Environ. Sci. Technol. 1982, 16, 525-532
Environment”; Frederick, E. R., Ed.; Air Pollution Control Association: Pittsburgh, PA, 1979; p 212. (53) Stevens, R. K.; Dzubay, T. G.; Shaw, R. W.; McClenny, W. A.; Lewis, C. W.; Wilson, W. E. Environ. Sei. Technol. 1980,
(45) Dixon, J. K. J. Chem. Phys. 1940, 8, 157. (46) Sadler, M.; Charlson, R. J.; Rosen, H.; Novakov, T. Atmos. Enuiron. 1981, 15, 1265. (47) Bennett, C. A.; Patty, R. R. In “Second International Topical Meeting on Photoacoustic Spectroscopy”;Berkeley, CA, June 1981; Paper TuB22. (48) McClenny, W. A.; Rohl, R. In “SecondInternational Topical Meeting on Photoacoustic Spectroscopy”;Berkeley, CA, June 1981; Paper TuB25. (49) Griffing, G. W. Atmos. Environ. 1980, 14, 577. (50) Whitby, K. T. Atmos. Environ. 1978, 12, 135. (51) Ensor, D. S.; Charlson, R. J.; Ahlquist, N. C.; Whitby, K. T.; Husar, R. B.; Liu, B. Y. H. J. Colloid Interface Sci. 1972,
14, 1491.
(54) Ho, W.; Hidy, G. M.; Govan, R. M. J. Appl. Meteor. 1974, 13, 871. (55) Stelson, A. W.; Seinfeld, J. H. Environ. Sei. Technol. 1981, 15, 671.
(56) Currie, L. A.; Klouda, G. A.; Cooper, J. A. Radiocarbon 1980, 22, 349. (57) Rosen, H.; Hansen, A. D. A.; Dod, R. L.; Novakov, T. Science (Washington, D.C.) 1980,208, 741.
39, 242.
(52) Stevens, R. K.; Paur, R. J.; Lewis, C. W.; Dzubay, T. G. In “Ozone/Oxidants: Interactions with the Total
Received for review October 22,
I n Situ Rapid-Response Measurement of H2S04/(“&SO4 Virginia
1981.
Accepted April 8,1982.
Aerosols in Rural
Ray E. Welss,” Tlmothy V. Larson, and Alan P. Waggoner
Department of Civil Engineering, Environmental Engineering and Science Program, FC-05, University of Washington, Seattle, washington 98195
rn We are reporting measurements of the chemical composition and degree of hydration of haze particles in rural Virginia during the summer of 1980. A new system of instruments, based on detecting relative changes in particle size by measurement of optical scattering, was used to determine with 5-min time resolution the fine particle sulfate mass concentration and NH4+/S0z- molar ratio. We found sulfate and ammonium ions to average 58% of the mass of particles smaller than 1 pm. The particle molar ratio ranged composition in terms of the “,+/SO$ from 0.5 to 2 with strong diurnal variation. The particles were most acidic at 1500 EDT and least acidic in the period 0600-0900 JZDT. The ratio of scattering at ambient RH to that at 35% RH depended only on relative humidity and not on particle chemical composition or increasing vs. decreasing relative humidity. The water contained in ambient aerosol particles was more strongly associated with sulfate and ammonium ions than with the remainder of fine particle mass; these two ions and their associated water caused about 70% of optical scattering.
Introduction We are reporting the results of aerosol measurements taken during July and August of 1980 at a rural location about 100 km southwest of Washington, D.C. A set of instruments measuring changes of particle optical scattering in response to changes in instrumental air temperature and/or relative humidity (RH) enabled us to characterize the aerosol particles in terms of sulfate ion concentration and the ratio of ammonium to sulfate ions. These measurements, made once per hour during a 5-min interval, are compared to more traditional chemical analysis of particles on filters. We also present data on the response of particle light scattering to ambient relative humidity. This latter measurement was made to detect the possible presence of particle supersaturation under decreasing RH (1,2).Our measurements were made as part of a cooperative study; the other participants were from General Motors and the EPA. The site was located in the Shenandoah Valley approximately 0.5 km east of Highway US. 340,15 km north of Luray, VA (population 3600), 22 km south of Front Royal, VA (population 8200), and less than 1km west of 0013-936X/82/0916-0525$01.25/0
the Shenandoah National Park. The site was at an elevation of 275 m. As reported by Ferman and co-workers (3), surface winds were generally from the southwest in the morning and were light and variable during the rest of the day. Flow, averaged from the surface to the 850-mbar level, generally was from the Gulf of Mexico and passed through the lower Midwest prior to arrival at the site. Radiation inversions occurred most evenings and usually broke up by 0900.
