of the air with only 40 V ac across the preheater. When operated in this manner, the two instruments reported statistically indistinguishable values of b,, a t ambient relative humidities below 60%.At higher relative humidities the ratio of the values measured by the instrument without the preheater to the values measured by the instrument with the preheater increased significantly. This reduction in light scattering by reducing the relative humidity of an aerosol has been described and measured previously by Covert, Charlson, and Ahlquist (12).
S u m m a r y and Conclusions The two models of commercially available integrating nephelometer currently in use will report different values of scattering coefficient for the same aerosol. Since the ratio between the measured values will depend on the size distribution of the aerosol, which is variable from place to place and day to day and seldom known, it is not possible to prescribe conversion factors that could be used to precisely convert the measurements by one model into values that would be measured by the other. Because of this it is essential to clearly specify either the spectral response of the instrument or the manufacturer’s model number when reporting data from an integrating nephelometer. The potential uncertainties associated with the conversion of the scattering coefficient to a visual range (in addition to the limited‘circumstances in which the simple Koschmeider equation is valid) leads us to suggest that data from an integrating nephelometer should always be measured and reported as b,, and not as bscat or as a visual range. Acknowledgment We acknowledge many helpful conversations with N. C.
Ahlquist, R. J. Charlson, and T. V. Larson during the development of this paper.
Literature Cited (1) International Association on Meteorology and Atmospheric Physics, Radiation Commission, “Terminology and Units of Radiation Quantities and Measurements”; National Center for Atmospheric Research: Boulder, CO, 1978. (2) Middleton, W. E. K. “Vision Through the Atmosphere”; University of Toronto Press: Toronto, Ontario, Canada, 1952. (3) Weiss, R. W.; Waggoner, A. P.; Charlson, R. J.; Thorsell, D. L.; Hall, J. S.; Riley, L. A. “Studies of the Optical, Physical, and Chemical Properties of Light Absorbing Aerosols,” in “Proceedings: Carbonaceous Particles in the Atmosphere” (CONF 7803101); Novakov, T., Ed.; Lawrence Berkeley Laboratory: Berkeley, CA, 1979; p 257. (4) Charlson, R. J.; Covert, D. S.; Tokiwa, Y.; Mueller, P. K. J . Colloid Interface Sci. 1972,39, 260. (5) Rabnioff, R. A.; Herman, B. M. J . Appl. Meteorol. 1973, 12, 184. (6) Harrison, A. W. Atmos. Enuiron 1979,13, 645. (7) Henry, R. C. “Psychophysics and Visibility Values”, in “Proceedings of the Workshop in Visibility Values”; Fox, D., Loomis, R. J., Green, T. C., Eds.; U S . Department of Agriculture, Forest Service, 1979; p 74. See also Land, E. M. Sci. Am. 1977,237 (6), 108. (8) Penndorf, R. J . Opt. Soc. Am. 1957,47, 176. (9) Harrison, A. W. Can. J . Phys. 1977,527, 1898. (10) Inadvertant Modification of the Stratosphere (IMOS) Task Force, “Fluorocarbons and the Environment”; U.S. Council on Environmental Quality, 1975. (11) Waggoner, A. P.; Ahlquist, N. C.; Charlson, R. J. “Recent Developments in Nephelometers”, in “Atmospheric Aerosols: Their Optical Properties and Effects” (NASA CP-2004); U.S. National Atmospheric and Space Administration: Langley Research Center, 1977; p TuA4-1. (12) CoGert, D. A.; Charlson, R. J.; Ahlquist, N. C. J . Appl..Meteorol. 1972,1I, 968.
Received for review June 30,1980. Accepted October 20,1980. This work was supported in part by U.S. Environmental Protection Agency Grant No. CR807376010.
Assessment of the Oxidant-Forming Potential of Light Saturated Hydrocarbons in the Atmosphere Hanwant B. Singh,* J. Raul Martinez, Dale G. Hendry, Raphael J. Jaffe, and Warren B. Johnson SRI International, Menlo Park, California 94025
Smog-chamber and field data were analyzed to assess the oxidant-forming potential of light saturated hydrocarbons (LSHCs), which consist of Cx-C6 alkanes, in urban, suburban, and rural atmospheres. Empirical, mechanistic, and computer-simulation approaches were used to estimate that LSHCs could produce from 25 to 125% as much oxidant as alkenes. The broad range of the estimate stems partly from uncertainty in the average daytime atmospheric abundance of the HO radical, which was estimated to vary from 0.5 X lo6 to 10 X IO6 molecules/cm3 (mean daytime HO concentration = 2.9 f 1.9 X lo6 molecules/cm3). The relative ability of LSHCs to produce oxidants is expected to be higher under “rural or transport” conditions when compared to “urban or no-transport” conditions. When present in equal carbon abundance, LSHCs are significantly less effective in oxidant formation than alkenes, an advantage partially offset by the dominant atmospheric abundance of LSHCs.
