Processes affecting concentrations of aerosol strong acidity at sites in eastern England

Atmospheric Environment Vol. 26A, No. 13, pp. 2389-2399, 1992.

0004-6981/92 $5,00+0.00 © 1992Pergamon Press Lid

Printed in Great Britain.

PROCESSES A F F E C T I N G C O N C E N T R A T I O N S O F A E R O S O L S T R O N G ACIDITY AT SITES I N EASTERN E N G L A N D ABDUL-MASSIHN. KITTO Institute of Aerosol Science, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K.

and RoY M. HARRISON* Institute of Public and Environmental Health, School of Biological Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. (First received 11 June 1991 and in final form 26 January 1992)

Abstract--Concentrations of aerosol strong acidity and related species have been measured at sites in eastern England using a sampler in which ammonia is pre-separated by a denuder. High concentrations occurred at a Coastal site and were associated with air advected over the North Sea. At inland sites, ammonia concentrations were higher and the aerosol was more substantially neutralized. Daytime concentrations of aerosol H + exceeded those measured at night, despite higher daytime levels of ammonia, presumably due to more effective production of HzSO4 during daytime hours. Concentrations of acidic aerosols were within the range 0-178 neqm -3, well below those observed at many eastern North American sites with lower concentrations of ammonia. Key word index: Acidic aerosol, ammonia, neutralization, denuder-filter pack sampler, Gran's titration.

INTRODUCTION Aerosol strong acidity is normally attributed to acidic sulphate aerosol, although few direct determinations have been made (Stevens et al., 1978; Larson et al., 1982; Morandi et al., 1983; Slanina et al., 1985); pH measurement or titration of the aqueous extracts of soluble particles collected on a filter (usually Teflon) are more commonly used (Brosset and Ferm, 1978; Ferek et al., 1983; Koutrakis et al., 1988; Pierson et al., 1980, 1989). The titration procedure of Gran (1952) and the development of Brosset (1978) has been widely used for measurements of strong acidity in the atmosphere (Ferek et al., 1983; Philips et al., 1984; McQuaker and Sandberg, 1988). A number of other compounds may also contribute to atmospheric acidity. Amongst these, gaseous hydrochloric and nitric acids may contribute significantly to total acid levels (Pierson et al., 1989; Harrison and Allen, 1990; Appel et al., 1991; Eldering et al., 1991). The dominant influence on their concentration is the reversible reaction with ammonia to form respective ammonium salt aerosols (Stelson and Seinfeld, 1982 a, b; Allen et al., 1989), although chemical kinetics and mass transfer limitations to the achievement of equilibrium may be important (Harrison and Mackenzie, 1990; Wexler and Seinfeld, 1990). Whilst methanesulphonic acid, derived from dimethylsulphide oxidation, may contribute signific* To whom correspondence should be addressed.

antly to strong acid in marine areas (Saltzman et al., 1983), its concentration is likely to be far lower than that of H2SO4 over the continental regions where SO2 emissions occur (Panter and Penzhorn, 1980). However, hydroxymethanesuiphonic acid which is formed by the reaction of C H 2 0 and dissolved SO 2 may contribute to strong acidity in fog and cloud water and dew (Munger et al., 1986; Pierson and Brachaczek, 1990). There is strong evidence that exposure to acidic species impairs human health (Schlesinger et al., 1983; Utell et al., 1983). A recently published symposium has highlighted the concern over acid aerosols in relation to adverse human health effects (Lippmann, 1989; Larson, 1989; Lioy and Waldman, 1989; Koenig et al., 1989; Raizenne et al., 1989). It was reported that aerosol H + determined during episodic conditions in southern Ontario indicated that respiratory tract deposition can exceed the effects level reported in clinical trials (Spengler et al., 1989). The same authors reported results from 9-month evaluation of human exposure in four U.S. cities. Strong acid is potentially highly reactive in atmospheric chemical processes. The most facile reaction is that with gaseous ammonia to form NH4HSO4 and (NH,)2SO 4. The coexistence of H2SO4 aerosol and NH3 gas in the atmosphere is indicative of the importance of mass transfer limitation in restricting the rate of this simple acid-base process (Huntzicker et al., 1980; McMurry et al., 1983). Acidic aerosols may be surrounded by an organic film (Gill et al., 1983;