Methods Air Sampling System. Figure 1A shows a schematic of our aerosol-sampling system. The system was housed in a mobile monitoring van. We designed this system to provide a flow of diluted sample air at a pressure slightly above ambient (AI’ about +10 cm of H,O), thereby avoiding problems of leaks in the sampling lines. Due to a large bypass flow, the residence time in the 2-m sampling stack was approximately 0.5 s. Dry (0 “C dew point), filtered, NH3-freeair was forced through a venturi, creating suction that was used to draw ambient air from the stack (see Figure 1B). In this way, the dilution ratio of dry air to sample air was a constant determined by the geometry of the venturi and was independent of small fluctuations in the dry-air flow rate. By use of an Aitken nuclei counter (GE Rich counter with modified electronics) and a switching valve, this ratio was continuously monitored and remained constant a t a value of 2.1 parts of dilution air to 1part of sample air. Since the outside dew point was relatively constant over a 24-h period (average 19.2 OC, range 17.1-22.9 “C), the resultant RH of the sample air after dilution was also relatively constant (average 32% RH, range 30-35% RH). Once an hour, filtered air was introduced into the ambient air sampling line, with the excess flow going back into the stack. This allowed instrument zeroing. Particle Size Measurement. A Royco 220 optical particle counter (OPC)with sheath air inlet ( 4 ) was used to measure particle size (in terms of equivalent scattering) in 16 channels between 0.3-and 10-pm diameter. The approximately isokinetic sampling inlet was placed directly downstream of the venturi. Accompanyingthe instrument was a 16-channel pulse height analyzer of our own design.
0 1982 American Chemical Society
Envlron. Sci. Technol., Vol. 16, No. 8, 1982 525
AEROSOL INLET
qF1cmo'D' Aolmin
- 2 . 4 cm I . D .
11000
A
BLOWER
-
U
riooollmin
i L ow RESTRICT~ON
4 4 VARIABLE
OOLER
HUMIDOSTAT
PORT FOR C L E A N AIR AND T E S T AEROSOL INJECTION
SAMPLE
STACK 13Ollmin
I I1
TEST AEROSOL
I I
400 I/min
t
AI.
O F DRY
I
F I L T E R E D , NH3 F R E E AIR
FLOW STRA IGHTENER
I I I
I I
130 I I m i n I -
L--l
BLOWER
Flgure 1. Schematic of the aerosol sampling system (A), including detail of the venturi dilutor (B)[abbreviations: humid neph = humidograph nephelometer, TGH = thermidograph, EAA = electrical aerosol analyzer, OPC = optical particle counter, CN = condensation nuclei; 3h = three wavelength].
