Separation and analysis of aerosol sulfate species at ambient concentrations

Atmospheric Environment

Vol.

11, pp.

955-966.Pergamon Press 1977. Printed in Great Britain

SEPARATION AND ANALYSIS OF AEROSOL SULFATE SPECIES AT AMBIENT CONCENTRATIONS R. L.

TANNER, R. CEDERWALL, R. GARBFB, D. LJZAHY, W. R. MEYERS, M. PHILLIPS and

MARUIW,

L. NEWMAN

Atmospheric Sciences Division, Department of Applied Science, Brookhaven National Laboratory, Upton, N.Y. 11973, U.S.A. (First received 29 September 1976 and in final form 7 March 1977)

Abstract-Sampling and analysis techniques appropriate for the determination of the chemical composition of sulfate in aerosol particles are described. These techniques are applied to the speciation of sulfate in ambient air-borne particles with time resolution of one hour or less and with size discrimination in the size range below 0.25 ,um. Initial East Coast data are reported indicating the dominance of sulfuric acid and its ammonianeutralization products in ambient sulfate-containing particles. Data from a comparison study of several analytical techniques for aerosol sulfate and related species are highly correlated. Diffusion sampling experiments for size-segregated chemical composition determination indicate that l/2 of airborne sulfate is in particles ~0.3 pm dia. Hourly variations in sulfate chemical composition at 3 RAMS sites in and near St. Louis, MO. are documented for a one-day intensive experiment and are correlated with air-mass backward trajectories and area primary sources.

INTRODUCHON

Analysis of sulfate and other oxyanions of sulfur in airborne particles has attracted increasing attention from air pollution control authorities since sulfate has been shown to be a more serious health hazard than its principal atmospheric precursor, sulfur dioxide (SO,) (Rall, 1973). Furthermore, reduction in SO1 emissions has not resulted in proportionate decreases in ambient sulfate concentrations in the U.S.A. (Altshuller, 1976). Developing an effective control strategy for atmospheric sulfate will require, analogously with other secondary pollutants, a more thorough understanding of the generation, transport, transformation and removal processes of sulfur oxides in the atmosphere. Increased understanding of transformational aspects of this cycle will not be forthcoming from sulfate measurements alone-important as such measurements are. It will be necessary (a) to determine the specific chemical nature of the airborne sulfate, (b) to make these determinations as a function of particle size from molecular or cluster dimensions to the “boulder” regime (> 10 pm dia.), and (c) to determine chemical composition as a function of particle size with a time discrimination of one hour or less. We report significant progress in determining chemical composition of the sulfate fraction in airborne particles from a variety of sampling locatiods with sub-optical size fractionation and time discrimination of one hour. Analyses for acid, ammonium, soluble sulfate and nitrate in acidic aqueous extracts of airborne particles are used, analogous with the methodology of Brosset et al. (1975), to identify air masses in which the sulfate portion of the aerosol

particles is dominated ammonia-neutralization

by sulfuric acid and/or

its

956

R. L. TANNERet al.

ul~a~nically leached from the filter, and the Sow rate through even the FA-type (1.0~ pore size) Fluoropore is limited to about 0.2 m3 h-’ cm-‘-unsuitably slow for high volume sampling. The quartz filter medium used for most of this work was prepared in groups of four 8 x 10 in. sheets from Pallflex tissue quartz (Pall Products Corp., Putnam, Conn.). Without prior rinsing, the quartz filters were heated to 700” for ca. 12 h to eliminate volatilizable impurities. The filters were then cooled, placed on a specially fabricated 9 x 11 in. glass-mesh filter sunport atop a Buchner funnel (lined with-two pre-washed GF filters for additional sup port) and washed with about 21 of distilled water. The filters were then dried, replaced on the filter support, and treated with hot (85°C) phosphoric acid for 30min. The filters were then removed to a large Pyrex dish and rinsed 3 times in distilled HZ0 to remove excess H3P0& After being replaced in the glass-mesh support, the filters were washed in about 61 of distilled water and dried to 110°C in an oven, whereupon the filters were ready for use. The resultant filters are suitable for high volume or low volume sampling with representative flows as follows: 8 x loin. filter, Staplex HiVol, 1.0-1.1 m3min-i; 1Ocm diameter circle, Staplex HiVol, 0.4sO.5 m3 min-‘; 4.7 cm circle, Gast IVBF-lo-Ml~X pump, 50 1min-‘. In addition, no evidence of oxidation of SO2 to sulfate on the treated quartz in the presence or absence of collected airborne particles.has been found for the following set of experiments: (a) HiVol sampling for cu. 5min on preloaded and clean filters, with addition of ppm quantities of SO,; (b) parallel HiVol sampling for 2-5 h of ambient air and ambient + IO-30 ppb additional SO, with 2 quartz filters in series; and (c) parallel low volume sampling of ambient air and ambient + l@-30ppb SO,. (b) Extraction methodology Barton and McAdie (1970) have reported removal of H2S0., from Nuclepore filters by extraction with isopropanol. Unfortunately isopropanol also dissolves ammonium bisulfate, and it has been demonstrated by Leahy et al. (1975) that benzaldehyde is the preferred extractant and does selectively remove H,SO, in the presence of busulfates and sulfates as shown by the data in Table 1. Microgram samples of 33S-H,S04 were placed on a variety of filter materials, dried in uuctw at <4O”C, contacted with l&l5 ml benzaldehyde, centrifuged to remove the filter and undissolved particles to the bottom of the benzaldehyde layer, a portion of the extract removed and back-leached with water, and the sulfate recovery determined by ~-counting of a portion of the dried aqueous extract. Sulfuric acid has been removed from Tef-

