2 Cumulative size distributions for aerosol components
PM10 Mass Sulphate Nitrate Chloride Ammonium Acidity
Fig. 4. Size distribution of chloride, based on percentage mass in each size range at the ground floor site. Fig. 5. Size distribution of nitrate, based on percentage mass in each size range at the ground floor site. Fig. 6. Size distribution of sulphate, based on percentage mass in each size range at the ground floor site. Fig. 7. Size distribution of ammonium, based on percentage mass in each size range at the ground floor site.
of of of of
shows results for 8 weekly samples at the urban sites and 4 at the rural site. For 3 of the weeks the rural chloride is 75]80% of the urban value which is consistent with the uniform distribution expected for sea salt. The slight urban excess of chloride may relate to local HCl emissions or possibly the resuspension of sea salt deposited on roads or other paved areas. For the 4th week in January 1996 the difference is much larger, indicating an additional road salt contribution of the order of 1 mg Clrm3. 3.4. Size distribution of ions The plots of DMrDlogD for the three anions and ammonium at the ground floor site are shown
20% below
50% below
80% below
0.5 mm 0.5 0.9 1.2 0.6 0.5
1.5 mm 1.1 1.6 3.2 1.0 1.3
5.0 mm 3.0 4.5 6.0 1.8 4.0
in Figs. 4]7, which are the averages for 8 sampling weeks. Sulphate ŽFig. 6. appears bimodal with one main peak in the region of 1 mm and another smaller one between 3 and 6 mm. These can tentatively be assigned to ammonium sulphate or bisulphate and metal sulphates such as CaSO4 2H 2 O. Nitrate is more uniformly spread over the whole range up to 6 mm. Chloride aerosols are relatively coarse with a peak in the size distribution in the 4]6-mm range. The cumulative size distribution curves for the urban sites have the features indicated in Table 2. Note that the recorded sizes could relate to either dry particles or droplets } in whichever state the aerosols exist at the prevailing relative humidity during sampling. The mass of both sulphate and nitrate peak on the 1.1-mm stage of the impactor but, relative to sulphate, nitrate is depleted in the finest particles - 0.65 mm and enhanced in coarser particles G 2.1 mm. This is a well known phenomenon and relates to the atmospheric chemistry of the sulphur and nitrogen species. Žsee Willison et al., 1985.. SO 2 is converted in the atmosphere either in the vapour phase or in the droplet phase to H 2 SO4 . The vapour pressure of sulphuric acid is very small and it reacts rapidly and non-reversibly to form non-volatile sulphates Žmainly ammonium and some calcium.. NO x is converted to nitric acid vapour which is present at significant levels in the atmosphere. It may react to form ammonium nitrate but the reaction is reversible especially at higher summer-time temperatures. A much higher proportion has a chance to react with other minerals such as sea salt Žproducing NaNO 3 . or calcium carbonate Žproducing CaŽNO 3 . 2 .. Since these tend to be fairly coarse a higher proportion of nitrate ends up associated
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
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Žwrb 9 August 1995. and one winter week Žwrb 17 January 1996. are shown in Table 3. Significant features include:
with coarser particles. Similar findings have been reported from Hungary where the ammonium and sulphate ions have peaks in the 0.5]1.0-mm range and the nitrate, in summer, has a peak in the coarse mode ŽMeszaros et al., 1997.. The data obtained for ammonium are summarised in Tables 1 and 2 above. The ammonium size distribution has a very strong peak near to 1 mm but very little mass at larger sizes ŽFig. 7.. This is reasonable because it will be primarily associated with sulphate and nitrate. The only large ammonium particles are expected to be those containing fine ammonium salts which have grown by coagulation with other particles. On the other hand there may be large sulphate and nitrate particles in compounds which are not ammonium salts as discussed in the previous section. Table 2 indicates that ammonium is the component with the finest size distribution. An attempt has been made to apportion the sulphate and nitrate to the categories of fine and coarse ammonium salt, fine and coarse metallic salt and Žin the case of sulphate. fine and coarse acidic species Ži.e. H 2 SO4 , NH 4 HSO4 .. Fine in this context refers to impactor stages collecting particles - 2.1 mm and coarse particles 2.1]10 mm. To do this it is necessary to assign the ammonium in each impactor size range to the sulphate and nitrate present. The ammonium is assigned pro-rata to the number of equivalents of SO42y and NOy 3 . This approach is not necessarily correct but is the best that can be done. The fraction of metallic salts is obtained by difference. The results of this analysis for one summer week
v v
v
v
v
an absence of coarse ammonium in summer; a much higher proportion of ammonium salts in winter; the dominance of metallic nitrates in summer changing to a balance between ammonium and metallic nitrates in winter; free sulphuric acid and bisulphate are a small proportion of the total sulphate; and rural aerosols are better neutralised than urban ones.