A Thermo Systems 3030 electrostatic aerosol analyzer (EAA)was used to measure particle size in terms of electrostatic mobility over the range 0.01-0.3-pm diameter. The EAA was modified to continuously scan through the full range of precipitator rod voltages each 2 min. The current for each rod voltage was digitized and averaged over an hour to give an hourly average size distribution. These data were combined with the OPC data as hourly averages of particle number or volume concentration vs. particle size. Note that particle size was measured in sample air dried by dilution, not in ambient air. Three-Wavelength Nephelometer. This nephelometer, as well as each of the other nephelometers used in this study, is optically similar to the MRI 1590 series (Meteorology Research Inc.). In addition, this research instrument has a rotating optical shutter (37 Hz) for continuous span and dark-current correction. The instrument measured the particle optical scattering extinction coefficient at three 10% bandwidth channels centered on wavelengths of 445,538, and 678 nm. Humidity Response Measurement (Humidograph). This system uses a nephelometer to measure the response of particle optical scattering to increases in RH. The change in size for an initially dry particle with increasing RH is a function of the particle's chemistry (2,5). We have previously described the use of this system in field experiments not only to determine the presence of sulfate compounds in the ambient aerosol but also to discriminate (NH4),S04from other more acid sulfate compounds (6-8). Data consist of an x-y plot of light scattering vs. RH. Temperature Response Measurement at Controlled RH (Thermidograph). In this system, particles were humidified to an RH above 80% and then rapidly heated to a peak temperature. Particles decompose or evaporate at a given peak temperature depending on their chemical composition. After heating, sample air containing the particles was rapidly returned to a dry bulb temperature such that the RH of the air was between 65% and 70%. This sample air subsequently enters a single-wavelength nephelometer (530 nm) for measurement of particle light scattering. The residence time of any one particle was about 0.1 s in the heating and cooling sections and about 0.5 s in the nephelometer. The peak temperature in the heater was slowly increased from 20 "C (no heating) to 380 "C in about 5 min. The response of this system to labo526
Envlron. Scl. Technol., Vol. 16, No. 8, 1982
.z/
Dl 0
,
,
,
,
,
,',Le,
200 300 lEPIPERRTUREL°C I
100
, 400
I 0 '
,
, , , , ,\e~ 100 ZOO 300 TEnPERRTUREI 'C I
, 400
Flgure 2. Thermidograms of test (NH4),S04 aerosol generated at the site (A) and ambient aerosol with excess NH3 added to the sample airstream (B). Light-scattering extinctlon at temperatures a-e are used to calculate the sulfate concentration (see text).
ratory-generated H2S04/(NH4)2S04aerosols has been previously described (9). Data consist of an x-y plot of light scattering vs. peak temperature. Downstream of the three-wavelength nephelometer and upstream of the humidograph and thermidograph, gaseous NH3 was periodically mixed with the sample air, raising the NH, concentration of the mixture to about 100 ppbv. The NH3 was added to the sample air to convert particulate acid sulfate compounds to (NH4)2S04.At the beginning of each hour, both the humidograph and thermidograph sampled ambient air; NH, was then added, and the humidograph and thermidograph were run again, sampling chemically neutralized aerosol. The NH, was turned off for about 40 min before another set of ambient runs were made. Sulfate Concentration Measurements. From our thermidograph system, we obtained quantitative information on the mass concentration of particulate sulfate compounds. Consider a plot of normalized particle scattering coefficient vs. peak heater temperature for a laboratory-generated (NH4)2S04aerosol with added excess NH3 (Figure 2A). The decrease in scattering at about 35 "C (from point a to b) is due to efflorescence, a decrease in particle size owing to loss of water as the particle changes from a supersaturated droplet to a dry crystal. Heating to about 175 OC (from point b to c ) does not change the aerosol scattering. The subsequent rise in scattering above 175 OC (point c to d) is due to a chemical change as (NH4)2504 particles thermally decompose to NH4HSOl particles in the heater. For RH values between 30% and 80% and equilibrium conditions, (NH4)$O4 is a dry crystal but NH4HS04is a wet droplet. As the NH4HS04particles
its associated water. Although this assumption is supported by filter sampling data discussed later, in principle any other compound evaporating in this temperature range would produce a positive interference. If we further assume that the difference in scattering between points b and e can be related to total dry fine mass concentration by the same scattering efficiency as before, we have ~ ~ 0 4 2 - [b,,(d) - b,,(e)l F=-Pfine
-
[b,,(a) - b,,(b)l
b,,(b) - b,,(e)
96
(132) (2)
a
F
E
v)
n \
n
t-
a 0 "
L,.
1600 8/ 9/80
Oi
'
I60
"\,
lCRC1.5
.zL
.75CRCl
1203
,!