0013-936X/81/0915-0113$01.00/0
@ 1981 American Chemical Society
Introduction The control of photochemical air pollution has been defined to mean the achievement of a 120-ppbhourly ozone standard not to be exceeded more than once a year on the average. Current control strategies depend on hydrocarbon (HC) abatement as the primary means of controlling photochemical air pollution. Because HCs differ in their ability to produce oxidant‘, a strategy based on the control of those HCs that manifest themselves most strongly in smog formation would constitute a potentially superior technical approach that could also be cost-effective. Based on such thinking, the principle of “substitution” was devised, which states that the emission of more reactive HCs are controlled by substituting them with less reactive ones. In California such a substitution principle has been applied for more than a decade in the form of “Rule 66” ( 1 ) . In recent years, the recognition of the rural photochemical air pollution problem has led to additional smog-chamber Volume 15, Number 1, January 1981 113
studies. These studies appear to suggest that the longer reaction time associated with multiday transport of polluted air masses can lead the less reactive HCs to play a significant role in oxidant production downwind of the pollutant source. Accordingly, it has been suggested that existing control strategies incorrectly distinguish the more reactive organics from the less reactive ones and underestimate the photochemical potential of less reactive HCs. This has brought forth proposals to control HCs of low reactivity as stringently as the more reactive HCs (1). Light saturated hydrocarbons (LSHCs), which are defined to include C1-Cs alkanes, are a subgroup of the less reactive HCs that have been subject to lesser controls under Rule 66 type control strategies. The participation of LSHCs in oxidant formation has been exclusively determined from smogchamber studies (1-5) whose applicability in real atmospheres has been questioned (6, 7). Various field studies have been conducted in the last 5 years, but no attempt has been made to assess the ability of LSHCs to produce oxidant (8-14). The current study was undertaken to analyze existing data and to develop a unified picture regarding the oxidant-forming potential of LSHCs in urban, suburban, and rural atmospheres. Thus, we analyzed both smog-chamber and field data with the help of empirical, mechanistic, and computer-simulation approaches to assess the role of LSHCs in oxidant formation. Three major parameters were necessary to estimate the contribution of various HCs to oxidant formation: atmospheric abundance, reactivity, and ability of the fragments to oxidize NO to NO2, which in turn is photolyzed to form ozone. Major uncertainties that prevent an objective assessment of the oxidant-forming potential of LSHCs were also studied. Atmospheric Abundance of Light Hydrocarbons (LHCs) A preliminary analysis of available field data suggests that 12 LHCs in three categories are most characteristic of urban, suburban, and rural environments: (1) alkanes or LSHCs-ethane (C2H6), propane (CSHS), butanes (i- and n-C4Hlo), and pentanes (i- and n-CSH12); (2) alkenes or re-
active hydrocarbons (RHCs)-ethylene ( C Z H ~ propylene ), ( C & j ) , and butenes (1-, 2-, and i-CdH8); (3) acetylene. Methane was not included in our analysis, because its extremely low reactivity and high natural background (1500 ppb) cause it to be a hydrocarbon of little interest from a control standpoint. Hydrocarbon data measured at 16 locations in the United States were analyzed. The 16 sites are described in Table I, and Table I1 shows the average concentrations of LHCs measured a t these locations. The data provided in Table I1 represent the average of data collected over a period of 2-20 weeks and generally represent 6-9 a.m. concentration averages. Based on these data, the following observations can be made: (1) On a volumetric basis, the average abundance of alkanes is 3.9 ( f 1 . 4 ) times that of alkenes and is somewhat higher a t the remote site 14 when compared to the urban sites. When ethane is excluded, this ratio is 2.9 ( f l . O ) . On a carbon-atom basis, the ratio of alkane to alkene is -5.5 (f1.9) and 4.6 ( f 1 . 6 ) when ethane is excluded. ( 2 ) C2H6, ( n i) C4H10, and ( n i) CsH12 are each about equally abundant and together account for -80% of the alkanes. The remaining 20% is largely C3H8. (3) In the rural areas (site 14) or under clean meteorological conditions (site 9), the abundance of ethane is generally greater than that of butanes or pentanes. (4) For the butanes, n-CdH10is about twice as abundant as i-CdH10. The reverse is true for pentanes where i-CbH12 is about twice as abundant as n-C5H12. (5) On the average, the "alkene" group is made up of 70% ethylene, 20% propylene, and 10% butenes. In relatively rural or remote locations, butenes largely disappear and C2H4 can be closer to 80%of the alkenes.
+
+
Chemical Reaction of Light Hydrocarbons and the HO Radical Abundance in the Boundry Layer Alkanes are almost exclusively depleted by reaction with HO. Alkenes, on the other hand, react with 0 , 0 3 , and HO, with the HO playing a dominant role. Because our knowledge of these reactions is based largely on laboratory and smogchamber studies, we studied the depletion of alkanes and al-
Table l. Site Descriptions for Field Data site type a
study period
12' 02'
u-s
June-Sept, 1975
U
June-Sept, 1975
North latltudeWest longltude
slte no.
slte name
Long Beach, CA
1
El Monte, CA
2
33' 46'-118' 34' 04'-118'
Upland, CA
3
34' 06'-117'
38'
U
June-Sept, 1975
Canton, OH
4
5
25' 25'
u-s
Malone College, OH
40' 50'41' 40' 52'-81'
July-Aug, 1974 July-AUg, 1974
Houston, TX
6 7
29' 39'-95'
17'
S S
8 9
29' 52'-95' 29' 49'-95'
20' 39'
u-s u-s
29' 49'-95'
39'
Chickatawbut Hill, MA Danvers, MA Boston, MA
10 11 12
42' 13'-71' 42' 18'-70' 42' 22'-71'
04' 59' 5'
S
Medfield, MA Elkton, MO
13 14
42' 11'-17' 37' 40'-93'
17' 22'
S R
July-Aug, 1975 Aug-Sept, 1975
Lawrenceville, IL
15
38O 45'45'
45'
R
IL
16
38' 45'47'
45'
R-S
Houston, TX Houston, TX Houston, TX
Lawrenceville,
a
U = urban; S = suburban: R = rural.