lon (Mitex 5 pm and Fluoropore 0.2~) and HzPO,treated quartz filters, with recoveries increasing from about 75% for 1Oflg samples to about 95% for 1OOjfg samples. Determinations of non-radioactive sample recovery by turbidimetry were also consistent. Ammonium bisulfate samples are clearly not extracted by ~n~ldehyde (Table 1, row 7) although for mixed H#O,-NH,HSO, samples, the % NH,HSO, coextracted may be as much as 5%. Methanol and isopropanol will quantitatively extract bisulfate in the presence of sulfates, but isopropanol is preferred since methanol will partially extract ammonium sulfate from the filter as well as bisulfates (Leahy et al., 1975). Preliminary ex~~ments have been conducted to determine the optimum procedure for determination of both H,SO, and bisulfate on the filter sample: sequential extraction with benzaldehyde for H,S04, then isopropanol for bisulfate; or parallel extraction of two portions of the filter, benzaldehyde for H,SO., and isopropanol for the sum of H,SO, and bisulfate with bisulfate calculated by difference. The latter procedure is experimentally simpler but optimization experiments are still in progress. (c) Acidity and soluble ion measurements In addition to the specific methodology for HzS04 and bisulfate discussed above, a second approach to sulfate speciation has been adopted from the approach of Askne et al. (1973). who have determined the acid and soluble ion contents’ of acidic aqueous extracts of airborne particle samples. The total titratable acid in an extract of the particle sample is determined by incremental titration with strong base with correction for the leach solution contribution. The data are plotted in exponential form (Gran, 1952) as shown in Fig. 1. to permit determination of strong acid (K, 2 10w3) in the presence of weaker acids by extrapolation from early points in the titration curve. Determination of as little as 0.1 geq (1 peq = 10m6 equiv~ent) with 10% precision is possible. The manual titration has been replaced by a coulometric titration (Liberti et al., 1972; A&e, 1973) using a PAR 179 Digital Coulometer (Princeton Aonlied Research. Princeton, N.J.) in control I mode, in which OH- is generated at a Pt working electrode in 0.04M KBr with a Ag/AgBr counter electrode and the pH continuously monitored by an Orion Ionalyser (Orion Research Corp., Waltham, Mass.) with recorder. The titration of a properly stirred extract can be conducted in less than 15 min with negligible operator attention. The data analysis is now being conducted on an HP 9101B Programmable Calculator {Hewlett-Packard Corp., Avondale, Pa.), using a modified linear regression program in which the raw mV data are

Table 1. Benzaldehyde extraction of HzS04 and NHIHSOd from filters Amount

Sample

10/G

HW,

20-40 Irg

H2S04

HtSG4 H,SGe

No. of determinations

2 2

30 pg 20-40 @g 70-80 pg

H2S04

1 4 4

1OOpg

H2S04

5

1O-70 fig

NH4HS04

3

34 peg

NH4HS04

2

* Soluble sulfate determined by turbidimetry. t Extraction by methanol.

Filter material

% Recovery

Method

quartz Mitex 5 pm Fluoropore 0.2 /ml quartz quartz any of the above Fluoropore 0.2 pm Fluoropore or quartz

75 71

=S “5s

90 78 86

35S 35s sot

94 + 8

35S

1.2

3sS

1117

35S

Separation and analysis of aerosol sulfate species at ambient concentrations 6

,

1

PQU,

I

I

5/14/75

5

4 0 ‘0 g

w ZL

3

2

B = SAMPLE POD& 5114175

I

MICROLITERS

OF BASE

Fig. 1. Gran titration plot of aqueous extract of ambient aerosol particles collected at BNL, May 14, 1975. converted to lOElKdata and plotted vs. ~1 of added base and the best fit straight line drawn by an HP 9125B Plotter (ibid). Complete determination of acidity in 610 filter samples per analyst-day is possible. The aqueous extract is further analyzed for ammonium by the indophenol calorimetric method (Bolleter et al., 1961) nitrate by hydrazine reduction, derivatization and calorimetry (Mullin and Riley, 1955), and sulfate by turbidimetry (Sulfate Method VIb.. , 1959). A portion of the 10cm diameter filter samples was used in some cases to determine the total sulfur content of the airborne particle sample, by the reduction- ii”Ag,S technique of Forrest and Newman (1973). The combined sampling and analysis precision ( f lc) for the wet chemical methods for ammonium, nitrate, sulfate and total sulfur is 5 + 10% at ambient levels. Sulfate determinations have also been done on many samples using a new flash volatilization-flame photometric techniaue (Husar et al.. 1975; Roberts and Friedlander, 1976). The basis of this technique is the flame photometric measurement of sulfate which has been volatilized from a metal strip or boat following rapid heating to about 1200°C by capacitance discharge. The apparatus used for this technique has been modified in this laboratory to improve the sensitivity, simplify the output signal (eliminating the need for integration of the response) and extend the life of the flash volatilization sample boat. Reduction of the volume of the volatilization chamber from about 150 cm3 to a near-minimal value of about 30 cm3 increased the speed of response and decreased the width of the response peak dramatically. With the redesigned chamber, the peak height of the output signal was found to be proportional to the square of the total quantity of volatilizable sulfur. The new chamber allows the use of unprocessed air flow supplying the flame photometer system to carry the volatilized sample to the detector. A troublesome feature of the stainless steel strip used by Roberts and Friedlander (1976) or the tungsten boat used by Husar et al. (1975) is the limited lifetimes of these