81 7/83 '
200
'
3;>
460
'0'
.zt
'
IO0
7;
,
'
260
R,-UND
'
3>
433
% ,> ,-
8/ 5/80
0'
100
200
303
400
TEMPERATURE ( " C ) Figure 3. Thermldograms with and without NH:, added to the sample airstream: (A) calibration aerosois of HZSO4, NH4HSO4,and (NH4),S04 generated at the site; (B-0) range of ambient aerosol thermidograms and the corresponding assigned molar ratio, R , of (NH4+)/(S0?-); (H) period when no value could be assigned to R . The hour and date of each pair of thermidograms is listed in the lower left of each figure.
return to both ambient temperature and 65% RH in the cooler, they accrete water forming droplets. At a relative humidity of 65 % , subsequent chemical neutralization of NH4HS04droplets with added NH3 downstream of the cooler results in supersaturated (NH4)2S04solution droplets of similar size. The drop in scattering from point d to point e shows that (NH4)2S04particles have totally decomposed or evaporated by 300 "C. Figure 2B shows a similar plot for a typical ambient aerosol to which excess NH3 has been added. In order to determine the mass concentration of sulfate from this plot, we have made the following assumptions: (1) the difference in scattering between pinta d and e is due to droplets containing only (NH4)2S04and associated water; (2) the difference in scattering between points a and b is due to the water associated with the (NH4)2S04in droplets containing both (NH4)2S04and other nonsulfate compounds; (3) the difference between these two differences is due to dry (NH4),S04; (4) this latter difference can be related to dry (NH4)2S04mass concentration by using a ratio of scattering to fine particle mass of 4.2 m2/g (based on data presented later in Figure 3). Thus the mass concentration of sulfate ion is given by eq 1. The factor 96/132 converts PSOP
bg/m3) =
from (NH4)2S04to sulfate ion concentration. We emphasize that eq 1 assumes that any change in light scattering between 225 and 300 "C is due to (NH4)2S04and
where b,,(b) - bsp(e)has been corrected from 65% to 50% RH by using the average humidograph curve (with NH3 added) for the entire experiment, a correction of about 10%. This correction was made because the filters were weighed at an RH of 50%. NH4f/S042-Measurement. In addition to quantitative information on sulfate mass concentration, we performed the following analysis to obtain semiquantitative information on the extent of NH3 neutralization of the acid sulfate compounds. Our system exploits the fact that H2S04 particles evaporate at a much lower temperature than NH4HS04or (NH4)2S04particles (see Figure 3A) and that particles containing mixtures of NH4HS04and (NH4)2S04 have different deliquescence points. In an earlier paper, we discussed the response of this system to laboratory-generated aerosols with NH4+/S042-ratios ranging from 0 to 2 (9). In this study, we assigned the aerosol a chemical composition in one of seven categories based on our laboratory studies of pure H2S04/(NH4)2S04compounds (9). When the thermidogram was the same with and without NH3 addition and when the curve with added NH3 displayed the characteristics of (NH4)2S04discussed earlier (see Figure 2), we assigned the aerosol an NH4+/S02- ratio of 2. An example is shown in Figure 3B. When the scattering difference due to efflorescence (a to b in Figure 2) increased by a factor less than 2 on NH3 addition and there was a rise (c to d in Figure 2), we assigned an NH4+/S042-category ranging from 1.75 to 2.0 (see Figure 3C). When the difference due to efflorescence increased by a factor of 2 or greater, we assigned an NH4+/S042-category ranging from 1.5 to 1.75 (Figure 3D). When the ambient aerosol showed no efflorescence step and no subsequent rise in scattering at high temperature, when both characteristics developed on NH3 addition, and when the two scattering values were equal at 225 "C, we assign a ratio category ranging from 1.0 to 1.5 (See Figure 3E). This latter large uncertainty is because we operated the thermidograph nephelometer at 65% to 70% RH, above the deliquescence points for compounds with this range of NH4+/S02- (5). The aerosol was assigned an NH4+/S02- ratio less than 1.0 if without NH3 there was no efflorescence step and if the scattering value at 225 "C (point d in Figure 2) was increased by addition of NH3. We attribute the difference in scattering at 225 "Cbetween the ambient case and that with added NH3 to the scattering associated with H2S04. Using these criteria, we assigned an NH4+/S042-ratio between 0.75 and 1.0 to case F in Figure 3 and a ratio between 0.5 and 0.75 to case G. When the S042-mass fraction of the aerosol was low (F< 0.35),we were not able to assign an NH4+/S042-ratio (Figure 3H). Ambient Scattering Measurement. In addition to the nephelometers already discussed, we operated a singlewavelength nephelometer (MRI Model 1560, wavelength 530 nm) on a separate sampling stack. The purpose of this system was to measure the particle-scattering extinction coefficient at ambient conditions. The 2-m high stack and Environ. Sci. Technoi., Voi. 16, No. 8, 1982
527