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A.-M. N. KITTO and R. M. HARRISON

D a u m e r et al., 1987) which may inhibit neutralization, as may the formation of a crystalline salt layer (Tanner, 1982; Bassett and Seinfeld, 1983), HzSO~ is typically present in very fine aerosol (Harrison and Pie, 1983; Koutrakis et al,, 1987; Spengler et al., 1989) which may coagulate with other aerosol particles leading to neutralization, e.g. C a C O 3 + H2SO 4. )CaSO4 + C O 2 + H 2 0 or to displacement of a more volatile acid (Appel et al., 1984; Clegg and Brimblecombe, 1985), e.g. NaC1 + NaNO 3+ 2NaCl + 2NaNO3 +

H2SO4--, N a H S O ~ + HCI H2SO4--,NaHSO4 + H N O 3 H2SO4--,Na2SO 4 + 2HCI H2SOg--,Na2SO4 + 2HNO3.

Thus the H2SO 4 observed in the atmosphere is a transient species in a short existence between formation from SO2 oxidation and neutralization by N H 3, or reaction processes such as those above. The atmospheric lifetime of H2SO4 is controlled by the rates of neutralization processes and is thus dependent upon the gas and aerosol composition of the atmosphere. The ambient concentration of H2SO# will be enhanced by rapid formation and slow removal by neutralization processes. Only few studies have been reported in the literature in which acidic aerosols were measured simultaneously at several nearby sites. Waldman et al. (1990) monitored the acidic aerosol in the Toronto metropolitan area. Concentrations of acidic aerosols showed spatial variations between the sites, with the lowest levels at the urban site. Pierson et al. (1989) investigated atmospheric acidity at two sites on the Allegheny mountains. High levels of acidic aerosols were observed at both sites and the levels at the two sites were comparable and highly correlated. Acidic aerosol exhibited the same pattern as sulphate and fully neutralized a m m o n i u m sulphate was never measured primarily due to low levels of ammonia. High concentrations of H + and S O ~ - were associated with winds from the west traversed over high emissions of pollutants. In this work, we have sought to measure H2SO4 (or more correctly aerosol strong acidity) at three sites simultaneously in eastern England with the aim of gaining a better understanding of the processes influencing aerosol strong acid concentrations in an area with relatively high levels of ammonia (Allen et al., 1988).

EXPERIMENTAL

Gaseous ammonia and acidic aerosols were collected using a denuder-filter pack sampling system at three rural sites in eastern England from May to December 1987 and March to December 1989. The denuder apparatus consisted of six borosilicate glass tubes in parallel, each 50 cm in length × 0.4 cm i.d. The last 35 cm of the tubes were coated together simultaneously with phosphorous acid (HaPOa) deposited from a 5% solution in methanol followed by drying with warm pre-cleaned air.

Citric acid was used instead of phosphorous acid on foggy days, to avoid condensation inside the denuder. The denuder was oriented vertically in the field, with entry at the bottom. Air was drawn for 24h or shorter intervals at 10~'min - t through the denuder and subsequently through a PTFE filter pack comprising 47 mm diameter Teflon (0.5 pm Whatman), nylon (Nylon 66; 0.45 #m Sartorius) and H3PO,,-impregnated Whatman 41 filters in series. Nylon and Whatman 41 filters were thoroughly washed with deionized water and dried before use in order to reduce the blank level. The Whatman 41 papers were impregnated with 5% H3PO4 and dried in a vacuum desiccator prior to use. This filter was employed to account for NH3 evolved from the Teflon filter. After sampling for 24h at 10fmin -1, the sampler was sealed immediately from both ends by Teflon caps to protect samples from neutralization. The denuder was extracted with 30 ml deionized water and analysed for NH2 by a OPAfluorescence flow injection procedure (Rapsomanikis et al., 1988). Teflon filters were extracted into 0.5 ml isopropanol with 20 ml deionized water added (Wolfson, 1980). Nylon filters were extracted into 10 ml of 2.95 mM Na2CO 3 and impregnated Whatman 41 filters into 10 ml deionized water. All extractions were carried out for 30 min on a mechanical shaker. Teflon and nylon extracts were analysed for CI-, N O r and SO~- by ion chromatography (Dionex 2000i/SP) and for NH2. Ten millilitres of the Teflon extract were used to estimate the strong acidity by Gran's titration. Extreme care was taken in handling the samples through transport and inside the laboratory to avoid neutralization of acidic particles. Most of the acidic aerosol measurements were made for 24 h sampling periods, with some measurements obtained for day and night periods separately. Twenty-four-hour sampling intervals may fail to detect brief peaks in acidic aerosol concentrations of shorter duration (Spengler et al., 1989).