957

materials under discharge conditions. The use of a platinum boat was introduced in this laboratory. This resulted in a volatilization system with a very long life (several thousand discharges). The use of platinum also slightly increases sensitivity, possibly due to more efficient volatilization of the sample. With the present system, the minimum quantity of sulfate which may be determined is approximately 1 ng S for a signal to noise ratio of 2:l (3 ng for f IO% precision). (d) Diffusion sampling The size discrimination innovations we report are in the use of a diffusion sampler consisting of a battery of diffusion cells designed by Sinclair (1972). Each cell is a highly collimated hole structure of 14,500 stainless steel tubes of 0.22 mm dia., creating a diffusion cell of cn. lo3 m equivalent length which is only 4 cm in dia. and 2.5 cm in length. Smaller particles diffuse preferentially to the walls, hence by sampling after increasing numbers of cells, removal of progressively larger particles is possible. Fractional penetration vs. log particle diameter curves can be calculated from the known conditions of volume flow, equivalent length and particle diffusion coefficients (Marlow and Tanner, 1976). Sampling after increasing numbers of diffusion cells with a condensation nuclei counter allows determination of particle size distributions. The innovation of Marlow and Tanner (1976) is to use filter sampling and chemical composition determination after diffusion sampling. The difference between the chemical composition of untreated and diffusion processed air after a given equivalent length of diffusion cells is the composition of the smallest particles (those captured by the diffusion battery). Furthermore, by comparing the chemical data for 2 (or more) simultaneously sampled ports, the composition of the fraction of particles sampled, as defined by the difference in the fractional penetration curves (at the extant volume flow) can be determined. (e) Air wrass trajectory calculation The method of calculation of the air mass trajectories used in this study has been described by Meyers and Cederwall (1975). The data base from which the RAMS stations trajectories were calculated included 12 h radiosonde data from stations at Peoria, Ill., Salem, Ill., and Monet, MO., and supplementary pilot balloon data taken in connection with the RAPS program at RAMS stations 152 and 144 in Alton, Ill., and Crescent, MO., respectively. The data at each site were averaged over the vertical layer specified; here a typical mixing layer of O-1000 m was used, and O-150 m was used for evaluation of surface trajectories. For the BNL traiectories 12 h radiosonde data from stations at JFK International Airport, Chatham, MA, Albanv. N.Y.. Dulles International Airnort and Wahoos Island:‘VA., and supplementary ground data from atop the Brookhaven National Laboratory tower and JFK International Airport were averaged over a mixing layer from O-500 m. The averaged data at each site were then interpolated by inverse square weighting of the distance to the 3-h trajectory segment being calculated. It should be noted that the 1 0 standard error bar for trajectory location is no more than 1.8 km h-i of trajectory age. Since time and space scales of this study are smaller than those used in the trajectory calculation, the use of the trajectories for interpretive purposes must be done with caution. RESULTS AND DISCUSSION

The analytical chemical, sampling and particle size discrimination techniques described above have been applied to samples collected in power plant plume, urban and rural ambient environments at both coas-

958

R. L. TANNERet al.

alents of NH:, although this is relatively rare and, indeed particles may be net basic to the pH4 leach used (and appear as a negative [H’] in the Table). The precision limits for simultaneous sampling on identical filters are f l&30%. It is clear from this summary that sulfuric acid and/or its ammonia-neutralization products frequently dominate the sulfate portion of the ambient aerosol sampled at the semi-rural BNL location (on Long Island, 100 km ENE of New York City), since equivalents of strong acid and ammonium usually equal equivalents of sulfate, and nitrate equivalents are only a few % of sulfate. This is consistent with the data of Brosset et al. (1975) taken at a rural location on the West Coast of Sweden, and also with the data of Charlson et al. (1974) obtained at both rural and urban sites in the St. Louis area. Within experimental error, (NH: -t H+) equivalents rarely exceed sulfate equivalents in airborne particle extracts, but 20% excess sulfate over (NH,’ + H+) equivalents is not uncommon, especially when, as on the lo/IS sampling date, the airmass back-trajectory from BNL passed through the metropolitan New York area. Acid sulfate particles with equivalents of strong acid exceeding equivalents of ammonium are relatively rare in non-size-segregated airborne particle samples, and appear to be confined to episodic conditions in stagnant air masses.

tal and continental locations in connection with several sampling programs during spring, summer and fall, 1975. The emphasis in these studies was on developing analytical methods for chemically speciating sulfate in ambient airborne particles, applying these methods in field studies designed to characterize ambient sulfate particle composition as a function of particle size and meteorological variables, and then beginning to evaluate transport and transformational implications of the resulting data. The results from some of these experiments are described below. (a) Brookhaven National Laboratory