the inlet line were insulated in order to keep the sample air as close to ambient conditions as possible. Flow rate through this nephelometer was increased to about 20 L/s to minimize heating of the sample by the light source. Additional heat shielding was added inside the nephelometer to minimize radiative and convective heat transfer from the light source to the inlet tube of the nephelometer. Dry bulb temperature was measured inside the nephelometer, close to the scattering volume. The absolute difference between this temperature and the ambient temperature was nominally within 0.5 OC. If this temperature difference exceeded 2 OC, the ambient scattering data were considered invalid. A heater on the sample inlet that raised the air temperature about 17 "C above ambient was turned on for 15 min every 2 h. This heating dried the sample, and the resultant dry scattering coefficient was compared with the value measured in the dilution system. Filter Sampling. We took filter samples of air that was drawn through a cyclone to remove large particles. The cyclone (Bendix Model 18)was modified for a smaller cut size at our flow conditions by reducing the cross-sectional area of the inlet (10). The 50% cut point of optical diameter as measured by the OPC was set at the site to be 1.0 pm (ag = 1.6) by regulating the pressure drop across the cyclone. The 12-h samples were taken starting at 0800 and 2000 EDT. The flow rate through the 25-nm Nuclepore (0.4-pm pore) filters was approximately 10 L/min; total sampling volume was measured with a dry-gas meter and corrected for pressure changes. Once a day, the filters were dried in a dessicator for 12-24 h and then weighed on-site with a microbalance (Cahn Model 4700). The filters were subsequently analyzed for total sulfur loading via PIXE (analysis provided by T. Cahill and associates, Crocker Nuclear Laboratory, University of California, Davis). Data Acquisition System. Two Apple microcomputers were used to record data in our system. One Apple with our hardware and software was used as a 24-channel, 12-bit data acquisition system. Two of the channels were connected to ground and to a +10.000 V DC reference. The other channels were used to record analog voltages. One-minute averages were determined consisting of about 2400 scans of each channel. The ground and +10.000 V DC were used as zero and span reference points for linear interpolation to cover a range of -1 to +11 V DC for the other channels. Six l-min averages for all 24 channels were recorded in a single block on 7-track tape. The data system also included a battery-powered clock to provide time on each data block and to trigger the once per hour thermograph and humidograph measurements. The second Apple was used to record thermograph and humidograph measurements. Optical scattering was recorded in terms of photon counts as an array in memory as a function of peak temperature for the thermograph (temperature is measured by using a type k thermocouple) and against relative humidity for the humiograph (RH is calculated by using a cooled-mirror dew point temperature and dry bulk temperature). For each system the nephelometer is first filled with filtered air to measure optical background and then with sample air to measure initial aerosol scattering. Background is substracted from all subsequent measurements, and initial scattering is recorded along with the data array of scattering vs. temperature or RH. Data are recorded on tape.
Results Table I contains a summary of our measurements in terms of seven parameters averaged over 12-h day and night periods. S042-, fine particle mass, and particle light 528
Environ. Sci. Technol., Voi. 16, No. 8, 1982
Table I. Summary of Measurementsu measured parameter
units
bSp(ambient RH) m-' bsp(- 35% RH) lo-' m-l pdDp 61 pmIb pg/m: pso 4z-(filter)c p~o,z-(thermidograph)~ [",+I /[SO4*-1" molar ratio % relative humidityf
;$z3
17.5 (9.7) 12.9 (6.5) 30.7 (13.2) 14.2 (7.2) 13.9 (7.5) 1.25 (0.5)
22.5 (13.5) 11.3 (7.0) 25.3 (13.9) 11.8 (7.7) 11.1(8.2) 1.46 (0.5
69 ( 1 5 )
89 (10)
standard deviation). Gravimetric determination of the mass concentration of particles with Inferred sulfate from PIXE measure. diameters 61 wm. ments of sulfur for particles with diameters 51 pm. Sulfate concentration from thermidograph (see text). e From thermidograph (see text). From once per hour measurement using a cooled-mirror dewpoint sensor. a
12-h mean
day and night time av 0800-2000 2000-0800
1r - - - - - - i
400 350
0
(2
250
'0
';E300 +
SHENANDORH JULY-RUGUST 1980
t
4
r =0.94
0' 0
10
20 30
40 50 60 70 80
FINE PRRTICLE MRSS
1pim3)
Figure 4. Scattergram of fine particle mass concentration and the light scattering extinction coefficient. A cyclone prefilter was used to remove particles with dmmeters larger than about 1 pm. Lght-scattering was measured at 538 nm in the diluted airstream where the RH < 35%. The measurements marked 1 and 2 correspond to anomolous mass or sulfate measurements (see text). The correlation coefficient, r , and the slope, b , with and without measurements 1 and 2 are r = 0.94, b l = 4.3 m2/g and r 2 = 0.96, b 2 = 4.2 M2/g, respectively.