Gran's procedure Ten millilitres oftbe aerosol solution was introduced into a thermostatted titration vessel (25°C), equipped with a Teflon-coated stirrer. N2 was flushed for 15 rain into the sample to remove CO2. The sample was mixed with 0.5 ml of 1M KCI solution. The titration was carried out at 25°C with constant stirring in an N2 atmosphere. A micro-burette was mounted in such a way that its tip could be dipped for 1 s in the sample, in order to insert the last drop. Twenty increments of NaOH were added to the sample. After each addition, 2-3 rain were required to allow pH readings to stabilize. A pH electrode (combination electrode) was inserted into the solution. The electrode was connected to an Orion Research model 701A/digital Ionanalyser pH meter which can measure the pH of the solution with a sensitivity of 0.001 pH units. Since the concentration of NaOH solution (the titrant) was 100 times stronger than the titrand, the total titrant amount can be summed in a suitable number of additions, each with a volume of a few microlitres. This ensures that the volume of the titrand solution was kept constant and hence the measured hydrogen ion concentration at each stage will be constantly proportional to the remaining amount of hydrogen ions from the measurement of pH. The hydrogen ion concentration (Ca+) was calculated from the formula Cn+ = K 10 -pa. The constant K was determined by the titration of a strong acid of known concentration, using Gran's plot and by extrapolation of its relevant pseudo-linear part to the abscissa. It is always an advantage to titrate from the acidic end. Therefore, occasionally when the sample was originally not sufficiently acidic (pH > 4.3), a precise amount of strong acid was added to the sample and the measured value was then corrected for this addition. The titrant solution (NaOH) was

Processes affecting aerosol acidity stored in a CO2 free environment and standardized before every batch of samples were titrated. The relative standard deviation of Grafts titration was 1-2% and the minimum detectable level was 0.5 neqm -3. The average blank for the ammonia denuder was 0.3 +0.4 #g per denuder (n = 12) and 0.18 +0.16/~g per filter (n = 10) for the impregnated Whatman 41 filter. The detection limit calculated from 3~ of the blanks was 1.6neqm -3 (10~'min-1, 24h sampling) for the ammonia denuder and 2 neqm-3 (10E rain-1, 24 h sampling) for the Whatman 41 filter. The ambient concentration~ were well above the limits of detection for all the species measured.

COLEHESTER

o

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Sampling sites A group of three sites was operated simultaneously between May and December 1987 when samples were collected over 24-h periods. These were chosen to cover a range of local environments in southeast England (Fig. 1) and are; (1) Walton Pier, Grid ref. TM 260212. Sampler at the end of the pier, 0.8 km offshore, 1 m above pier structure, 28 km ESE of Colchester town. (2) Essex University, library roof, Grid ref. TM 029239. An open site, 25 m above ground level, semi-rural area southeast of Colchester town. Light industrial activities 1 km SW. (3) Great Domsey, Grid ref. TL 887218. Sampler I m above ground level at a crop research station, 10 km WNW of Colchester town. Rural area, arable farming practice (4) Ardleigh reservoir, Grid ref. TM 037282. An open site, semi-rural, 5 km northeast of Colchester. Sampler 1 m above the ground. Separate measurements were obtained at this site for day and night periods.

--~ RESULTS AND DISCUSSION

o

scale

Fig. 1. Map of study area. Site 1: Walton Pier; 2: Essex University; 3: Great Domsey; 4; Ardleigh. The shaded areas are towns.