(BNL)

sampling

A data summary which reports, on an equivalent basis, the cation-anion balance in the soluble ion portion of selected airborne particle samples collected at the BNL sampling site in 1975 (mostly at about 125 m above ground level atop the Ace meteorology tower) is given in Table 2. The data are used to compute a cation-anion balance, comparing the H+ + NH: content on an equivalent basis with the sulfate and nitrate content in order to identify air masses in which the airborne sulfate is dominated by HzS04 and its ammonium salts. It is important to note that equivalents of H+ + NH: equal equivalents of SOi- -t NO; to within *20% for most of the air masses sampled, independent of filter material used. The equivalents of HC may be found to exceed equiv-

Table 2. Selected chemical composition data from airborne particle samples collected at BNL, 1975

CH’I Duration 518 24 h S/14 24 h 5119 24h 619 24 h 6119 6h (day) 7/l 24h 718 24h

9115 6h HiVol lo/15 3 h HiVol 11/4 5h HiVol 11/20 2h HiVol

Location BNL tower BNL tower BNL tower BNL tower BNL tower BNL tower BNL ground level BNL tower BNL tower BNL ground level BNL tower

* Soluble sulfate by turbidimetry;

Filter

CNHZI 3

3

[so:-]* 3

[NO;1 3

33

74

125

13

298

191

456

13

141

339

432

7

quartz

22

42

69

18

quartz

890

770

2010

60

51

108

170

ND

85 + 12

117 + 40

Mitex 5 pm quartz Fhroropore 0.2 pm

quartz Fluoropore [mean of triplicate samples]

241 + 24

[variable] 21

quartz

20

33

86

6

quartz

48

286

494

28

159

141

15

228

419

12

quartz quartz

-6 40

1 neq mm3 = 0.048 pg mm3.

Separation and analysis of aerosol sulfate species at ambient concentrations Table 3. RAPS sulfate data from Fluoropore fine fraction MDS filters by turbidimetry and X-ray fluorescence; samples 201-206, 210, 216 and 218 obtained at urban RAMS Station No. 106; samples 801-806, 810, 816 and 818 obtained at rural RAMS Station No. 124 Turbidimetric sulfate, pg me3

Sample No.

7.01

201 801 202 802 203 803 204 804 205 805 206 806 210 810 216 816 218 818

XRF sulfur as sulfate, pg me3 4.90 2.10 9.46 4.09 20.1 21.0 23.5 21.2 14.3 10.9 6.35 4.32 12.9 11.3 23.4 18.7 13.3 NW

2.30 11.8 4.90 24.0 NAt 26.1 25.1 15.2 7.14 6.42

NAt 11.8* 10.8 29.9 29.5 17.1 19.8

* Sample collected on treated quartz. t Data not available. (b) Intercomparison of aerosol sulfate analysis methods

In connection with an extensive intercomparison study of methods for the physical and chemical characterization of atmospheric aerosols conducted by U.S. EPA/ESRL personnel at RAMS Stations 106 (St. Louis urban location) and 124 (rural location S. of St. Louis), manual dichotomous samplers (MDS) (Dzubay and Stevens, 1975) operating in parallel at each site were used to collect 24 h total, fine fraction, and coarse fraction samples during the period of 8118 (Day 1) to 9/4/75 (Day 18). We obtained MDS fine fraction samples collected on Fluoropore (47 mm, Type FALP) at both sites on sampling days 14, 10, 16, and 18 as well as MDS fine fraction and total samples collected on H,PO,-treated quartz filters at both sites on days 10, 16 and 18. From the resultant Table 4. Comparison

A.E.

11 ‘IO--F

959

analytical data a direct comparison of sampling methods and sulfate analysis techniques was possible. In addition, sulfate speciation data were obtained for some samples, and the percentage of sulfate in the fine fraction was determined. A direct comparison of the turbidimetric sulfate data from samples collected on Fluoropore filters with data obtained by T. G. Diubay of EPA/ESRL using X-ray fluorescence (XRF) (Dzubay and Stevens, 1975), is shown in Table 3. Considering that the one c precision for sampling and analysis of each of the two methods is ca. f lo%, the agreement between methods is good. A linear regression plot of the turbidimetric data vs. the XRF data (x-axis) yields a slope, a = 1.2, an intercept, b = - 0.5 (pg/m3) and a correlation coefficient R = + 0.955, and indicates that the XRF data are -20% low or the turbidimetric data are -20% high. Table 4 lists speciated sulfate analyses for 3 sampling days at both sites. As noted above, both total and MDS fine fraction samples were collected on acid-treated quartz filters and preserved in sealed plastic containers until analyses were performed. The data show that on average more than 90% of the soluble sulfate is in the fine particle range. In fact, a plot of total sulfate vs. fine fraction sulfate yields the following linear regression statistics (defined above) when the origin is included: a = 0.96, b = 1.18, R = + 0.965. Fine fraction samples usually contain more strong acid than total particulate samples, indicating inhomogeneity in the distribution of extractable strong acid or base with respect to particle size, as originally noted by Junge and Scheich (1969), and found as well by Marlow and Tanner (1976) in smaller size ranges. Contributing to this apparent inhomogeneity is the presence of net-basic particles in the coarse particle range as noted in section (e) below. (c) Difision battery sampling The results of preliminary experiments with the diffusion-sampling method at BNL (Marlow and Tan-

of total and fine fraction airborne sulfate speciation data; sample collection on treated quartz

Sample No.