3.5. Acidity The absolute amounts of acidity are shown in Table 1 and the cumulative size distribution is indicated in Table 2 above. The acidity distribution is close to the sulphate distribution. There is a steadily increasing trend to higher values at smaller sizes although there is more acidity in the medium and coarse particles than one might expect. The rural aerosols have the lowest acid concentrations. There appears to be an enhancement of acidity at the roadside relative to the fifth floor and AUN site but only in the finest particle size ranges - 0.65 mm Žlast stage and back-up filter. and the excess is only 1]2 neq.rm3. The most acid particles seem to be the smallest ones ŽF 0.65 mm. and those in the mid-range Ž2]5 mm.. The existence of freshly generated fine sulphuric aerosols from vehicles and possibly oil
Table 3 Sulphate and Nitrate apportionment for one summer and one winter week Sulphate %
Nitrate %
Summer
Coarse NH4 Coarse metal Coarse acid Fine NH4 Fine metal Fine acid
Winter
Summer
Winter
Urban
Rural
Urban
Rural
Urban
Rural
Urban
Rural
0 7 10 43 27 13
0 9 9 34 33 15
12 4 3 75 1 5
24 20 2 45 6 3
0 74
0 60
25 11
29 28
12 14
16 24
59 5
37 6
A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
combustion in urban areas is to be expected. Homogeneous nucleation of very fine sulphuric acid droplets may also occur. The apparent peak in the mid-range is unusual but is consistently shown at all sites. As discussed in the experimental section, determination of acidity on quartz filters is not ideal and we would consider the values approximate. On the other hand the combination of the ammonium and acidity measurements confirm earlier work by the Leeds group ŽClarke et al., 1984. that the aerosols are generally well neutralised. For example, 28 samples taken by dichotomous samplers onto teflon filters in winter 1988]1989 gave average acidity of 9 neq.rm3 in the presence of 122 neq.rm3 of sulphate ŽClarke et al., 1989. while 21 samples in winter 1990 gave average acidity of 4]5 neq.rm3 in the presence of 129 neq.rm3 of sulphate ŽClarke and Karani, 1992..
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reactions of nitric acid with atmospheric particles while in winter there was more ammonium nitrate. There is a significant urban enhancement of nitrate. 5. Chloride is generally in the coarse mode and derives from seasalt. The occasional influence of road salt is seen in winter. 6. The aerosols are well neutralised. Levels of strong acidity are low.