Summaries of the results of acidic aerosol and ammonia measurements are listed in Tables 1 and 2. The efficiency of a m m o n i a collection by the phosphorous acid denuder was measured as 92% (Rapsomanikis et al., 1988); ammonia results are corrected by this factor. Simultaneous measurements at the three sites in eastern England have shown that acidic aerosol concentrations were higher at the Walton site than the Essex University and Great Domsey sites. This may be due to lower concentrations of ammonia at Walton

Table 1. Concentrations of acidic aerosols (24-h ave) measured at three sites simultaneously* (neqm-3; n=33)

H+ SO 2NH~ NH 3

Walton Pier Average _+a (range)

Essex University Average + a (range)

Great Domsey Average + ¢ (range)

28.7+24.1 (1.5-110) 119 -+97.5 (27.5--407) 147+133(13.5-588) 51.9+48.4 (4.6-180)

18.4-+16.2(1-68.1) 113 +92.7 (25.4-425) 164-+134(16.5-666) 72.7-+47.4 (7.5-204)

11.5-+11.7(0-50.5) 121 -+ 100 (29.1-487) 182-+156(28.3-847) 161 + 170 (5.2-919)

* Measurements were obtained once a week from May to December 1987.

Table 2. Other measurements of acidic aerosol concentrations in the study area (neq m - 3) Ardleigh Essex University* 24-h average + a (range) a +

SOINH2 NH3

2.58+32.5 (0-178); n=62 173 + 170 (25.4-806); n = 62 293 -+297 (16.5-1448); n = 62 75.2-+ 54.7 (5.6-220); n = 62

Dayt Average 4- a (range) 22.0-1-15.5 (0-64); n=23 177 :L 159 (29.1-629); n-- 23 216_+ 190 (45.8-770); n--23 178_+88.4 (78.7-456); n=23

Nightt Average :L¢ (range) 10.1-1-6.5 (0-23); n=23 143 + 125 (25.3-557); n = 23 252+ 186 (62.7-674); n=23 141 +62.2 (43.2-308); n=23

* These results were obtained from March to December 1987. tDay and night measurements were obtained during May, June and July 1989. The duration of day and night samples varied according to the times of sunrise and sunset which were the temporal boundaries adopted.

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A.-M. N. KITTOand R. M. HARRISON

than the other two sites (Table 1). The equivalent ratio of H+/SO 2- was in the range 0.03-0.9, 0.0-0.7, 0.0-0.4 (n = 33) at Walton, Essex University and Great Domsey, respectively. Elevated concentrations of acidie aerosol generally corresponded with low concentrations of ammonia. Figure 2 shows results for Walton; comparable behaviour was observed at the other sites, but with lower H + concentrations. A plot of H+/SO 2- equivalent ratio versus SO 2concentration for the Essex University site (Fig. 3) shows that higher H+/SO 2- ratios were frequently associated with low to moderate sulphate and not the extremely high sulphate concentrations. Other researchers have also reported that the highest H+/SO 2- ratios do not occur at the highest sulphate concentrations (Ferek et al., 1983; Morandi et al., 1983; Clarke et al., 1984; Waldman et al., 1990). However, still other workers have reported high H+/SO 2- ratios concurrent with high sulphate concentrations (Koutrakis et al., 1988; Pierson et al., 1980, 1989; Keeler et al., 1991). It is possible that the data showing the latter behaviour may have encountered H + losses in the sampling or analytical procedure which are more critical at lower H + and SO 2concentrations. Whilst some workers have not used an ammonia pre-denuder, others reporting this behaviour have done so (Koutrakis et al., 1988; Keeler et al., 1991). The apparent existence of two contrasting forms of behaviour of H÷/SO 2- ratios relative to SO 2- concentration requires further consideration. Whilst an analytical or sampling artefact could provide one possible explanation (see above), there may be genuine differences relating to atmospheric chemistry. At sites with very low gaseous ammonia concentrations (e.g. most of the eastern U.S.), a large proportion of the acid aerosol remains unneutralized and it is physically reasonable for high concentrations of sulphate to be largely in the form of HzSO 4 and hence associated with a high H+/SO 2- ratio. Our sites are not of this type. At our sites, ammonia is typically high, and well in excess of aerosol acidity. This being the case, high H+/SO 2- ratios might be generated

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Fi 8. 2. Relationship of aerosol acidity to gaseous ammonia concentration at Walton site (n = 33). These results

were obtained from May to December 1987.

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Fig. 3. Relationship of H+/SO] - equivalent ratio to SO 2- concentration at Essex University site (n=62). These samples were obtained from March to December 1987.

