Environ. Sci. Technol. 1982, 16, 514-525
Wallops Flight Center, Wallops Island, VA, 1979. Dumbauld, R. K.; Rafferty, J. E.; Bjorklund, J. R. "Prediction of Spray Behavior Above and Within a Forest Canopy", Special Report, Methods Application Group, Forest Insect and Disease Management, Forest Service, USDA, Davis, CA, December 1977. Warren, L. E. World Agric. Aviation 1976, 3, 23-28. Akesson, N. B.; Yates, W. E. Report to Forest Service on Contract No. P.S.W.-3,21-39S, Douglas-fir Tussock Moth Control Project, 1976-1977, Berkeley, CA. Yates, W. E.; Akesson, N. B.; Cowden, R. E. "Atmospheric Transport of Sprays from Helicopter Applications in Mountainous Terrain", Paper No. 78-1504, American Society of Agricultural Engineers, Chicago, 1978.
(9) Seber, M. W. "Linear Regression Analysis"; Wiley: New York, 1977. (10) Feldmann, R. J.; Maibach, H. I. Toxicol. Appl. Pharmacol. 1974,28, 126-132. (11) Campbell, K. I. Clinical Toricol. 1976, 9, 849-921. (12) Guyton, A. C. "Medical Physiology"; Saunders: Philadelphia, 1979. (13) South Carolina Epidemiologic Studies Center, Medical University of South Carolina. "Measurement of Exposure to the Carbamate Carbaryl", Maine Carbaryl Study, 1979; interim report, 1979.
Received for review September 15, 1981. Revised manuscript received February 15, 1982. Accepted April 9, 1982.
Visibility and Aerosol Composition in Houston, Texas Thomas 0. Drubay," Robert K. Stevens, and Charles W. Lewis
Environmental Sciences Research Laboratory, US. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1 Don H. Hern
Systems Applications, Inc., San Rafael, California 94903 William J. Courtney, John W. Tesch, and Mark A. Mason
Northrop Services, Inc., Research Triangle Park, North Carolina 27709 Relationships between light extinction coefficients, visual range, and aerosol mass and composition were studied in Houston, TX. Light extinction coefficients, measured with a telephotometer and black-box targets, agreed accurately with sums of light scattering and absorption coefficients. The light-scattering coefficient due to particles in heated air was highly correlated ( R = 0.987) with the mass concentration of fine particles ( b,,(3.5 " C ) > b,,(12.5 "C) (13) where the numbers in parentheses refer to the difference between the integrating nephelometer air temperature and ambient air temperature. The left-hand inequality indicates that b, constitutes only a portion of beXt,which is consistent w t h eq 1. The right-hand inequality indicates a volatile component in the aerosol. Differences between be=$,b, (3.5 "C) and b,,(12.5 OC) are most pronounced at 0800 8 D T when the average RH was 82%. When air having this value of RH is heated in the nephelometer to 3.5 and 12.5 "C above ambient, the RH is reduced to 67% and 40%, respectively. Thus, previous reports that the scattering coefficient rises steeply as RH rises above 60-70% (43, 44) lead us to infer that the temperature dependence indicated in eq 13 is actually a humidity dependence. Accuracy of Telephotometer Results. Figure 2a is a plot of 8-day averages of hourly NW and NE measurements using the Gamma Scientific telephotometer. Differences between values of bextfor the NE and NW targeta 518
a integrating nephelometerb
0.46[N02] b,c light transmissiond b, + bsp(3.5 "C) + t a g + bap telephotometer
coefficient, Mm" 13 178 -i. 1 6 11 * 2 30 (+O,-15) 232 ( + 18, - 22) 232
*
23
From Penndorf (19). Average of 10-min averages coincident with telephotometer measurements between 0758 and 1815 CDT. The coefficient 0.46 has units of Mm" ppb-' and was deduced from data by Dixon ( 4 5 ) . Average for daytime aerosol samples.