114
Environmental Science & Technology
u-s U
July, 1976 July, 1976
ref
remarks
11 11 11 12 12 13
July, 1976
13 13
July, 1976
13
July-AUg, 1975 July-AUg, 1975 July-AUg, 1975
14, 15 14, 15
selection of the cleanest days from site 8
14, 15 14, 15 16
an open forested rural site
June. 1974
17
June, 1974
17
sample from 600 m above ground level background samples 18 km away from the refinery
Table II. Average Atmospheric Concentrations (ppb C) of Light Hydrocarbons and Their Percentage Volumetric Abundance *nee sompd
1 U-S
Z U
OU
ethane ethylene acetylene propane propylene i-butane %butane I-butene i-and2-butenea i-pentane Rpentane
56.0 26.0 22.0 12.0 10.0 36.0 83.0 3.0 7.0 86.0 49.0
114.0 60.0 72.0 101.0 30.0
51.0 40.0 43.0 48.0 14.0, 30.0 88.0 4.0 8.0 91.0 47.0
52.0 111.0 8.0 24.0 171.0 87.0
4U-S
58.0 31.0 29.0 40.0 13.0 38.0 82.0 7.0 21.0 92.0 36.0
5 5
6 5
47.0 17.0 11.0 28.0 7.0 19.0 43.0 3.0 10.0 41.0 21.0
29.0 56.0 19.0 68.0 33.0 54.0 58.0 4.0 12.0 63.0 44.0
7U-S
BU-S
29.0 22.0 11.0 58.0 13.0 35.0 62.0 4.0 12.0 59.0 32.0
44.0 21.0 33.0 119.0 36.0 67.0 194.0 8.0 22.0 186.0 160.0
0
1 0 s 11 U-S
7.0 4.0 4.0 6.0 0.0 5.0 8.0 4.0 2.0 2.0 4.0 4.0 1.0 9.0 0.0 0.0 0.0 0.0 4.0 10.0 3.0 5.0
U = urban: S = suburban: R = rwal. For Site desaiption see Table i. Since the rate constant of together lor brevity. FC2H, data not available due to instrument mailunction.
7.0
c
15 R
16R-S
% "01 abundanca
10.0 5.0 10.0 7.0 1.0 0.0 6.0 13.0 5.0 7.0 3.0 4.0 1.0 3.0 7.0 5.0 2.0 4.0 19.0 14.0 3.0 16.0 0.0 0.0 0.0 0.0
13.0 13.0 1.0 11.0 2.0 3.0 9.0 0.0 0.0 6.0 3.0
20.9 f 7.7 14.0f6.1 7.4f 5.1 14.8f4.4 4 . 2 f 1.5 6.5f 1.8 1 3 . 2 f 4.9 0.5f 0.5 1.5f1.4 1 1 . 0 f 3.0 6.0f 2.5
12U
7.0
c
13s 1 4 R
5.0
6.0 10.0 2.0 8.0 13.0 0.0 0.0 0.0 0.0 15.0 23.0 21.0 8.0 14.0 20.0
CC and 26&
5.0
c 2.0
0.0
0.0
3.0 2.0
13.0 3.0
with HO is about the same. these were lumped
and (0.6 f 0.3) X IO6 moleculeslcm? in the urban and rural environments, respectively. Oxidant-Forming Potential of LSHCs
I
: -
~
,
~
n
I 1
7
.
3
HO
5
d
1oc
6
2
"
9
1
0
cl
Figure 1. Average calculated daytime HO concentration in the mixed
layer from field data. kenes in the real atmosphere. T o reconcile observations and theory, the depletion rates of HCs were used to determine average HO radical abundances. The atmospheric determination of HO abundance was accomplished hy using data from the 1973 Los Angeles Reactive Pollutant Program (LARPP), and the Da Vinci balloon study (15,16). The LARPP was primarily concerned with urban pollution, whereas the Da Vinci study was largely concerned with the nonurhan atmosphere. The procedure utilized by Calvert was used to estimate atmospheric HO concentrations (17,181.Hydrocarbon concentrations were normalized with respect to C2H2 to correct for atmospheric dilution. Corrections were made for ozone and O(3u) reactions hv using- rate constants obtained from the literature (19,20): The estimated HO levels are disnlaved in Fieure 1. which " plots the mean HO concentrations and the standard errors of estimate from this study as well as the estimates of Calvert ( I 7) derived from a single day of LARPP data (operation 33). Figure 1 shows that HO estimates are strongly dependent on the hydrocarbon used to obtain them. Both estimated HO levels and the uncertainty associated with them increase with decreasing reactivity of the HC. The daytime average HO concentration based on all data was (2.9 f 1.9) X IO6 molecules/cm3. However, the uncertainties are large, and a range of HO concentrations between 0.5 X 106 and 10 X loe molecules/cm3 can be calculated. The least variable HO estimates, obtained from CsHs and C4H8 depletion data, are found to be (0.9 f 0.4) X 106 molecules/cm3 .