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Fig. 4. Relationship of SO2-/NH + equivalent ratio to SO 2- concentration at Essex University site (n--62).

either by rapid near-ground formation of HzSO 4, or by downward mixing of H2SO,-rich air from the ammonia-depleted higher layers. In either case, kinetic constraints limit the neutralization of the H + input, which will impact appreciably on H+/SO 2- ratios only at low concentrations of SO 2-. This behaviour appears physically reasonable and may be widespread in western Europe where NH3 levels are mostly high. A plot of equivalent ratio of S O 2 - / N H 2 versus SO 2- (Fig. 4), similar in form at all sites, clearly demonstrates that at high sulphate concentrations, the equivalent ratio did not exceed unity, reflective of substantial neutralization of sulphuric acid aerosol with gaseous ammonia to form ammonium bisulphate or ammonium sulphate (some of the ammonium ion is incorporated with chloride and nitrate in reversible reactions), whereas at lower sulphate concentrations the ratio sometimes exceeded unity, reflecting that sulphate aerosols were acidic, which suggests that H2SO4 aerosols and/or NH4HSO4 were present. Typically only a small portion of the total sulphate aerosol was present as sulphuric acid due to the high ammonia concentration at our sites, although at Wal-

Processes affecting aerosol acidity ton, some samples showed a high H+/SO 2- equivalent ratio. It is interesting to note that equivalent ratios of S O 2 - / N H ~ were higher at the Walton site than the other two sites, reflective of less neutralization at Walton, primarily due to the low levels of ammonia at this site (Table 1). High sulphate concentrations were usually measured during the intense portions of a photochemical air pollution episode, which were typically associated with anticyclones and high temperatures. Successive sampling during these high-pressure periods has demonstrated that most of the acidic fraction of a smog episode tends to be in the build-up phase of the episode and the high NH ~- to be in the trailing part (for example see Table 3). The air masses during these sampling periods were characterized by slow moving of the high-pressure systems and by light winds. Thus the air parcels have greater opportunity to come into contact with local ammonia emissions. As the high pressure dominates the area for a prolonged period, the temperature rises which has a dual effect on ammonia concentrations. High temperature favours the dissociation of ammonium nitrate and ammonium chloride to their gaseous components (Harrison and Allen, 1990), whilst higher temperatures will increase the emission of ammonia from the ground surface. Indeed, experimental measurements of ammonia were high during these periods (Table 3). Measurements in summer 1989 were made during dry weather periods. Most of these samples were acidic (Tables 2 and 3). It is apparent that daytime concentrations of the acidic aerosol were higher than nighttime despite higher concentrations of ammonia during the day. This suggests that H2SO 4 aerosol was formed through the hydroxyl radical due to the photochemical activity during the day and implies that the formation rate of H2SO , was higher than the rate of neutralization. Photochemical activity was a distinct feature during the sampling campaign; high levels of nitric acid were observed during this period (Kitto and Harrison, 1992). It is interesting that nighttime concentrations of ammonium aerosol were higher than

2393

those of daytime. This is most probably due to the high temperature and low humidity during the day which favours dissociation of NH4NO3 and NH4CI. At night, the pH of the aerosol increases as a result of cessation ofproduction of fresh H2SO 4, and due to the low temperature and high relative humidity, ammonium nitrate and ammonium chloride are more stable.

Inter-site correlation

Inter-site correlations for aerosol acidity were high, despite the differences in mean H + concentration between the sites. Regression analyses for the data set are tabulated in Table 4. Although ammonia concentrations are strongly influenced by local emissions (Allen et al., 1988), aerosol acidity exhibited similar trends at all sites. This suggests that the source of aerosol acidity is due to long-range transport rather than local production. Ammonia concentrations were not correlated between the sites (see Table 4) reflecting localized influences. This local difference in ammonia concentrations is however reflected in spatial patterns of neutralization of the acidic aerosol. The lack of NH 3 inter-site correlations indicates that H + inter-site correlations in the absence o f N H a (g) variation would have been very high, as for sulphate. Indeed, high H + inter-site correlations have been documented at eastern U.S. sites where NHa(g ) levels are too low to cause appreciable neutralization (Spengler et al., 1986; Koutrakis et al., 1988; Pierson et al., 1989). Ammonium and sulphate concentrations were well correlated between the sites, reflecting their uniform distribution. As one would expect for a secondary aerosol with particle size 0.1-1 pm diameter (Asman and Janssen, 1987), these aerosols are subject to long distance transportation and may remain airborne for periods of the order of days to weeks (Irwin and Williams, 1988; Heintzenberg, 1989). Dry deposition velocities of sulphate and nitrate formed from the reaction with ammonia are likely to be small, 0.05-0.1 cms-1 (Garland and Cox, 1982; Nicholson