,>---o.,
c
Y
method
Envlron. Sci. Technol., Vol. 16, No. 8, 1982
range from 0 to 20% and are greatest during early morning and late evening when the sky radiance was measured close to the sun for the sunward target. Thus, for the early and late hours the recorded value of Bh may have been too large, causing the resulting extinction coefficient to be too small. In the remainder of this paper each reported value of best is an average of results for both target directions using the Gamma Scientific telephotometer. We estimate the overall accuracy of each average to be &lo%. Figure 2b shows plots of C(O)(l - f ) , which represents the combined effects of a target's intrinsic radiance and instrument flare. For the Gamma Scientific telephotometer, C(O)(l- f ) ranges from -0,998 to -0.978, which compares favorably with -1.000-the ideal value obtainable only with an instrument having zero flare cf = 0) and with black targets (C(0) = -1). The greatest deviations from ideal performance occur early and late in the day when the sun shines directly onto a target so as to illuminate its baffles and scatter light onto the back of the target. For the Gamma Scientific telephotometer we estimate that C(0) = -0.99 f 0.01 and f = 0.01 f 0.01. For the MRI Model 3010 telephotometer, we deduce a larger flare value, f = 0.055 f 0.025. If we had attempted incorrectly to determine extinction coefficients using eq 2 without considering flare, the bext results deduced using the Gamma Scientific and MRI telephotometers would have had positive biases of 34 and 190 Mm-l, respectively, for a 300-m target distance, equivalent to 15% and 82% errors on average (Table 11). Relationships among Optical Coefficients. To test the applicability of eq 1, we plot extinction coefficient vs. sum of scattering and absorption coefficients in Figure 3. Data on be, represent directionally averaged hourly telephotometer observations. The abscissa is based upon corresponding 10-min averages of b, and bag and 12-h averages of bap. The factor 0.46 Mm-P ppb-l was used to convert NOz concentrations to units of bag for 530-nm wavelength light (45). Data in Figure 3 are partitioned by time of day to indicate the relative importance of sun orientation and relative humidity. The morning data show the greatest amount of variability. Six cases in early morning when the relative humidity was high (RH I80%) are plotted as open circles. Three of those cases represent the largest deviations, which we attribute to differences in RH between ambient air and air warmed by 3.5 "C in the nephelometer. Reduction in RH in the nephelometer causes a reduction in b,,, which can be substantial at the highest values of RH. The afternoon data were measured over the same approximate range of solar angles as the morning data, but the afternoon relative humidities were much lower (see Figure l),and the amount of scatter is
I
o
1
A
I
y = -14 (51) + 1.10 (0.18) x
R = 0.984
E
=. K W
30
400
1
0
10
30
20
40
50
60
baP (NUCLEPORE),Mm-'
7
E
tB
10
1250-1814 HOURS y = 2 (22) t 0.92 (0.09) x
-
y = O.O(O.3) + 1.04(0.05)x VIX = 1.04(0.03)
R = 0.997 8-
-
6-
-
i
0
2
4
6
8
1
0
1
2
S (NUCLEPORE),fig m 3 0
200
bsgt bSp(3.5'C) t bag +
400
600
hap, Mm.'
Figure 3. Extinction coefficient vs. sum of scattering and absorption coefficients measured hourly during morning (A) and afternoon (B) in Houston between September 11 and 19, 1980. The open circles pertain to RH 2 80%. The lines represent y = x and are drawn to serve as guides to the eye.
much lower. Thus, we conclude that R H had more effect on the integrating nephelometer results than solar angle effects had on the telephotometer results. (Humidity effects due to heating could have been reduced if we had replaced the air blower provided by the nephelometer manufacturer with one that maintained a higher flow rate.) Data taken between 1048 and 1453 CDT had the least amount of scatter; for that period the correlation coefficient for extinction coefficient vs sum of scattering and absorption coefficients is 0.97. From a linear regression analysis the intercept was -9 (21) Mm-l and the slope was 0.96 (0.08) where the 95% confidence intervals (in parentheses) bracket 0 and 1, respectively. The sky was cloudy or partly cloudy for 37% of the 78 measurements represented in Figure 3. Although the nonuniform illumination associated with clouds can add error to telephotometer measurements of bext (8,17), adverse effects due to clouds were not noticed. Our use of averages for two directions (NW and NE) may have reduced the magnitude of such errors. Average values of optical coefficients are compared in Table 11. The average extinction coefficient measured by telephotometer agrees well with the average sum of scattering and absorption coefficients. Scattering by particles contributed 77% to the extinction coefficient when the air in the integrating nephelometer was warmed by 3.5 "C. Scattering by air, absorption by NOz, and absorption by particles contributed 5.6%,4.7%,and 6-13%, respectively.
Flgure 4. Comparisons between (A) paired measurementsof b, and (B) paired measurements of sulfur collected in the fine fraction on Teflon and Nudepore filters. The data represent 16 12-h sampiing periods in Houston between September 11 and 19, 1980. The numbers in parentheses represent 95 % confidence intervals.
