Acid generation in the troposphere by gas-phase ... - ACS Publications

Acid generation in the troposphere by gas-phase chemistry Computer simulations are used to evaluate the reactions

Jack G. Calvert William R. Stockwell National Center for Atmospheric Research Boulder, Colo. 80307

Understanding the nature and importance of the various chemical pathways to acid generation within the troposphere is one of the several prerequisites to the development of scientifically sound strategy for the control of acid rain. Other equally important and also incompletely understood components of this phenomenon add to its great complexity. These include: • the geographical distribution and composition of the S 0 2 , N O x , hydrocarbon, NH3 and other acid neutralizing compounds, and other emissions—both natural and anthropogenic; • an accurate description of the transport processes that distribute the emissions and their reaction products; • the mechanisms and rates of dry deposition of both initial impurities and the acids and other products formed in the chemical processes that occur in the troposphere; and • the role of clouds in mixing polluted air masses, capturing impurity gases, effecting rapid chemical production of acids, and delivering acid rain to the receptor sites. Scientifically sound regional acid deposition models must incorporate all of these important interrelated aspects 428A

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of the problem in order to account for the chemical nature and the concentrations of the acidic compounds deposited and the location at which these depositions occur. The National Center for Atmospheric Research and several other scientific institutions throughout the world have begun to develop complex transport-transformation models. In recent years, the scientific community has made reasonable progress in elucidating the chemistry as well as the other components of this problem, although many areas of uncertainty remain. In this article, we consider only a few of the many important aspects of the acid rain problem: identifying the important chemical reactions that form acids in the troposphere, the related reactions that generate and remove the reactive precursors required for these reactions, and the theoretical estimation of the rates at which acid generation is expected to occur in these reactions. We focus our attention on the role of gas-phase chemistry in acid development in the troposphere, but we attempt to clarify the many interrelationships that exist between the chemistries of the liquid phase and the gas phase. It has long been recognized that the major acids responsible for acid rain, sulfuric acid and nitric acid, are formed in the troposphere through the oxidation of sulfur dioxide and the oxides of nitrogen (NO, NO2), respectively, through a variety of chemical reaction pathways. In recent years, the seemingly dominant role of the solution-phase reactions in acid rain

development has been increasingly accepted. Indeed, the present authors also share this view. One must remember, however, that the rates of acid generation in the troposphere through gas-phase chemistry can be significant, and many of the reactants that drive the S 0 2 (HSOi") oxidation in the solution phase of cloud water are generated through gas-phase chemistry. It is clear today that a quantitative description of acid rain formation must include the interrelated chemistry that occurs in the gas phase, the liquid phase, and possibly on the surfaces of certain liquid and solid aerosols. In the present study, we have incorporated the relevant current rate constants and reaction mechanisms into a computer model of the complex chemistry of acid generation in the troposphere. Through the years, we and many others have used computer modeling of complex chemical systems as a guide to further experimental work in atmospheric chemistry. We are primarily experimental chemists and have no illusions about the qualitative nature of these results at this stage of our understanding of such a highly complex system as acid rain generation in the troposphere. We believe, however, that continually synthesizing and updating present knowledge in this area through computer modeling is one important link in the chain of ideas that connects experimental laboratory research, field observations, and theory related to the tropospheric acid problem. It is our belief that using a suitable series of

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1983 A m e r i c a n C h e m i c a l Society

Environ. Sci. Technol., Vol. 17, No. 9, 1983

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elementary reactions in such a mod­ eling effort is valuable for two reasons: It nearly duplicates observed atmo­ spheric chemistry, and it helps in rec­ ognizing previously unexpected and seemingly important results that have not yet been observed or tested exper­ imentally. Of course, in the long run, measur­ ing events accurately in the real at­ mosphere will bear most directly on our understanding of the problem. Theoretical predictions beyond ex­ periment can provide a useful guide to laboratory and field scientists, but such predictions can be accepted only when they are verified experimentally. We believe that developing a scientifically sound transport and transformation model of acid rain deposition, desired for reliable control strategy develop­ ment, requires a periodic reexamina­ tion of all the relevant science. This is necessary before we can arrive at a simple, yet reasonably accurate, de­ scription of each element of the science. It was in this spirit that we initiated the present work. We hope that it will provide some advance in the understanding of the very complex atmospheric processes that result in acid generation in the troposphere. Simulating gas-phase chemistry In the present study, we use a seemingly realistic chemical reaction scheme that our research group pub­ lished recently (/, 2). The reaction sequence involves 130 chemical reac­ tions that show in some detail the in­ organic and organic chemistry of the