Table 3. Atmospheric concentration measurements during two high pollution episodes (daytime)(neq m- 3) Date

Temp. (C° ave)

R.H. (ave)

HNOa

NH3

NH~

NO~

S O ,2-

H+

SO 2

Episode A 7.6.89 13.6.89 14.6.89 15.6.89 16.6.89

18 215 332 80 39

91 83 445 167 135

187 668 491 138 70

66 109 67 16 26

113 599 392 115 62

2 64 37 11 18

44 495 168 747 53

19 18 20 2~5 18.5

61 60 53 55 54

Episode B 26.6.89 3.7.89 4.7.89 5.7.89 6.7.89

174 60 64 42 334

96 192 188 204 206

192 99 46 147 770

24 43 18 47 143

204 83 31 104 629

30 37 11 14 8

118 168 57 53 871

17 16 18 20 16

62 61 40 55 60

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A.-M. N. KITrO and R. M. HARIUSON Table 4. Linear regression analyses for the data set of 24-h samples (n = 33) Site* H+ SO~NH~ NHs

X

Y

Slope + SDT

Intercept ± SD (neq m- s)

r2

1 1 2 1 1 2 1 1 2 I

2 3 3 2 3 3 2 3 3 2

0.60+0.05 0.43+0.04 0.65+0.06 0.93 + 0.04 O.99-1-O.05 1.06+0.04 0.99+0.04 1.05± 0.05 1.05±0.03 0.79+0.13 1.14-1-0.60 1.76-1-0.50

0.5+7.1 -1.3±5.5 -0.5+5.3 3.0 ± 21 3.8-1-28 t.5+21 20+31 29 ± 36 9.t +26 35±33 94±152 20± 136

0.83 0.80 0.81 0.95 0.92 0.96 0.95 0.94 0.97 0.55 0.11 0.28

1

3

2

3

* 1, 2 and 3 are Walton Pier, Essex University and Great Domsey sites, respectively. TSD = Standard deviation.

and Davies, 1986; Allen et al., 1991). Consequently their atmospheric residence time is relatively lon~ Relation with wind history Since acidic aerosols are present in fine particles, like ammonium sulphate they are transported for long distances in the boundary layer (Pierson et ai., 1989; Waldman et al., 1990; Keeler et al., 1990). Thus it is important to quantify the relationships between the observed concentrations and emission sources. This relationship is significant to both wet and dry deposition. Backward geostrophic air mass trajectories were used to identify sources of pollutants (Colbeck and Harrison, 1985). Trajectories were drawn for the air arriving at 12 h GMT on the initial and final days of selected sampling periods of high and low acidic aerosol concentrations. High acidic aerosol concentrations were observed with air masses originating from east through eastsoutheast sectors (Fig. 5a, b, c, d, e). For example, the H + concentration on the sampling period 8-9.3.1987 was 178 neq m ~ 3. Twenty-four-hour backward trajectories (Fig. 5c) suggested that the air masses arriving at our sites originated from the industrial provinces in north Italy and northern Yugoslav/a then passed over southwest Germany, the air masses traversing the high emission northwest Rhine region and Ruhr area and then the Netherlands before reaching the North Sea. Elevated levels of acidic aerosol were also measured during the sampling period 24-25.5.1987 (110neq m - 3). Twenty-four-hour backward trajectory analysis (Fig. 5e) demonstrated that the air masses originated from the industrial region of northwest Germany, then passed over the Netherlands and the North Sea. Primary pollutants (SO2 and NOx) undergo chemical transformation while they are being transported (Ferm et ai., 1984; Keeler et al., 1990). Since NH 3 concentrations over the North Sea are much lower than those

Fig. 5. Twenty-four-hour back trajectories arriving at 12 GMT on a: 5.3.87 (H+--5tneqm-3); b: 7.3.87 (H + = 114 n~t m- 3);c: 8.3.87 (H + = 178 neq m- 3);d: 7.5.87(H + =84neqm-3); ¢. 24.5.87 (H+--ll0neqm-3); f. 14.9.87 (H + --0 neq m- 3). The arrowheads are shown for 6-h intervals.