Fraction

[H’] as HZS04, M rn-’

210 210 810 810 216 216 816 816 218 218 818 818

fine total fine total fine total fine total fine total fine total

0.01 -0.9 0.37 0.23 1.57 0.64 1.3 2.8 0.3 -0.6 0.55 0.32

CNHdl pg m-3 3.8 3.3 2.2 2.1 13.1 10.8 6.0 3.7 3.7 4.5 5.8 5.0

Turbidimetric sulfate g m-3 11.8 14.4 10.5 10.7 28.3 26.1 23.5 26.9 13.7 17.0 18.4 15.0

Nitrate pg m-3 0.25 0.36 0.22 0.17 0.36 0.23 0.59 NA 0.31 0.27 0.60 0.44

R. L. TANNERet al.

960

ner, 1976), indicated that suboptical particles were frequently more acid than larger light-scattering particles; furthermore, passage of an air mass from a source region across a body of water to the BNL receptor site resulted in more acid particles in the suboptical size range than passage predo~n~tly over land areas. It may be hypothesized that lower ammonia concentrations over ocean areas decrease the neutralization rate of H2S04 droplets, and hence increase the proportion of acid in particles of a given size. It was desirable to test the method and the tentative conclusions derived from it in other sampling locations. Therefore, a nine-day, intensive, diffusionsampling experiment was conducted at Glasgow, Ill., a rural location 120 km N. of St. Louis, MO., during the period of July 22-30, 1975, in coordination with the RAPS summer 1975 field program. Twelve hour samples were taken each day and night with the diffusion battery ‘of Sinclair (1972) sampling ambient air (samples designated PQU), diffusion processed air with a 50”/, penetration diameter (d,,%) of 0.035 m (PQDB,), and diffusion processed air with d 5O%= 0.13 m (PQDB,). Size distributions were also obtained using an ll-port screen-type diffusion sampler in conjunction with a condensation nuclei counter. The sulfate concentration data presented in Table 5 demonstrate the powerful potential of the diffusion sampling method. Comparison of the data for unprocessed air (PQU) with that for air with the smallest particles removed (PQDB,) demonstrates that, as expected, sulfate concentrations are equal within experimental error. This indicates that little sulfate mass is present in particles of diameter less than a few hundredths of a micron. However, comparison of PQU data with data from air with most suboptically sized Table 5. Sulfate concentration in airborne particles from ambient and diffusion-processed air (1 fig me3 = 20.8 neq m-3); 12 h sampling at Glasgow, Bl., July 22-30, 1975 Sulfate concentration, Date 7122 i’f22-23 l/23 7/24 7124-25 7125 Y/25-26 7/26 T/26-27 7/27 7127-28 If28 l/28-29 7f29 7/29-30 7/30

PQU E* 604 542 E’ 132 165 240 ‘323 554 F 312 163 217 852 810 777

PQW 388 556 438 166 151 166 242 365 578 425 283 151* 225 E 744 944

neq my3

PQDB, 143 352 178 73 84 84 123 146 329 159 194 58 110 460 431 442

Average PQDB,/PQU = 0.97 + 0.19; average PQDB,/ PQU = 0.52 + 0.17. * Erroneous data obtained.

particles removed (PQDB,) indicates that of the order of 50”/, of the sulfate mass is found in particles with diameters less than the size limit for optical scattering techniques. Furthermore, this fraction is not strongly dependent on the total sulfate concentration for the reported sampling period and location. There is no evidence in this Eulerian experiment of temporally dependent growth of sulfate within a given air mass into larger size fractions. The detailed results of this experiment will be reported elsewhere. (d) Regional Air Pollution Study (RAPS) ground sampling experiments

An intensive experiment was designed to test analytical methodologies for speciated sulfate determinations with one-hour time resolution and to educe changes in the concentration and chemical composition of sulfate-containing airborne particles during passage of an air mass through the St. Louis urban area_ Hourly HiVol samples were taken on 4in. circles of H3P04-treated quartz at three Regional Air Monitoring Stations (RAMS), No. 125, 105 and 123, during the periods 1lo@-1500 and 180@-2200h, 7/28/75 and during 04O&O800h 7/29/75 (all times CDS). Hourly HiVol samples were analyzed for strong acid, a~onium, sulfate, nitrate and, in most samples, also for HzS04 and total sulfur by the methods described above. The locations of the three sampling stations are shown in Fig. 2. In addition, the backward trajectories from the RAMS 125 site are shown at 6 h intervals from 1300 h, 7/28 to 0700 h, 7/29. The 1300 h trajectory indicates that RAMS 125 is a background station for St. Louis during this period. During the evening sampling periods the backward trajectories reflected a wind direction change to SE and by early morning of 7/29 the trajectory passed progressively eastward with increasing encroachment on the urban St. Louis area. The chemical composition data for RAMS 125 are shown in Fig. 3. Sulfate incursion from nearby Labadie power plant is indicated in the first hour (cf. Fig. 2, trajectory I), but otherwise the RAMS 125 data appear to represent sulfate levels upwind of St. Louis. Equivalents of (H+ + NH:) equalled equivalents of sulfate for a Ieast half of the samples. Titratable acid concentrations included negative values (i.e., samples neutralized part of the pH 4 leach). High levels of titratable acid during the periods 1200-1300, 7/28 and 04OCKl600, 7/29 may not be associated with sulfate based on com~~son with sulfuric acid concentrations measured by benzaldehyde extraction and flash volatilization-FPD and listed in Table 6. The data from the urban station (RAMS 105) are shown in Fig. 4. Hourly variations in sulfate concentrations were greater than at RAMS 125 which is eon&tent with a primary source area near diverse industry. Trajectories from station 105 shown in Fig. 5 indicate that 105 was downwind from Baldwin power plant during part of the evening sampling