I
Since the reactivities of HCs cannot he quantitatively established from available field data, the ability of LSHCs to produce oxidants cannot he established on an absolute basis. Therefore, an alternate approach was used that involves determining the contribution of alkanes to oxidant formation relative to that of alkenes. Three techniques were used for this purpose: empirical analysis of smog-chamher data; modeling studies; and estimates based on field data. Empirical Analysis of Smog-Chamber Data. Experimental data from four smog chambers (4, 20-22) were ohtaioed and analyzed by using multivariate regression in an attempt to relate maximum 1-h 03 level to various combinations of selected explanatory variables. Initial concentrations of LSHC, reactive hydrocarbons (RHC), and nitrogen oxides (NO,) and the HC/NO, ratio were considered as explanatory variables. The HCs involved ranged from simple binary mixtures of CsHs and n-C4Hlo to surrogate atmospheric mixtures containing many HCs. In the latter case, individual HCs were divided into two mutually exclusive and exhaustive categories, one containing LSHCs and the other containing RHCs such as C3H6 and C4H8. In some data sets, the surrogate mixture contained toluene, whose reactivity lies between that of LSHCs and RHCs, but analysis revealed that its class assignment exerted negligible influence on the regression. Table I11 summarizes the characteristics of the experimental data obtained from Exxon Research and Engineering (Exxon R and E), General Motors Research Laboratory (GM), Gulf Research and Development (Gulf R and D), and Statewide Air Pollution Research Center (SAPRC) of the University of California a t Riverside. The irradiation times with constant light intensity ranged from 5 to 10 h. The shorter times correspond to the urbanhuburban scale (20-60 km) while the longer times may correspond to a transport scale of 60-120 km. Although a wide range of HC concentrations as well as HC/NO, ratios is represented, the validity of empirical analysis can only he applied within the concentration hounds defined in Table 111. Statistical analysis of data from all the smog chambers yielded the following regression equation relating maximum I-h ozone (Odmax)) to LSHC and RHC: Os(max) = 0.0628 (f0.0675)
+ 0.1450 (f0.038) RHC
+ 0.0139 (f0.0053) LSHC
(1)
Volume 15, Number 1. January 1981 115
Table 111. Smog-Chamber Data Summary source
Exxon R and E GM Gulf R and D SAPRC all SC data a
no. of runs
no. of polnfs
16 24
6a 5b
20 20
108 14a
80
35
Replicates averaged before regression.
RHC
LSHC
range, Ppm C
range, ppm C
range
Odmax) range, ppm
run
nciNox
ilme,
range
h
1.2-1.28
1.1-2.72
0.478-0.681
4.6-16
10
0.195-0.263
0-1.27 0-2.63
0.99-3.99 0-26.3 0-16.1
0.76-1.0 0-0.9 0- 1.o
9.1-12.4 4.0-42.9 2.6-84.6
6 5 6
0.045-0.40 0.15-0.65 0.073-0.745
0-26.3
0-1.0
2.6-84.6
5-10
0.045-0.745
0-3.25 0-3.25
Locus of maximum 0 3 for five compositions.
where RHC and LSHC are measured in ppm C, and O3 in ppm. The multiple correlation coefficient for this fit is 0.85, and the coefficients of RHC and LSHC are significant at the 0.001 level. However, the constant term is not statistically significant at the 0.05 level. The 95% confidence limits for the three regression coefficients of eq 1are obtained by adding the terms in parentheses to the coefficient. Despite the important role the nitrogen oxides are expected to play in oxidant formation, eq 1 shows no dependence of Odmax) on NO, or HC/NO,. When all four variables were included (LSHC, RHC, NO,, HC/NO,), the coefficients of NO, were never found to be significant a t the 0.05 level and the 11-to-1 ratio of the coefficients of RHC and LSHC was maintained. The lack of dependence of Os(max) on NO, is probably due to the fact that the matrix of our data is not truly orthogonal. A weak correlation was found to exist between the variables RHC and NO, (R = 0.54),and HC/NO, and LSHC ( R = 0.57). Thus, the empirical analysis of smog-chamber data suggests that RHCs are roughly 10 times more effective than the LSHCs in oxidant formation, when present in equal carbon abundance and subjected to relatively short irradiations (5-10 h). The regression relationship developed here accounts for -72% of the variance in Os(max). The corresponding uncertainty in the results is indicated by the fact that the 1 0 1 mean ratio of the coefficients of RHC and LSHC can vary from 6:l to 21:l when the 95% confidence limits on the regression coefficients are considered. This variation in part reflects the variation of reactivity of both RHC and LSHC used in individual chamber runs. Caution must be exercised in extrapolating smog-chamber results to atmospheric conditions. However, where the carbon abundance of LSHCs is 5.5 (f1.7) times that of alkenes, an alkane oxidant-forming ability of 36-75% of alkenes can be inferred. When the 95%confidence limits of the coefficients in eq. 1 are considered, a range of 18-125% can be calculated. Modeling Studies. Over the last few years SRI has developed a detailed 175-step chemical model of a n-C4H10 t C3H6/NOX photochemical system and validated it with SAPRC smog-chamber data (23).This model was used to perform simulations to estimate the effect of various combinations of n-C4H10 and C3H6 on ozone formation. To approximate atmospheric conditions, we dropped simulated wall sources from the model, and a variable sunlight intensity was used to simulate the sunlight cycle at the summer solstice. On the basis of LARPP data, 0.20 ppm of nC4HIO and 0.08 ppm of C3H6 were used to represent typical HC loading in Los Angeles. Because of great uncertainties in the photochemical mechanisms involving aromatics, the aromatic hydrocarbons have been represented by C3H6 on an equivalent reactivity basis. Initial concentrations of 2.0 and 0.24 ppm were used for CO and NO,, respectively, and an NO/N02 ratio of 2.0 to reflect the typical early morning conditions were used. All calculations were carried out by using the following initial concentrations in addition to HC and NO,: 0.010 ppm HCHO, 0.010 ppm CHsCHO, 0.010 ppm 116
FRACT
Environmental Science & Technology
BO
FRACT denotes the ratio of LSHC/(LSHC
t RHC).