polluted troposphere. All aspects of the chosen mechanism are based upon current knowledge of the chemistry of sunlight-irradiated, dilute mixtures of hydrocarbons, aldehydes, oxides of nitrogen and sulfur, and other common impurities of the polluted troposphere. It was necessary to simplify the hy­ drocarbon reaction sequences in order to avoid excessive numbers of reactions and species and unreasonable calcu­ lation times for simulations of events occurring over several days. Only three different hydrocarbon species were included: CH 4 , a "typical" alkene, and a "typical" alkane. The reaction rate constants for HO and O3 reactions with the alkene and alkane are aver­ ages (weighted by concentration and reactivity) that are derived from typi­ cal alkane, alkene, and aromatic hy­ drocarbon composition data observed in the early morning urban tropo­ sphere. The subsequent reaction pathways of the alkene and alkane are made to mimic those for propylene and propane, respectively. Thus C H 2 0 , CH3CHO, CH3COCH3, and C H 3 C O C H O are the only carbonyl products that appear. In our simplified reaction scheme, C H 3 C H O and CH3COCH3 represent all of the higher aldehydes and ketones, re­ spectively, that are formed in the complex mixtures of the real atmo­ sphere. The summer is the most productive season for acid deposition in the northeastern U.S., so we have re­ stricted our present simulations to summer conditions. The rates of the 16 different photochemical reactions in­

volved in the mechanism were varied to match reasonably well the diurnal variation of the sunlight near sea level for 40 °N latitude in midsummer (temperature, 25 °C; pressure, 1 atm; surface albedo, 0.20). The rate-con­ stant function for N 0 2 photolysis ( N 0 2 + hi/ — 0 ( 3 P ) + NO) matches well the desired rate vs. time variation. That for O3 photolysis (O3 + hv —O ( ' D ) + O2) underestimates the de­ sired rate early and late in the day so that the present results probably un­ derestimate somewhat the concentra­ tion of the OH-radical and the rates of SO2 and N 0 2 oxidation. On the whole, the treatment used to estimate the photolysis rates should be adequate to provide meaningful although qualita­ tive results. The absolute rate constants for a few of the reactions employed here have not been determined (e.g., the reactions of the Criegee intermediates, C H 2 0 0 and CH3CHOO). In these cases, estimates were made using ap­ proximate thermochemical kinetic methods together with experimental relative rate constant measurements. In this simplified model, only homo­ geneous gas-phase chemical processes are considered. For the intended pur­ poses of the present study, we did not include the subsequent neutralization reactions of the initial acidic products, such as HONO2, H2SO4, and H C 0 2 H , by NH3 and other basic compounds. The results show the maximum po­ tential for acid generation from these gas-phase reactions. The ground re­ moval of rcactants and products, and

TABLE 1

Initial concentrations and emission rates for pollutants used in the two-day simulations of tropospheric chemistry Concentration H O S 0 2 ( + M ) (1) HOS02 - - — H2S04 (2) HO + N 0 2 ( + M) ->HON02(+M)

(3)

Time (h) a

The initial pollutant concentrations are outlined in Table 1. Relative humidity for case A is varied from 100% (A-100 curve), 50% (A-50 curve), to 10% (A-10 curve); other cases are at 5% relative humidity. The concentrations of PAN in cases D, E, G, and H are too small to register on the ordinate scale. See also data of Table 3.

FIGURE 4

Theoretical time dependence of [H2O2] in the various two-day polluted air scenarios A - H a

-Ο

ô CM

Time (h) a

The initial concentrations are outlined in Table 1 (50% relative humidity). At 100% relative humidity, the H2O2 concentrations are greater than those shown here by a factor of - 2 (see text).

The detailed mechanism of the subsequent reactions of the H O S 0 2 transient radical product of Reaction 1, represented above by the overall, simplified Reaction 2, is uncertain. It is clear, however, that the H O S 0 2 species ultimately leads to the gener­ ation of sulfuric acid aerosol. Recent studies from our laboratory demon­ strate that the concentration of the OH radical in an irradiated dilute mixture of H O N O , CO, S 0 2 , and N O x in air at 700 torr, as monitored by the rate of C 0 2 generation [HO -I- C O ( + M , 0 2 ) ~* H 0 2 + C 0 2 ( + M ) ] , is insensitive to the concentration of S 0 2 in the mix­ ture (0-172 ppm) (18). This is the case even though up to one-half of the re­ acting H O radicals are removed by reaction with S 0 2 in the formation of H 2 S 0 4 aerosol. We have interpreted these results to indicate that the termination of H O H 0 2 radical chains by S 0 2 is un­ important. In our opinion, the oftenused, simplified overall reaction that results in the S 0 2 termination of chains (HO + S 0 2 - * H 2 S 0 4 ) cannot be correct and should be replaced in atmospheric models. Our results are consistent with the stoichiometry of Reaction 4 following Reaction 1: HOS02 + 0 2 — H02 + S03