961

Separation and analysis of aerosol sulfate species at ambient concentrations

/

.*

18’

.* ._

2#”

I

.**

I ..-

Fig. 2. Backward trajectories for RAMS Station No. 125 for the period 1300 7128175 to 0700 l/29/75 using winds from @-loo0 meters. Hatched area is urban St. Louis; triangles are power plant locations with symbol size proportional to SO, emission. I

I

I_. L

HOUR 12 DATE 7/20

14

16

16

20

22----;T29C6

08

Fig. 3. Chemical composition data for airborne particles collected at RAMS Station No. 125, July 28-29, 1975; -= soluble sulfate by turbidimetry, . . . . = ammonium, --- = H+ by titration, -.--. = benzaldehydeextracted H,SO, and 0 = total sulfur as sulfate. Table 6. Comparison

125-1 125-2 125-6 125-8 125-9 125-12

I

16

I8

20

I

22---04

Time

7128 7128 7128 7128 7129 7129

1100-1150 1200-1250 1900-1950 21W2150 0400-0450 07OWI750

I

I

I

06

7129

of titratable acid, extracted sulfuric acid, and soluble sulfate data from ground samplingsat RAMS Station No. 125, July 28-29, 1975

Sampling date

I

06

Fig. 4. Chemical composition data for airborne particles collected at RAMS Station No. 105, July 28-29, 1975; -= soluble sulfate by turbidimetry, . . . . = ammonium, --- = H+ by titration, -.-. = benzaldehydeextracted H,SO,, and o = total sulfur as sulfate.

Titratable acid Sample No.

I11111111b~11 HOUR 12 14 DATE 7/26

3 653 550 -59 64 945 43

Extracted HzS04, 3 25 33 18 27 40 33

Turbidimetric so:-, !zl 969 346 189 176 201 304

962

R. L. TANNERet d.

Fig. 5. Backward trajectories for RAMS Station No. 105 for the periods 1300 7/28/75 to 0700 7129175, using winds from O-1000 meters. Hatched area is urban St. Louis; triangles are power plant locations with symbol size proportional to SO2 emission. period, thus exaggerating the hourly variation in sulfate concentrations. High titratable acid concentrations (24 x lo5 neq m-j) were observed on four of the one hour samples and are mostly weak acids not associated with sulfate. Total sulfur concentrations were not always 2 soluble sulfate concentrations on an equivalent basis, hence the possibility of positive interference with the turbidimetric technique for some samples cannot be excluded. Sulfate concentrations were on the average 50-100~0 higher than background levels at RAMS 125 even when speciation data indicate’ that the sulfate is present mostly as ammonium sulfate. Chemical composition data from RAMS station 123 are shown in Fig. 6. Interpretation of these data was complicated by the presence of varying, large amounts of weak acid (> 2 x lo3 neq mm3) in all but one of the samples, and also by the complex meteorological situation. During the daytime sampling, the urban plume passed directly to the RAMS 123 sampling site in 4-6 h, as shown by the 1300 h 7/28 trajectory from station 123 in Fig. 7. Concurrently, hourly variations in sulfate concentration were substantial, and sulfate equivalents exceeded the sum of Hf + NH:. The sharp maximum in sulfate concentration observed at 123 during the 2000-2200 period may indicate the influence of Baldwin power plant (cf. trajectories 2 and 3 in Fig. 7) although total sulfur data is not confirmatory. The trajectory of 0700 h 7/29 skirted the St. Louis urban area to the East and South, and the chemical composition data in Fig. 6

indicate that >80% of the sulfate is present as (NH&S04. The fact that the sulfate levels at RAMS 123 are about twice as large as at RAMS 125 is not clearly explained by the backward trajectories using winds from 0-1OOOm or from surface trajectories using winds only from 0-150m. It is concluded that, when backward trajectories are used to deduce movement of air masses, the correlation between surface sulfate concentrations and SO2 emission sources is very strong. Nevertheless, it is difficult to explain all hourly surface concentration variations on the

600400

-

bD ..- .. ......

.

II 200 -

-

*

0 (,.,p.

__

-._.

___r.n (11 HOh I I 13 15----I8 DATE 7/20

_._q 20

._. ._ 22----

..-. y2;6

._. 08

Fig. 6. Chemical composition data r airborne particles collected at RAMS Station No. 1P3, July 28-29, 1975; -= soluble sulfate by turbidimetry, . . = ammonium, ---. = benzaldehyde-extracted H,S04, and 0 = total sulfur as sulfate.