,
,c
260
P
51
u
s
x
40
40
38
k
8
25 20
0 50 0
38
0 50 75 0 50 50 0 75 12 50 58 17 PERCENT HYDROCARBON REDUCTION
75 75 75
n-BUTANE PROPENE TOTAL CARBON
Modeled results showing variations in ozone dose due to changes in initial propene and +butane abundances (propenelsbutane = 1:2.5 in ppm). Figure 2.
H202, 0.010 ppm peroxyacetyl nitrate (PAN), 0.01 ppm HN02,0.005 ppm 03,and 20 000 ppm HzO (56%relative humidity). To determine the effect of n-C4Hlo and C3H6 on O3 formation under approximate atmospheric conditions, we performed a series of calculations using the diurnal sunlight intensity for June 22, the summer solstice, with NO2 photolysis data from Peterson ( 2 4 ) .An O3 loss rate of 1.0 X min-l and a dilution rate of 1.2 X 10-3 min-1 were assumed. The latter corresponds to a doubling of the mixing volume in 10 h. First, simulations were run a t standard initial concentrations defined above. Then, simulations were run in which both n-CdHlo and C3H6 were reduced independently to determine the effect on the ozone dose, while keeping all other initial concentrations constant. The ozone dose was calculated over a period of 10 h. Average HO levels during simulations were 3 X 106-5 X lo6 molecules/cm3, which are within the uncertainty range of inferred HO values in the atmosphere. These data indicate that reduction of either n-C4Hlo or will reduce O3 formation. If one starts with 0.08 ppm C3H6 and 0.20 ppm n-C4Hlo or one-half of both of these HCs, the same percent reduction in n-C4Hlo or leads to the same reduction in ozone dose, as shown in Figure 2. Figure 2 applies only to the case where the initial ratio of C3H6 and n-CdHlo is 1 to 2.5 (in ppm) and the concentrations of hydrocarbon and NO, are consistent with urban values. Changing the hydrocarbon ratio in favor of one HC will make the results more dependent on that HC. The first bar indicates the reduction observed by reducing the initial n-C4HlO from 0.2 to 0.1ppm, the second indicates the effect of reducing C3Hs from 0.08 to 0.04 ppm, and the third shows the reduction in O3 dose by reducing both n-CdHlo and C3H6 by 50%.Similarly, the last set of three bars shows the effect of reducing the hy-
I ame iv. Maximum uzone r u Hydrocarbons Subset---' ’NO Oxldlreda
compd
lnltlai b
final
C2H6
2
6
C2H4 CzHz
2 3
4 4
C3H8
3 2 3 3
8 10 11
C3H6 bC4H10 fiC4HlO 1-C4H8
7
2
10
2-C4H8
2
bC4H8
2
bC5H12
3
fiC5H1.2
3
10 9 13 14
a Number of NO molecules oxidized per molecule of hydrocarbondepleted due to HO attack. Short-term irradiationprior to reactionof aldehydemolecules (“urban or no-transport” conditions). Long-term irradiations, including reaction of aldehyde molecules (“rural or transport” conditions).