The data on absorption coefficient in Table I1 and Figure 3 pertain to fine particles collected on Teflon filters. Figure 4a shows a comparison of b, values measured for fine particles collected on Teflon and Nuclepore filters in two Beckman dichotomous samplers operated side by side. The results are highly correlated (R = 0.984),and the ratio of means is very close to unity. The data include five cases (plotted as open circles) in which two Nuclepore filters were needed in a sampling period because of clogging. Figure 4b compares sulfur concentrations measured by XRF on the same set of paired filters. The paired sulfur results are also highly correlated (R = 0.997),and the mean values agree within 4%. Such good agreement implies that any bias between Teflon and Nuclepore filter results is very small. In contrast, Sadler et al. recently found a factor of 2.9 bias in b, measured for aerosol collected on either quartz or Milripore filters compared to aerosol collected on Nuclepore filters (46). The present results suggest a need for confirming the finding of Sadler et al. Despite the excellent agreement between our b, measurements on Teflon and Nuclepore filters, we indicate a large asymmetrical uncertainty band for b,, in Table 11. This is needed to accommodate recent results on the comparison of light transmission and photoacoustical measurements of soot in particles collected on Teflon filters (39,47,48). Results obtained by light transmission were higher by a factor of about 2 due to optical effects by nonabsorbing particles. Visual Range and Light Extinction. In Figure 5 we plot bed-1 vs. visual range to test the reciprocal relationship predicted in eq 4. The visual range data were reported by Environ. Sci. Technoi., Vol. 16, No. 8, 1982 519
--
0
I I
I
I
Y = - O . 6 ( 1 . 2 ) +0.67(0.12)x
X/V= 1.6410.14) R = 0.79
I
1
I
I
.
30
t
I
I
0 0
R = 0.987
y/X= 3.5210.241 m2g.l
I
I
I
I
1
40
20
1
I
60
J 80
mf. iis m.3
Flgure 6. Scattering coefficient measured by a heated integrating nephelometer (12.5 O C above ambient temperature) vs. mass concentration of particles smaller than 2.5-pm diameter (see also Figure 4 caption).
values at 550 nm; the exact value depends upon the aerosol size distribution and the ratio of b, to b, (24). Comparison is further complicated by the absence of descriptions of calibration procedures in ref 4 , 5 , and 49 and the fact that the F-12 light-scattering coefficient used prior to 1981was about 15% too high (24). Particulate Mass and Visual Range. Previous investigators have found precise proportionality between fine-particle mass concentration and light-scattering coefficient (9-12). The same is true for our data in Figure 6. There is a high correlation ( R = 0.987), and the intercept of a linear regression fit is not significantly different from zero. The average ratio of means is b,,(12.5 "C)/mf = 3.52 (0.24) f 0.35 m2 g-l
(16)
where the number in parentheses represents the 95% confidence interval of the ratio's precision, and the uncertainty (k)represents the combined accuracies of the instrument calibrations for the &gauge and integrating nephelometer. The light-scattering coefficient to mass ratio shown in eq 16 is comparable to the value 3.0 m2 g-l derived from theoretical curves by Lewis ( 1 1 ) . I t is also close to a value 3.13 f 0.2 m2 g-l deduced from measurements by Waggoner and Weiss at six locations in the Western United States (9). However, their value should be reduced to 2.78 m2 g-l due to a temperature correction that was inadvertently omitted ( 11 , 24). From the above relationships we can derive an approximate relationship between visual range and fine-particle mass concentration. By taking the product of the following ratios mf/b,(12.5 OC) = 0.284 g rn-'
(174
b,,(12.5 OC)/bs,(3.5 "C) = 0.79
(1%)
b,,(3.5 OC)/beXt = 0.77
(17~)
beXtV= 1.64
(17d)
we obtain the following: mfV = 0.28 f 0.04 g m-2
(18)
Equations 17a, c, and d are results presented above, and eq 17b represents the ratio of our daytime (0700-1900 CDT) averages. 520
Environ. Scl. Technol., Vol. 18, No. 8, 1982
1 mc 20
I
500
I
400 -
-OPC --- EAA
c
I
I
I
I
I
I
1 1 . 1
I
Y
L
6
(1
E, 0'
D
s . b
10 I---
I
j j )
!
i h*
4
m
B
+
a . a
I
J *;*
0 0,005
0.01
0.02
0.05
0.1
0.2
0.5
1.0
2.0
5.0
10.0
D. pm
Figure 7. Aerosol size distribution measured 1130 CDT, September 15, 1980 (histogram) and specific scattering cross sections (circles).