Separation and analysis of aerosol sulfate species at ambient concentrations

963

Fig. 7. Backward trajectories for RAMS Station No. 123 for the periods 1300 l/28/75 to 0700 l/29/15 using winds from ~1000 meters. Hatched area is urban St. Louis; triangles are power plant locations with symbol size proportional to SO2 emission.

expanded spatial scale used in this experiment with the trajectory model employed, due to the limited time resolution of the basic meteorological data. In addition to the ground sampling done during the 7/B-29/75 intensive experiment, aircraft samples were taken once over the urban sampling location and once over the background location during each 4 h sampling period in midday, evening and early morning. Additional airborne samples were taken by the EPA/Las Vegas helicopter over the urban and background (station 125) sampling areas during the midday sampling period. The comparison data for aerosol sulfate are shown in Table 7 which lists 4 h averages at each ground location for each sampling period together with airborne data taken during the same period; the downwind (station 123) ground data are included for reference value. The observed sulfate concentration is higher at the urban location by a factor of 2 for both ground and airborne samples (except for the early morning airborne data). More interesting is the observation that ground level concentrations exceed airborne values for the midday sampling period, when ground level sources are maximized and good mixing within the boundary layer is expected. However, during the evening and early morning sampling periods, when a ground level inversion, which would inhibit boundary layer mixing, would be expected, the ground level concentrations of sulfate were lower than the airborne values at the background site, although equal (within

experimental error) at the urban site. Further interpretation of the data is not warranted at this time but, clearly, care must be taken in data interpretations which require the assumption of homogeneous aerosol sulfate concentrations within a well-mixed boundary layer. In view of the results of this experiment, we suggest that in the documentation of the frequency and severity of acid sulfate episodes, titratable acidity data should be supplemented by data from specific sulfuric acid and bisulfate methodologies such as the extraction scheme reported above or, for HzS04, the thermal volatilization technique of Mudgett et al. (1974). Secondly, additional experiments should be performed in other urban locations with sufficient time resolution and chemical speciation to determine both the amount of primary sulfate emissions injected by an urban area into a passing air mass, and also the time-species history of the air mass downwind to the point where the airborne sulfate is chemically and physically indistinguishable from the regional background. It is recommended that continuous meteorological data be used when available to construct trajectories since these trajectories would better explain the large variations in sulfate observed on the fine time and space scales used here. In addition, to better understand the transport and transformation of airborne sulfur oxides in the study area, a suitable air quality model should be exercised which should incorporate

R. L. TANNERet al.

964 Table 7. Comparison

of airborne and ground level aerosol sulfate data, St. Louis, MO., July 28-29, 1975 Sampling Data

Turbidimetric

cm-1 Site

Location

M mm3

1100-1500

RAMS 125 Airbome# RAMS 105 Airborn& RAMS 123

Bkgd Bkgd Urban Urban Downwind

17.0* 10.2 33.4 22.1 37.9t

1800-2200

RAMS 125 Airborne RAMS 105 Airborne RAMS 123

Bkgd Bkgd Urban Urban Downwind

8.8 13.2 25.6 25.9 32.0

0400-0800

RAMS 125 Airborne RAMS 105 Airborne RAMS 123

Bkgd Bkgd Urban Urban Downwind

11.7 21.4 20.2 20.9 26.2

Period

7/28:

7/28:

7/29:

* Average concentration excludes data from llW1150 sampling period since RAMS 125 was directly downwind from Labadie power plant during that period. t Average concentration excludes data from 1400-1450 sampling period due to erroneous analytical data. $ Average of aircraft sample taken from 1210-1305 h and helicopter sample taken from 1317-1332 h. § Average of aircraft sample taken from 14151515 h and helicopter sample taken

-

from 135&-1412h.

the St. Louis regional emissions inventory. This is planned for the future and would be a natural extension of this initial study. (e) Ldzgrangian experiments for sulfate speciation Several experiments have been conducted to document the chemical evolution of the sulfate portion of the ambient aerosol in an air mass as it passes from a source-dominant urban area downwind into a rural area. Ground sampling experiments were conducted utilizing simultaneous high volume sampling at an urban location (roof of U.S. ERDA’s Health and Safety Laboratory, 376 Hudson St, New York) and at the BNL rural location, with wind speed and direction appropriate for transport of the New York

urban plume to the*BNL location in 4-6 h. The data for the 9/10/75 Lagrangian experiment, shown in ,Table 8, include data taken aboard a ship traversing parts of the continental shelf 30-60 km S. of Shinnecock Inlet, Long Island The most noteworthy aspect of the dam is their internal consistency. BNL 1 represents the regional background (about 3.5 ,ng mW3) before passage through New York City since the back-trajectory (Fig. 8, 1100 trajectory) passed north of the city and through upstate New York with relative high wind speeds. The urban sulfate concentration increased during the day to from 50% to 200% larger than background, and this increase is reflected in the concentrations measured at BNL (3 and 4) and, to a

Table 8. Speciated sulfate data from Lagrangian experiment conducted Sept. 10, 1975 Sample Description Date 9/10

FVFPD

Time

Location No.