drocarbons by 75%. Thus, while the same reduction in O3 is observed with the same percent change in n-C4Hlo or C3H6, the reductions in total initial carbon are quite different because of the higher concentration of n-C4Hlo. Thus, 50%reduction of n-CrHlo represents a 38% reduction in total carbon while 50%reduction in C3H6 represents a 12% reduction in the total carbon. Analysis of simulation results indicates that at the end of the 10-h simulation run eqqal moles of n-C4Hlo and C3H6 are reacted and about equal contribution is made to O3 formation. Because of complex 0 3 destruction processes this cannot be interpreted to mean that once reacted both n-C4Hlo and C3Hs are equally efficient in O3 formation. This will become more clear in the next section. For the purpose of applying the simulation results to urban atmospheric conditions, we shall make the assumption of equal efficiency. This assumption is likely to result in underestimation of the oxidant-forming potential of LSHCs. The relative rate of depletion of n-C4H10
by n-C4Hlo) would be 25-50% (0.12 X abundance) that of the alkenes. Estimates Based on Field Data. Field data from experiments that sought ozone formation in plumes containing an overwhelming abundance of LSHCs was studied. Such data could directly relate LSHCs to ozone production in the atmosphere. In one instance, involving a refinery plume, 0 3 production was not observed (14) possibly because of insufficient reaction time. The data needed to explain these results in detail were not available. Therefore, these preliminary observations could not be used to absolve LSHCs of oxidant-forming ability. This led us to consider indirect means of assessing the role of LSHCs in oxidant formation. To estimate the contribution of various HCs to O3 formation, one must consider the relative concentration, the HO reactivity, and the ability of the fragments to oxidize NO to N02, which in turn is photolyzed to form 03.Although the first two factors are directly measurable, the third factor is more complicated. The maximum ozone-forming ability of various HCs can be estimated from the anticipated mechanism of reaction. If one uses the reaction mechanism for n-C4Hlo developed by Hendry et al. (23),the following overall reactions would occur as a result of HO attack: C4H10
HO + 3 N 0 --+ 2CH3CHO + 3N02
However, the acetaldehyde reacts further, and the following net reaction results: HO
C4H10 + 11NO --+2C02
+ 2CO + llNO2
Thus, considering only the HO attack on n-C4Hlo yields an initial net oxidation of three molecules of NO, which becomes a net oxidation of 11 NO when the additional reactions of products of the initial HO attack on hydrocarbon are included. Table IV lists for several hydrocarbons the net number of NO molecules oxidized first upon initial attack and then assuming that all the products except CO have reqcted by HO at’tack. From Table IV the ability of two hydrocarbons to contribute to O3 formation once they are attacked by HO may be
Table V. Estimated Oxidant Formation Potential of Light Hydrocarbons
compd
C2H6 C2H4 CzHz C3H8 C3H0 bC4H10 *C4HlO l-C4H8
band 2-C4H8 bC5H12 *C5H12
[alkane (LSHCs) O3potential] / [alkene O3 potential]
percentage contribution of 0 3 a based on actual average HC abundance based on equal carbon abundance “urban or “rural or “urban or “rural or no-transport” transport“ no-transpoit” transport” conditions conditions condltlops condltlons
0.9 f 0.4
f 11.4 0.3 f 0.2
22.2
7.3 f 2.8
f 7.6 f 0.8 9.0 f 3.4 3.5 f 3.0 12.9 f 11.9 8.2 f 2.2
23.8 3.1
8.8 f 2.9 0.60 f 0.20
f 0.4
0.2
0.2
13.5 f 8.8
9.6
4.6
0.1
0.2
0.1
5.7
2.1
1.3
23.9
14.8
0.8
f 0.1 f 2.7 24.3 f 9.4 3.0 f 0.8 9.4 f 3.8 4.6 f 4.0 17.0 f 15.3 10.1 f 2.6 11.5 f 3.4 0.70 f 0.25
1.5
1.2
2.2
1.9
23.6 31.1
25.7 37.0
1.9 3.7
2.0 4.2
a Due to HO, 0 3 , and q 3 P ) attack. The corrections assume average HO, 0 3 , and O(3P) concentrationsof IO6 molecules/cm3,50 ppb, and 2.5 X IO5 molecules/cm3, respectively.
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117
compared. While initial attack for each alkane (except ethane) and alkene leads to oxidation of 3 and 2 mol of NO, respectively, the overall contribution of each alkane and alkene to NO oxidation more closely reflects the total carbon number once the contribution of products is considered. For example, initial attack on n-CdHlo leads to 50%more O3 than attack on C2H4, but once the initially formed aldehydes have reacted, the difference is increased to 175% more. Recent measurements by Niki et al. (25) confirm the yield of two molecules of NO oxidized for each molecule of alkene reacted, in agreement with the yield derived by us from mechanistic considerations. The oxidant-forming potential of LHC species is proportional to [C,][k,][HO][a], where [C,] represents the volumetric concentration of a chemical species and includes all isomers. The rate constant of individual chemical species with HO is represented by k,. The quantity [a] describes the number of NO molecules oxidized per molecule of hydrocarbon depleted (Table IV) under “urban or no-transport’’ and “rural or transport” conditions, respectively. The estimated relative oxidant-forming potential of LHCs is shown in Table V. Corrections were made for the alkene loss due to O3 and 0 attack based on our best estimates for HO, 0, and O3 