(4)

S03 + H20 — H2S04 We have noted previously, however, that thermochemical estimates of Benson (20) suggest that Reaction 4 may be endothermic by about 6 kcal mole - 1 , and hence it may be slow at tropospheric temperatures ( / / ) . The hydration of the H O S 0 2 intermediate 432A

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product may be important as Friend et al. suggest (14); if so, then Reactions 5 and 6 may be a better choice to de­ scribe the mechanism: HOS02 + H20 — H0S02-0H2 (5) HOS02-OH2 + 0 2 — H2SO4 + H 0 2 (6) In theory, Δ Η 6 = - 1 8 . 7 - Δ Η 5 kcal m o l e - 1 ; obviously if Δ Η 5 > — 18.7, as is likely, then Reaction 6 is exothermic. Thus Reactions 5 and 6 are an alter­ native mechanism choice to Reaction 4. The H O - S 0 2 reaction used in this work was written simply as Reaction 7: HO + S 0 2 (+02,H20) — H 2 S 0 4 + H 0 2 (7) Among"the many reactive transients formed in the troposphere, there are a large number of potentially good oxi­ dants, yet it is surprising to find how few of these react measurably with S 0 2 in the gas phase even though the potential reactions are highly exo­ thermic. For example, the S 0 2 reac­ tions in the gas phase with O3, H 2 0 2 , CH3C002N02, N03, N205, H02, 0 2 (>Δ Ε ), and 0 2 ( 1 Σ £ ) , are all insig­ nificant in the cloud-free troposphere (11, 17). However, in addition to Re­ action 1, S 0 2 can be oxidized by other gas-phase pathways. In the tropo­ sphere S 0 2 can be oxidized by 0( 3 P)-atoms (11), Criegee interme­ diates (CH2O2, CH3CHO2, RCHO2, etc.) formed in the a l k e n e - 0 3 reac­ tions (3-5,15,16), and presumably by CH3O2 radicals (21). 0(3P) + S 0 2 (+M) — S 0 3 (+M) (8) R C H 0 2 + S 0 2 — R C H O + SO3 (9) S03 + H20 — H2S04 C H 3 0 2 + S 0 2 — CH3O2SO2

(10) (11)

CH302S02 + 0 2 (NO,H20,R02) — H 2 S 0 4 (etc.) (12) Reaction 11 appears to be revers­ ible, and the effective rate constant for S 0 2 oxidation by C H 3 0 2 radicals is in theory a complex function of the epoxy radical and N O * concentrations, temperature, and other parameters. The qualitative rate data related to Reactions 11 and 12 suggest that they are not an important source of H 2 S 0 4 under most tropospheric conditions. Therefore, we have omitted the CH3O2-SO2 reaction in this study. In theory, nitric acid can arise in several gas-phase reactions in addition to the very important Reaction 3:

HO + N 0 2 (+M) — HONO2 ( + M )

(3)

Two other favored pathways involve the reactions of NO3 and N2O5 inter­ mediate species, which are generated from NO2-O3 reactions: N 0 2 + O3 — N 0 3 + 0 2

(13)

N 0 3 + N 0 2 (+M) ^ N 2 0 5 (+M) (14) Morris and Niki have shown that the NO3 species can react with acetaldehyde, and presumably with the other aldehydes by analogy (22). In experiments in our laboratory, we have recently demonstrated the somewhat obvious point that these reactions in­ volve the abstraction of an H-atom from the aldehyde: N 0 3 + CH3CHO ^ H O N 0 2 + CH3CO

(15)

N 0 3 + C H 2 0 -* H O N 0 2 + HCO (16) In the present work, we have taken k| 6 equal to the value of k] 5 measured by Morris and Niki (22). In a further study, Morris and Niki measured the rate constant for the N 2 0 5 - H 2 0 reaction (k, 7 = 1.3 X 1 0 - 2 0 cm 3 molec - 1 s _ 1 ) , which can in theory also lead to nitric acid (23): N 2 0 5 + H 2 0 -* 2 H 0 N 0 2

(17)