We also computed the ratio of the mean mfl to the mean daytime visual range for Hobby Airport in Houston and obtained Vmf = 0.34 (0.09) g m-2
(19)
where the number in parentheses is the 95% confidence interval. The linear correlation coefficient for V vs. mf' was 0.90. Equations 18 and 19 are based upon visual ranges less than 30 km and are not appropriate in particle-free air. A result similar to that in eq 19 was obtained by using visual range data from Houston Intercontinental Airport. Recently Lodge et al. derived the relationship (13) Vmf = 1.25 g m-2 (20) by assuming that eq 5 is valid and that b,, measured in heated air equals be* The Coefficient in eq 20 exceeds ours in eq 18 or 19 by more than a factor of 3 because eq 5 does not accurately describe the airport visibility in Houston and because b,, measured in heated air only accounts for 61% of the extinction coefficient in Houston. Particle Size Results. Data acquired from the EAA/OPC system between September 13 and 19 were transformed to volumetric particle size spectra by using the simple steps given previously (27,28). Figure 7 shows one of the 140 composite spectra thus obtained. There is some discontinuity between the EAA and OPC spectra at their point of overlap. The average ratio of the OPC ordinant for the 0.56-pm size to the EAA ordinate for the 0.42-pm size was 2.0. The fact that the averaging times for acquisition of the OPC and EAA spectra were so different (60 and 2 min, respectively) undoubtedly had some effect on the difference in results from the two instruments. However, the agreement was not noticeably better during long periods of stable pollution levels, so the results seem to be more related to genuine differences in the instruments' response. In any case, no attempt was made to normalize the two types of spectra to one another. In general, the size distribution of the Houston aerosol was found to be bimodal, with the geometrical parameters of the modes being similar to those found at many other sites in the U.S. (50). Also shown in Figure 7 superimposed on the experimental size distribution is the theoretical specific scattering cross section 71. (i.e., cross section per particle volume u ) for the geometrical mean of each measured particle size bin. The cross sections were calculated from Mie theory for a 530-nm wavelength and for a complex index of refraction 1.5-0.02i. An approximation to the scattering
0
100
200
300
400
500
600
bSpi3.5"C), Mm.' MEASURED
Figure 8. Light-scattering coefflcient calculated from aerosol size distributions vs. light-scattering coefficient measured by using an unheated nephelometer. The numbers in parentheses represent 95% confidence intervals.
coefficient corresponding to each measured particle size distribution duld log D can be calculated from (bsp)caicd
= Cai[du/d 1% D~IoPc[Alog D~IOPC +
Cai[dv/d log D~IEAA[A log D i l (21) ~ where the sums are over all particle size bins shown in Figure 7. Using eq 21, we computed individual b,, values from all 140 EAA/OPC measurements made between September 13 and 19,1980. In Figure 8 these results are compared with b,, measurements taken at the same time as each EAA measurement. This type of matching seemed to be the most reasonable since the EAA portion of the whole particle size distribution generally contributed more to the calculated scattering coefficient than the OPC portion. The ratio of the means of the calculated and measured quantities is seen to be 0.84. Alternately, the straight line represents an unweighted least-squares fit to the data and has a slope of 0.71 (compared to an ideal slope of unity) and a nearly zero intercept. Our results can be compared with those of a similar experiment reported by Ensor et al. (51). A straight line fit to their resulta presented in the same manner as Figure 8 gave a smaller slope of 0.43, and a larger intercept of 50 Mm-l. Aerosol Composition. The average aerosol composition that we measured in Houston is presented in Table I11 and is similar to that measured earlier in Houston (52) and other locations (21,39,53).Sulfate is the major component of the fine fraction, and crustal matter is the major component of the coarse fraction. Most of the chemical species have larger average concentrations during the day than at night. Exceptions are nitrate in the fine fraction and chlorine m d bromine in both fractions. These species are labile, and their evaporation is enhanced by the presence of hydrogen ion, which turns out to be most abundant during daytime. Nitrate is more abundant in the coarse fraction, where alkaline species can serve as sinks for gaseous forms of nitrate. The carbon data in Table I11 represent total carbon. Volatile carbon accounted for 80% of the total carbon in the fine fraction. The data in Table I11 are based on samples collected in three dichotomous samplers. Figure 9 shows that XRF analysis of sulfur collected in the fine fraction on one set of Teflon filters agrees extremely well with ion chromatographic analysis of sulfate on a second set of filters. The Environ. Sci. Technoi., Vol. 16, No. 8, 1982
521
Table 111. Average Mass,Elemental, and Ionic Concentrations for Aerosol Samples Collected at the University of Houston in Houston, TX, from September 11 to 19, 1980a*b fine fraction, ng rn+
day 42500 67 7600 250 4300 300 95 200 16700 19 120 150
mass H’ C NO; NH,’ Na’ A1 Si S0,Z-
c1
K Ca Ti Cr Mn Fe Ni cu Zn Br Sr