concentrations. These are defined in Table V. It is further assumed that, subsequent to O3 and 0 attack, the oxidantforming potential of free radicals is similar to that due to HO attack. Table V presents the estimated O3 formation potential of individual LHCs under conditions of measured atmospheric abundance and equal carbon abundance. In addition, the estimated ratio of the oxidant-forming potential of LSHCs and alkenes is provided in Table V. I t is clear from Table V that the average oxidant-forming potential of LSHCs is -60 and 70% that of alkenes under “urban or no-transport” and “rural or transport” conditions, respectively. Considering the range of variabilities defined in Table V, the oxidant-forming potential of LSHCs can be determined to be between 40 and 100%of that of alkenes under urban, suburban, and rural conditions. It is also evident that the oxidant-forming potential of alkanes relative to alkenes increases by -15% when the same air masses are subject to long-term (multiday) irradiations. C2H6 and C2H2 do not make an important contribution to oxidant formation. Butanes and pentanes alone account for -80% of the oxidant formed by LSHCs. Under conditions of equal carbon abundance, alkenes are shown to be significantly more effective in oxidant formation than alkanes. The oxidant-forming potential of alkanes when compared to alkenes is dependent on the HO level. Our analysis is conducted with the best determined level from field data of lo6 molecules/cm3. However, the greater the HO levels the greater the 0 3 potential of alkanes relative to alkenes. For HO levels of 0.5 X lo6 molecules/cm3,the contribution of alkanes ranges from 25 to 125%of that due to alkenes. Abundance of NO, was also studied whenever data were available. For both urban and suburban locations, the NO, abundance was adequate to sustain photochemistry. Simulations with C2H6 and C ~ Hmixtures ~O suggested a significant O3 production from a wide variety of HC/NO, ratios that were found to exist in nearly all urban and suburban locations we studied. In very remote locations, however, NO, may be deficient and may limit the oxidant formation chemistry of all HCs (26). Results and Conclusions Available field data are both insufficient and inadequate to establish a precise quantitative estimate of the direct involvement of LSHCs in oxidant formation. The reactivities of all HCs are subject to a large uncertainty. The HO freeradical concentration necessary to initiate the involvement 118
Environmental Science & Technology
Table VI. Estimated Ozone Formation Potential of LSHCs Relative to Alkenes
iechnlaue used
smog chamber kinetic model field data and simplified chemistrya
esilmates of percent 0 3 iormatlon poientlal of LSHCs relatlve io alkenes urban rural envlronmeni environment
36-75 25-50 35-70 (25-100)
45-100b (35-125)
a Represents maximum potential. * Assumes HO level of IO6 molecules/cm3. Estimates including the uncertainty in the HO abundance 0.5 X 106-10 X IO6 molecuies/cm3.
of LSHCs in oxidant formation was estimated to be between 0.5 X lo6 and 10 X lo6 molecules/cm3. The lower the reactivity of the HC used to calculate HO, the higher the calculated HO abundance and the uncertainty associated with it. Any attempt to devise reactivity scales based on the reaction with HO must consider this uncertainty. Analysis of field data indicates that the atmospheric abundance of alkanes is significantly higher than that of alkenes. Our best knowledge of smog chemistry suggests that, as long as LSHCs react in the atmosphere, they would participate in smog chemistry in a way similar to other more reactive HCs. On an equal weight basis, alkenes are significantly more effective in oxidant formation than LSHCs. However, the reduced photochemical involvement of LSHCs is partially offset by their dominant atmospheric abundance. Taking into consideration the atmospheric abundance of alkanes and alkenes and the uncertainties of HO levels (0.5 X 106-10 X lo6 molecules/cm3), we estimate that the contribution of LSHCs to oxidant formation, in urban, suburban, and rural atmospheres, ranges from 25 to 125%of that of alkenes. Estimates of the oxidant-forming potential of LSHCs relative to alkenes were summarized in Table VI. The relative contribution of LSHCs to oxidant formation is higher under rural conditions when compared to urban conditions. Among the various LSHCs present in the atmosphere, ethane makes an insignificant contribution to photochemical smog while pentanes and butanes together account for 80%of the oxidants formed by LSHCs. Acknowledgment The suggestions of the API-LSHC task force are much appreciated. Literature Cited (1) Dimitriades, B. Proc.: Hydrocarbon Control Feasibility: Its Impact Air Qual., Spec. Conf., 1977 1977,111-21. (2) Altshuller, A. P. et al. J. Air. Pollut. Control Assoc. 1969, 19, 787-90. (3) Gay, B. W., Jr.; Bufalini, J. J., Enuiron. Sci. Technol. 1971,5, 422. (4) Farhi, J. T.; Powers, T. R.; Wigg, E. E. Proc. Hydrocarbon Control Feasibility: Its Impact Air Qual., Spec. Conf., 1977 1977, 12232. (5) Farhi, J. T.;Powers, T. R., presented a t the Air Pollution Control Association Meeting, Houston, TX, Paper No. 78-10.5, 1978. (6) Bufalini, J. J.; Walter, T. A.; Bufalini, M. M., Enuiron. Sci. Technol. 1977,11, 1181-5. (7) Chang, T. Y.; Weinstock, B. In “Proceedings, International Conference on Photochemical Oxidant Pollution and Its Control”; Raleigh, N.C., 1977; pp 451-65. (8) Mayrsohn, H.; Kuramoto, M.; Crabtree, J . H.; Solterne, R. D.; Mauo, S. H. “Atmospheric Hydrocarbon Concentrations” JuneSept, 1975, CARB, Jan 1976. (9) Westberg, H. H. “Measurement of Light Hydrocarbons in the Field and Studies of TransDort of Oxidant Bevond an Urban Area”; 1974 Ohio Study; EPA Contract 68-02-1232; WSU, 1975. (10) Westberg, H. H.; Allwine, K. J.; Robinson, E. “Measurement of