scattering at 35% RH have little day/night difference. Nightly elevated RH increases ambient particle scattering during the period 2000-0800 EDT over that measured during daylight hours. The NH4+/S042-ratio has day/ night differences with the 2000800 EDT period less acid. Fine Particle Mass vs. Scattering. Figure 4 shows the measured relationship between fine-particle mass and dry particle scattering coefficient. Each scattering value is a 12-h average of 60 12-min averages. A least-squares linear regression analysis gives a slope of 4.2 m2/g (I^ = 0.96). This regression analysis excluded points 1and 2 in Figure 4 for reasons discussed later in the text. The scattering coefficient was measured at 538 nm by the three-wavelength nephelometer. Freon-12 span values of and 1.93 x lo4 m-l were used to calibrate the 1.82 x 538-nm channel and the MRI Model 1560 nephelometer, respectively (11). The relative humidity in the threewavelength nephelometer was approximately 35%. The cyclone in the filter sampling system was operated such that the filters collected 50% of the ambient mass of particles having an optical diameter of 1.0 pm and 85% of the mass of particles having a diameter of 0.6 pm. The filter mass implicit in Figure 4 was measured at approximately 50% RH.
1.5 SHENANDORH
SHENENDORH 1980
.
6
JULY-AUGUST 1980
E
a:
W x 3 -I
> 0
4
a
i
/ e
TI:
0
E z 0
*
COflPRRlSONS OF MERS. THEFVIDOORRPH IS07 I TO FILTER SRnPLES
0
ib-2
2
5
10-1
2
5
I00
2
5
101
DIRMETER(uUIII)
Figure 5. Range of measured ambient size distributions of particle volume. Particles with diameters less then 0.3 pm were sized with an electrical aerosol analyzer; particles greater than 0.3 pm and less than 10 pm were sized with a Royco (Model 220) optical particle counter. a is the corresponding scattering wavelength dependence (see text). On the basis of continuous measurements of a, the site average mean diameter of the fine particle mode was 0.33 pm (a +0.07, -0.05 pm). Also shown is the transmission efficiency of the cyclone used for removing large particles prior to collection on filters.