over the continent (Ottley and Harrison, 1991), acidic particles are less neutralized. This explains the elevated concentrations of acidic aerosols at the eastern coastal site (see Table 1). As aerosols are carried over the land, acidic particles are progressively neutralized by ammonia, to an extent and at a rate which depends on the availability of ammonia. Similar observations have been reported from studies based in continental Europe (Brosset, 1978; Elshout et al., 1978: Ferm et al., 1984). Whilst high levels of acidic aerosols were associated with air masses originating from east through eastsoutheast, low levels of pollutants (including H +) were

Processes affecting aerosol acidity associated with west through northwest sectors. Low concentrations of acidic aerosols were measured during the sampling period 14--15.9.1987. The 24-h back trajectory (Fig. 5f) clearly demonstrated that the air masses originated from the Atlantic Ocean, then passed over south Wales. The high SO z emission from the London area was traversed by the trajectory, but low concentrations of H ÷ were observed, suggesting that the oxidation of the SO2 to produce H~SO4 was slow and/or that vertical mixing was very efficient. Time-weighted pollution roses

Time-weighted pollution roses, which show how pollutant concentrations vary with surface wind direction, were constructed according to the method of Harrison and Williams (1982). Wind directions were

2395

obtained from a wind vane operated on the roof of the library, a five-storey building at the University of Essex. The time-weighted mean concentration of a species in a given wind sector is calculated using the formula: ~ (tl,, x c~) (TWMC), = i= l ti, n

i=1 where t~.n is the number of hours for which the wind was in sector n during the ith sampling period, c~is the concentration of the pollutant for the ith period, ra is the number of sampling periods. Pollutant concentration roses at Essex University (Fig. 6) clearly demonstrate that the highest concen-

J J

3'o 4o0

Fig. 6. Time-weighted pollution roses at the Essex University site (n=62) for SO~-; aerosol H+; NH3; and NH2 (concentrations in neq m - 3).

2396

A.-M. N. KITTOand R. M. HARRISON

trations for S O ~ - , H + and N H 2 occur in the east- by vertical mixing in the absence of a ground-level ern and southeasterly sectors (Fig. 6), with the source, chemical removal by reaction with strong lowest concentrations in air masses from the sectors acids, and possible deposition to the sea. Although the southwest through northwest, an indication that these winds from north-northeast sector originate from the species are transported from continental source re- North Sea, high ammonia concentrations were measgions. The surface winds showed that the winds from ured since the winds have traversed over local land southwest through northwest were relatively clean before reaching our sites. compared to those from east through south sectors, Ammonium aerosol exhibited highest concentraanalogous to the continental air masses from east and tions in the east-southeast sector, with the lowest from maritime air masses from the Atlantic Ocean as shown west-southwest through north-northeast sectors (Fig. in Fig. 5. Both sulphate particles and aerosol acidity 6), suggesting that ammonium aerosol is formed in exhibited remarkably similar distributions with the the atmosphere via the reaction of ammonia with highest concentration from east through east-south- strong acids, associated with air masses originating in east, suggesting that strong acidity measured at our continental Europe. This is consistent with H ÷ and sites was being formed over the continent or over the SO 2- patterns. North Sea through either homogeneous or heterogeneous oxidation of SOz while it is being transported to Britain, and due to low levels of ammonia over the C o m p a r i s o n with published data North Sea, aerosol acidity remains unneutralized. The majority of published acidic aerosol measureAmmonia exhibited a quite different behaviour (Fig. ments have been carried out in U.S.A. and Canada; 6). The highest concentrations were from west-south- few measurements have been made in Europe. A list of west through west-northwest sectors and the lowest some of these measurements appears in Table 5. Most concentrations from east-northeast sector. Although were obtained for short periods and limited to a ammonia concentrations are influenced by local emis- specific research programme. A small number of longsions over the short term (Mien et al., 1988), long-range term measurements are reported (Pierson et al., 1989; processes may be significant. Ammonia is incorpor- Waldman et al., 1990; Keeler et al., 1991). A broad ated into ammonium aerosols, which are subject range of acidic aerosol and sulphate have been found. to long-range transport. Ammonium nitrate and The highest peaks in acidic aerosol concentrations ammonium chloride are unstable compounds; high were measured with short sampling periods (Pierson temperature and low relative humidity favours the et al., 1989; Ferris and Spengler, 1985). Levels of acidic gaseous components (NH 3 and HC1 or HNO3). Air aerosols are generally higher in North American sites masses arriving at our sites directly from the North than in Europe. This is due to the low neutralization Sea (ENE) are affected by the depletion of ammonia capacity of air at many North American sites, due to over the Sea. Low concentrations of ammonia over the low levels of ammonia (Pierson et al., 1989; Keeler et North Sea (Ottley and Harrison, 1992) are explained al., 1991).