103&1150 120&1320 1330-1450 15OG1620

NYC NYC NYC NYC

1 2 3 4

1003-1313 1318-1611 1615-1918 1922-2226 9/10-l 1 2230-0834

BNL BNL BNL BNL BNL

1 2 3 4 5

9110

9110 9/W11

1252-1910 1915XWO

OCEAN 1 OCEAN 2

Turbidimetric

neq m-3

neq me3

neq mm3

Total S as SOineq mm3

84 95 112 98

108 204 154 192

109 128 155 198

110 163 156 282

4 22 21 33 10

17 32 89 78 86

70 108 177 184 101

69 122 155 206 120

69 129 117 126 147

-7 9

28 66

118 132

101 142

NA NA

CH’I neq me3 -44 -35 -32 -62

CNHfl

w-1

cm-1

Nephelometric Mass, lrg mW3 8.2 8.5 17.9 28.4 17.9

Separation and analysis of aerosol sulfate species at ambient concentrations

965

MSSACHUSETTS

6

CONNECTICUT

symbol

Tra,ectory

Arrival

-

1100

EDT

9/10/75

______

1700

EDT

9/10/75

0200

EDT

9/10/75

... ...

Time

numbers are trafectory age in hours.

Fig. 8. Backward trajectories for Brookhaven National Laboratorjr site for the periods 1100 and 1700, 9/10/75 and 0200 9/11/75 using winds from &500 meters.

lesser extent, offshore (OCEAN 1 and 2) during the period when the back-trajectory passed through the urban area 4-6 h previously (Fig. 8, 1700 trajectory). During the sampling period for BNL 5, the wind direction change to SW. in late afternoon shifted the back-trajectories to south of New York City through the Philadelphia and central New Jersey areas (Fig. 8, 0200 trajectory). In addition, the air mass arriving at BNL during sampling period 5 was more stagnant during passage through New Jersey (earlier in period 5) and metropolitan Philadelphia (later in period 5). Hence it is not surprising that the sulfate background had increased by over 50% of that observed in BNL 1. The concentrations of airborne particulate sulfur calculated as sulfate by three methods are in excellent agreement for most measurements. Occasional incursions of sulfur compounds not analysed as soluble sulfate may be expected in airborne particles collected in an urban region; this would cause the total S measurements to exceed those of the other two techniques as in sample NYC 4. The observation of negative acid concentrations (i.e., net base present) in the urban samples, which are not present in the BNL samples, indicates that the urban source region contains large basic particles which are lost by sedimentation in the 4-6 h transit time to the BNL location. In fact there is a moderate amount of titratable acid in the samples (BNL 2,3,4) with back-trajectory through the urban area, even though the sulfate con-

centration exceeds the sum of [H’] + [NH;] by 40-100neq rnw3 (2-5 M mm3). This experiment is clearly valuable in simultaneously deducing the changes in the concentration and chemical composition of sulfate which are induced by passage of an air parcel through a large urban area, and possibly in distinguishing between the urban sulfate contribution and the regional background. The results of this and other experiments indicate that an increase of 48 g mm3 of sulfate at the BNL site over regional background commonly occurs due to passage of an air mass through the New York metropolitan area when the trajectory to BNL is reasonably direct and the transit time is of the order of 2-8 h. The changes in chemical composition of sulfate induced in the urban plume are more difficult to identify and quantify, although the analytical techniques for doing so have now been developed

SUMMARY

Techniques for sampling and analysis of airborne particles, for the purpose of determining the chemical composition of the sulfate in them have been described. This analytical methodology has been applied to speciation of airborne sulfate in several locations and sampling regimens with time resolution of 1 h or less and with size discrimination in the suboptical particle size range.

966

R.

L. TAIwm

The significant results and conclusions of this study include the following : (a) the sulfate mass at the semi-rural BNL sampling site is frequently dominated by sulfuric acid and/or its ammonia-neutralization products; (b) more acidic sulfate may be present in small particles at coastal receptor sites than at continental sites; (c) aerosol sulfur and sulfate levels measured by several techniques under carefully controlled conditions may vary by as little as +20x; (d) aerosol sulfate fractions in the fine particle range are about 90% and may consist of > 80% ammonium sulfate; (e) a significant portion (cu. 50%) of sulfate mass exists in the particle size region below 0.3 pm; (f) hourly variations in the amount and species composition of sulfate in airborne particle samples from St. Louis (upwind, urban and downwind) have been documented, and the value of air mass back-trajectories and emission source information in explaining these sulfate concentration variations has been demonstrated. (g) the proportions of regional particulate sulfate and New York City urban-induced sulfate in an air mass reaching the BNL sampling site were quantitatively documented. Acknowlecigements-Support by Division of Biomedical and Environmental Research, U.S. ERDA and the U.S. Environmental Protection Agency is gratefully acknowledged; we thank J. Forrest and S. E. Schwartz for many helpful discussions, and the remainder of our analytical group for performing the analyses; we also thank Dr. D. Sinclair of the U.S. Health and Safety Laboratory (HASL) for the use of the diffusion battery, P. Krey and L. Toonkel of HASL for assistance in obtaining the urban New York samples, and T. G. Dzubay of U.S. Environmental Protection Agency for his cooperation and permission to use the X-ray Fluorescence data from the intercomparison study in St. Louis. This work was performed under the auspices of the U.S. Energy Research and Development Administration under contract No. E(30-I)-16.

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