Acknowledgements The financial support of the UK Department of the Environment Žnow the Department of the Environment, Transport and the Regions. for this work is gratefully acknowledged. References
4. Conclusions
1. The roadside site gave small particle mass concentration enhancements of 5]10 mgrm3 relative to other urban sites on a long term average basis. The average urban level was 15]20 mgrm3 above the rural site. On average 10]20% of the urban mass concentration is in the finest particles is - 0.43 mm, 50% below approximately 1.5 mm, and 80% of the PM10 mass is - 5 mm. 2. Rural aerosols are depleted in the coarsest and finest size ranges relative to urban situations. On the one hand this relates to the elevation of coarse dust levels by human activity and on the other to the presence of sources of fine particles in the city such as traffic. 3. Sulphate is mainly in the fine particles although there is a significant amount of coarse sulphate which is presumed to be metal salts. There is little urban to rural difference. 4. Nitrate is in general coarser than sulphate. In summer there appeared to be a preponderance of metal salt arising from secondary
Allen G, Sioutas C et al. Evaluation of the TEOM method for measurement of ambient particulate mass in urban areas. J Air Waste Manage Ass 1997;47:682]689. Baik NJ, Kim YP, Moon KC. Visibility study in Seoul, 1993. Atmos Environ 1996;30:2319]2328. Clarke AG, Karani GN. Characterisation of the carbonate content of atmospheric aerosols. J Atmos Chem 1992;14:119]128. Clarke AG, Willison MJ, Zeki EM. Aerosol neutralisation by atmospheric ammonia. Physico-chemical behaviour of atmospheric pollutants. In: Versino B, Angeletti G, editors. Proceedings of the Third European Symposium ŽCOST 61A bis.. Dordrecht: D.Reidel Pub. Co., 1984:331]338. Clarke AG, Karani GN, Lambert DR. Acid]base relationships of atmospheric particles and their influence on rainwater composition. In: Man and his ecosystem. Proceedings of the Eighth World Clean Air Congress. The Hague, vol. 3. Amsterdam: Elsevier, 1989:639]644. Clarke AG, Chen J-M, Pipitsangchand S, Azadi-Bougar GA. Vehicular particular emissions and roadside air pollution. Sci Total Environ 1996a;189r90:417]422. Clarke AG et al. Final report to DoE on the sources and chemistry of atmospheric particles. Research Contract EPGr1r5r44, November 1996. Francois F, Maenhaut W et al. Intercomparison of elemental concentrations in total and size-fractionated aerosol samples collected during the Mace Head experiment. Atmos Environ 1995;29:837]849. Harrison RM, Deacon AR, Jones MR, Appleby RS. Sources and processes affecting concentrations of PM10 and PM2.5 particulate matter in Birmingham ŽUK.. Atmos Environ 1997;31:4103]4117.
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A.G. Clarke et al. r The Science of the Total En¨ ironment 235 (1999) 15]24
Horvath H, Kasahara M, Pesava P. The size distribution and composition of the atmospheric aerosol at a rural and urban location. J Aerosol Sci 1996;27:417]435. Lee YH, Brosset C. The slope of Gran’s plot: a useful function in the examination of precipitation, the water-soluble part of airborne particles, and lake water. Water Air Soil Pollut 1978;10:457]469. Metcalfe SE, Whyatt D, Derwent RG. A comparison of model and observed network estimates of sulphur deposition across Great Britain for 1990 and its likely source attribution. Q J Royal Met Soc 1995;121:1387]1412. Meyer MB, Rupprecht E, Patashnick H. Considerations for the sampling and measurement of ambient particulate mass. Presented at Particulate matter: health and regulatory issues. Pittsburg, PA, 4]6 April, 1995. Meszaros E, Barcza T et al. Size distributions of inorganic and organic species in the atmospheric aerosol in Hungary. J Aerosol Sci 1997;28:1163]1175. Morandi MT, Kenip TJ, Cobourn WG, Husar RB, Lioy PJ. The measurement of H2SO4 and other sulphate species at Tuxedo, New York with a thermal analysis flame photometric detector and simultaneously collected quartz filter samples. Atmos Environ 1983;17:843]848.
Otley CJ, Harrison RM. The spatial distribution and particlesize of some inorganic nitrogen, sulphur and chlorine species over the North Sea. Atmos Environ Part A 1992;9:1689]1699. QUARG. Quality of urban air review group, third report. Airborne Particulate Matter in the United Kingdom, 1996. Rickard A, Ashmore MR. The size fractionation and ionic composition of airborne particulates in the London Borough of Greenwich. Clean Air 1995;26:37]42. Smith S, Stribley T, Barratt B, Perryman C. Determination of PM10 by TEOM, ACCU, and cascade impactor instruments in the London Borough of Greenwich. Clean Air 1997;27:70]73. Stedman J. The secondary particle contribution to elevated PM10 concentrations in the UK. Clean Air 1998;28:87]93. Willison MJ, Clarke AG, Zeki EM. Seasonal variation in atmospheric aerosol concentration and composition at urban and rural sites in Northern England. Atmos Environ 1985;19:1081]1089. Zou LY, Hooper MA. Size-resolved airborne particles and their morphology in central Jakarta. Atmos Environ 1997;8:1167]1172.