Table 5. Concentration ranges of aerosol SO~- and H + (neqm -a)

Study location Allegheny Mountain Laurel Hill Toronto, Canada Nova Scotia, Canada Txedo, New York St. Louis, MO St. Louis, MO St. Louis, MO Kingston, TN Newtown, CT Los Angeles, CA Onsala, Sweden The Netherlands Northwest England Southeast England

Sample duration (h) 7, 10 7, 10 8, 16 24 1-12 QC§ QC 24 24 24 12 24 24 24 24

Concentration range (neq m- 3) SO~H+ 35.9-946 46.0-1156 0-1562 0-542 20.8-916 62.5-521 104-896 0-833 0-583 0-542 62.5-208 72.9-896 10.4-583 39.6-475 27.1-996

Method

8.4-617 pH meas. 9.1-844 pH meas. 0-388 pH addition* 0-184 Titration t 2 0 . 4 - 1 7 8 F.P.D/Titr :~ 0-143 F.P.D. 0-694 F.P.D. 0-122 pH addition 0-286 pH addition 0-204 pH addition 12.2-65.3 Titration 2-204 Titration1" 2-53.1 Extractionll 0-22.4 Titrationt 0-178 Titration1"

* Addition of perchloric acid to solution before pH measurement. t Titration according to Gran's method. :~Flame photometric detector or titration. § Quasi-continuous measurements. IIH,S04 was extracted from filter by isopropanol.

Reference Pierson et al. (1989) Pierson et al. (1989) Waldman et al. (1990) Smith-Palmer and Wentzell (1987) Morandi et al. (1980) Huntzicker et al. (1984) Ferris and Spengler (1985) Koutrakis et al. (1988) Koutrakis et al. (1988) Keeler et al. (1991) John et al. (1985) Brosset (1978) Eishout et al. (1978) Harrison and Pio (1983b) This work

Processes affecting aerosol acidity Measurements of sulphuric acid aerosol in England started after the historic L o n d o n smog episode of December 1952. The highest daily concentration is reported to have been 347 #g m - 3 recorded in December 1962 (Ito and Thurston, 1989). Since then levels of SO2 and sulphuric acid aerosol have been much lower, following the introduction of the Clean Air Acts. Concentrations of acidic aerosols in northwest England have been found to be relatively low (Harrison and Pio, 1983b) with the highest concentrations (22 n e q m - 3) from the east-southeast sector. It is interesting to compare levels of acidic aerosol presented here with those measured in Europe. In the Netherlands, concentrations of aerosol H + were similar to those measured in southeast England but they were lower than those in Sweden. This may be due to the comparable concentrations of atmospheric a m m o n i a in the Netherlands and southeast England (Erisman et al., 1988; Allen et al., 1988); much lower a m m o n i a concentrations have been observed in Sweden (Brosset, 1978).

CONCLUSIONS Acidic aerosol collected at three sites in southeast England showed similar concentration trends, with the highest concentrations at a coastal site. Acidic aerosols were progressively neutralized while they were transported inland. This is presumably due to high a m m o n i a concentrations which were spatially unevenly distributed, with the highest concentrations inland. High equivalent ratios of H+/SO 2- were commonly associated with low to moderate sulphate, rather than with high sulphate concentrations which generally corresponded with high levels of a m m o n ium. Summer measurements have shown that aerosols were acidic despite high a m m o n i a concentrations, implying that the rate of H2SO4 formation exceeded the rate of neutralization. High levels of acidic aerosols were associated with continental air mass back trajectories, demonstrating that acidic aerosols were transported from the continent or were formed during transport. Low a m m o n i a levels over the North Sea have an important influence at our sites, higher levels of unneutralized aerosol acidity persisting in air masses from this sector.

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