Particle Size Measurements. Figure 5 shows three plots of fractional particle volume per log (decadic) diameter interval. The measurements represent hourly averages made in the diluted airstream at about 35% RH. These plots were chosen to illustrate the extreme range and mean size distribution (center plot) of the accumulation mode. The average accumulation mode volume mean diameter was 0.3 pm as measured by the OPC-EAA system. The extreme range was 0.2-0.55 pm. Also measured was a related parameter, the wavelength dependence of particle scatter in terms of a parameter a defined by a = -In (b,,(445 nm)/b,,(538 nm))/ln (445/538)
(3)
a is a sensitive indicator of the mass mean diameter of the accumulation mode (12). Based on continuous measurements of a, the site average mean diameter of the accumulation mode was 0.33 pm (a = +0.07, -0.05 pm), in good agreement with the OPC-EAA measurements. Sulfate Mass Concentration. Figure 6 shows a comparison of particle S042- mass concentration determined from analysis of thermidogram data and from Nuclepore filters analyzed by P E E for S. We have assumed that all of the sulfur is as sulfate, consistent with the results of Ferman et al. (3). A 12-h filter data observation is compared to the average of 12 hourly thermidogram values. Three measurements, marked 1,2, and 3 in Figure 6, are significantly different from the remainder of our measurements. The period corresponding to point l occurred when the particle size was unusually small and the measured ratio of scattering to mass was low (3 m2/g compared to an average of 4.2 m2/g). The period corresponding to point 2 had unusually high particle mass concentration (inferred from bSp)during the first half of the period followed by low mass concentration during the last half of the period. The filter apparently clogged during the first half of the sampling period giving an unrepresentative average mass concentration and an anomalously high ratio of scattering to mass (6 m2/g). Point 3 is from a period of high variability. S042- was determined once per hour during a period of a few minutes and variability adds uncertainty in comparing thermidogram S042- to 12-h averaged filter S or mass. We have calculated the regression with and without these three points. With these points,P = 0.94 and the slope is 0.76. Removing these three
-0
10 FILTER
20 SO,
30
40
(1~gidI
Figure 6. Comparison of filter and thermidograph measurements of sulfate concentration. Each filter was sampled 12 h, and only particles with diameters less than about 1 pm were collected. Sulfate was inferred from PIXE sulfur measurements. Corresponding thermidograph measurements of sulfate concentration are averages over each filter period of 12 measurements (one per hour). The measurements marked 1,2, or 3 are anomalous (seetext). The correlation coefficient, r , with and without measurements 1, 2, and 3 is 0.94 and 0.98, respectively.
points, the slope in Figure 6 is 0.95 and the correlation increases to P = 0.98. Temporal Variation. Parts A and B of Figure 7 show the variation of SO-: concentration and S042-fraction of fine-particle mass as inferred from the thermidograph (1 point/h) and from 12-h filter samples (the horizontal bars). Figure 7C shows temporal variation of the NH4+/S042molar ratios as inferred from the thermidograph. Figure 7D shows the humidograph characterization of the aerosol as acid (H2S04or (NH4)HS04)or salt ((NH4),S04),with use of criteria and instruments described elsewhere (6, 7). Figure 7E shows the temporal variation of light-scattering extinction by ambient and dry particles. The dry scattering coefficient is that measured in the diluted, dried sample airstream and corrected for the dilution ratio. Figure 8 shows an expanded view of the same five variables of Figure 7 during a 2-day period, August 7 and 8. As evident from Figures 7 and 8, the apparent molar ratio of NH4+ to S042- has a diurnal variation. This is shown more clearly in Figure 9A, where we have plotted the average value for each hour of the day. The bars in Figure 9 include f l standard deviation about the mean. Although not evident from Figures 7 and 8, the fraction of sulfate mass to total fine particle mass also varies diurnally. This is shown in Figure 9B, where we have determined the average deviation from the daily mean sulfate mass fraction. For comparison we have also include the average diurnal variation of RH (Figure 9C). Effect of R H on Ambient Scattering. In Figure 10, we have plotted the ratio of ambient particle scattering coefficient to dry-particle scattering coefficient vs. the ambient RH (symbols). The RH values are 1-min averages taken a t the beginning of each hour. The corresponding scattering ratios are 12-min averages of data taken continuously 6 min before and 6 min after the beginning of each hour. The hatched area in Figure 10 represents f l standard deviation around the average humidogram (no NH3 added).
Discussion and Conclusions We have observed a strong correlation (r = 0.94) between dry-particle scattering coefficient and dry fine particle Environ. Sci. Technol., Vol. 16, No. 8, 1982 529
OO2H
7/25
7/29
7/27
8/2
7/31
JULY-RUGUST
8/4
1980
816
8/8
8/10
8/12
DFITE Figure 7. Time series of measured parameters: (A) thermidogram (points) and filter (bars) measurements of SO4'- concentration; (B) fraction : O S to fine particle mass concentration; (C) thermidograph determined molar ion ratio of (NH,+/SO?-) (the vertical bars represent the range of assigned to each of SIXcategories of ion ratios); (D) humidograph responses (salt responses displayed deliquescence without the addition of NH,; acid responses displayed deliquescence only after the addition of NH,); (E) light-scattering extinction measured at ambient RH and at RH