Light Hydrocarbons and Studies of Oxidant Transport Beyond Urban Areas”; 1976 Houston Study; EPA Contract 68-02-2298, WSU, 1975. (11) Ruff, R. E.; Gasiorek, L. S.; Shigeishi, H. Master Data File from the Summer 1975 Northeast Oxidant Transport Study; EPA 901/9-76-004; SRI International, Menlo Park, CA, 1977. (12) Lonneman, W., EPA, North Carolina, private communication, 1977. (13) Rasmussen, R. A.; Chatfield, R. B.; Holdren, M. W. “Hydrocarbon Species in Rural Missouri Air”; WSU, 1977. (14) Westberg, H. H.; Allwine, K. J.; Robinson, E. “Measurement of Light Hydrocarbons and Studies of Oxidant Transport Beyond Urban Areas”; 1974 Lawrencville Study; Contract 68-02-1232, WSU, 1975. (15) Parker, R. 0.;Martinez, J. R.; Los Angeles Reactive Pollutant Program (LARPP);Data Archiving and Retrieval, Doc. P-1464-W; ERT Inc., Santa Barbara, CA, 1975. (16) Zak, B., personal communication; unpublished Da Vinci balloon data, Sandia Laboratories, NM, 1977. (17) Calvert, J. G. Enuiron. Sci. Technol. 1976,10, 256-62. (18) Singh, H. B.; Martinez, J. R.; Hendry, D. G.; Jaffe, R. J.;Johnson, W. B., “Estimates of the Oxidant-Forming Potential of Light
Saturated Hydrocarbons”; Menlo Park, CA, July 1979,SRI Project 6395, Contract API-LSHC-1977. (19) Anderson, L. G . Reu. Geophys. Space Phys. 1976.14, 151-71. (20) Pitts, J. N. et al. “Mechanisms of Photochemical Reactions in Urban Air”; Chamber Studies, EPA-600/3-77-0146; University of California: Riverside, CA, Feb 1977; Vol 11. (21) Heuss, J. M. “Smog Chamber Simulation of the Los Angeles Atmosphere”; General Motors Research Labs, GMR-1802, Feb 1975. (22) Spindt, R. S., Gulf Oil Co., Pittsburgh, PA, personal communication, July 26,1977. (23) Hendry, D. G.; Baldwin, A. C.; Barker, J. R.; Golden, D. M. “Computer Modeling of Simulated Photochemical Smog”; EPA 600/378-059; SRI International, CA, July 1979. (24) Peterson, J. T. Atmos. Enuiron. 1977,11, 689-95. (25) Niki, H. et al. J . Phys. Chem. 1978,82, 135-7. (26) Singh, H. B.; Ludwig, F. L.; Johnson, W. B. Atmos. Enuiron. 1978,12, 2185-96.
Received for review July 13,1979. Accepted September 29,1980. The financial support of the American Petroleum Institute ( M I )is much appreciated.
NOTES
Calculation of Overall Particulate Penetration from Control Equipment Clayton P. Kerr Tennessee Technological University, Cookeville, Tennessee 3850 1
A way of using Gauss-Hermite numerical integration to calculate the overall mass penetration of particulate control devices is presented. The log normal size distribution is used. The method is compatable with a variety of ways for determination of penetration as a function of size. The method is illustrated with an example. Sound design of devices for control of particulate emissions requires that the overall mass penetration be estimated. The overall mass penetration is the fraction of total incoming particulate material to the control device that is not collected. To estimate the overall mass penetration, the particulate size distribution function and the penetration as a function of particle size must be known. This note presents a computationally simple method for determining the overall mass penetration. The particulate size distribution function can be expressed by using the log-normal distribution function ( I ) . n ( u )=
e-(u-im2u,2
( 2ir ) 1’2 uu u is the natural logarithm of the particle diameter; n ( u )is the number of particles between sizes u and u du; N is the total number of particles; C T ~is the natural logarithm of the ratio d,/d,; d, is the diameter for which the cumulative distribution has a value of 0.841; d, is the diameter for which the cumulative distribution has a value of 0.5; E is the natural logarithm of d,. Both d, and d, are generally determined experimentally. The size penetration, p , is a function of the type of control device and its operating and design parameters. When one applies the definition of the overall penetration, P , the following expression can be written:
+
0013-936X/81/0915-0119$01.00/0
@ 1981 American Chemical Society
If we define u as (3)
eq 2 can be written as
(4)
In this form, the integration can be easily done numerically by using the Gauss-Hermite method ( 2 ) .
1:
c U 2 f ( u du )
=
2 w;f(u;)
i=O
The w,and u, are respectively the weight factors and the roots, and f is a function defined as f = pe3u21/2cTu (6) For most design applications, a five-point formula has been found to be suitable. For this case, the roots and weighting factors are listed in Table I. If more points are desired, the roots and weighted factors can be found elsewhere ( 3 ) .The
Table 1. Roots and Weighting Factors for Five-Point Gauss-Hermite Integration V/
Wi
f2.02018 f0.95857
0.01995 0.39361 0.94530
0
Volume 15,Number 1, January 1981
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