Not all atmospheric chemists agree that Reaction 17 is a homogeneous, elementary gas-phase reaction. This is not surprising in view of the complexity of the atom shuffling that presumably occurs in the passage from reactants to products. The observation, however, that the experimental rate constant for Reaction 17 is independent of both the nature of the wall of the reaction vessel and the total gas pressure supports the opinion that it may be a homogeneous gas-phase reaction. Atkinson et al. report k ] 7 < 3 X 10~ 21 cm 3 m o l e c - ' s - 1 , and they attribute even this somewhat smaller upper limit to a heterogeneous reaction (24). In view of the uncertainty in the true value of this rate constant, we carried out two sets of simulations in this work to test the theoretical influence of k 17 on the H O N 0 2 generation rates. We have taken ki 7 to be either at its pos­ sible upper limit suggested by Atkin­ son et al. or at its lower limit of zero. Our present computer simulations of the tropospheric chemistry include Reactions 1-17 together with the di­ verse series of reactions of NO*, hy­ drocarbons, aldehydes, and other compounds that are characteristic of

smog chemistry. These reactions con­ trol the levels of the important HO, H 0 2 , and R 0 2 transient species that eventually convert N O to N 0 2 and generate 0 3 , C H 3 C 0 0 2 N 0 2 , and other species. In the following sections of this work, we analyze these results to evaluate anew the current theoreti­ cal significance of the various path­ ways to H 2 S 0 4 , H O N 0 2 , and other acids in the chemistry of the tropo­ sphere. Rates of SO2 gas-phase oxidation to H2SO4 in simulated, polluted tropo­ spheric air masses. The theoretical rates of S 0 2 gas-phase oxidation (%/h) as a function of time of the day are shown in Figure 5 for the various polluted air scenarios defined in Table 1. In each case, the relative humidity is fixed at 50%. Note that in every case the maximum rates occur near the noontime solar maximum. They vary from a maximum value of about 5%/h for the relatively clean air of case G to less than 0.07%/h for the NO-rich, hydrocarbon-poor atmospheric mix­ ture of case E. The rate for a highly polluted at­ mosphere with a typical NO^-hydrocarbon composition, case A, 1.5%/h maximum, rises as the air mass is di­ luted by a factor of 10 in case F, 3.4%/h maximum, and a factor of 100 in case G, 5.4%/h maximum. Further dilution of the initial impurity present in case G by another factor of 10, case H, results in a somewhat decreased rate of S 0 2 oxidation, 2.6%/h maxi­ mum. As we might anticipate, the rate of S 0 2 oxidation drops precipitously during the nighttime hours to values less than 0.1 %/h as the oxidizing agents (largely H O radicals generated photochemically) decrease in concen­ tration. The rates do not go to zero, however, because the thermally con­ trolled nighttime chemistry of peroxynitric acid, NO3, N 2 0 5 , and PAN generates small but not insignificant quantities of the H O radical. The theoretical percentages of the total rate of gaseous SO2 oxidation (at the time of the maximum rate) that occur by the various reaction pathways can be compared in Table 2. As ex­ pected from many previous studies, we see that the H O radical is the major oxidant for most of the cases consid­ ered. Only in those experiments with the highest pollution levels and the highest hydrocarbon-to-NO* molar ratios arc the Criegee intermediates important as oxidants for S 0 2 (cases A - C ) . Even in these cases, the uncer­ tainties in the rate constants for the reactions of the Criegee intermediates make their quantitative role in S 0 2 Environ. Sci. Technol., Vol. 17, No. 9, 1983

433A

FIGURE 5

Theoretical rates of SO2 oxidation to H2SO4 vs. time for the various two-day polluted air scenarios A-H"

£^ g •g

"x ο CM

Ο

CO

tr

Time (h) a

The initial compositions are outlined in Table 1 and the relative humidity is 50%.

FIGURE 6

Theoretical rates of HO radical generation from the various reaction pathways for the highly polluted air of case A (Table 1) at 50% relative humidity

ο χ

c

Έ

Ε α.

c ο œ Ε ο Χ Ο "ο

Key Reaction sequence initiated by (a) CH2O photolysis (b) O3 photolysis with 0( 1 D) formation (c) HONO photolysis (d) CHaCHO photolysis (e) NO3-CH2O reaction (f) H2Û2 photolysis

Β 15 ce

Time (h)

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Environ. Sci. Technol., V o l . 17, No. 9, 1 9 8 3

oxidation speculative at this time. The commonly encountered lower pollutant levels in the nonurban troposphere are more closely represented by cases F-H, and here only the H O - S 0 2 reaction appears to be significant in theory. Cases D and Ε have atmo­ spheres very rich in ΝΟ Λ , typical of a stack gas plume in a fairly early stage of its dilution; here the 0 ( 3 P ) reaction shows its highest percentage contri­ bution to the S 0 2 oxidation rate. For these conditions, however, little total SO2 oxidation is anticipated theoreti­ cally (, respec­ tively, for 100% relative humidity. As

TABLE 2

Theoretical fraction of the gaseous S0 2 oxidation that occurs by different reaction pathways (at time of maximum rate) Relative humidity

c

Total rate (%/h)

0.4 0.7 0.4

45.9 17.8 8.4

1.94 1.47 1.62

79.1 56.0 94.9

0.04 0.02 2.0

21.3 44.0 2.9

1.35 0.35 0.18

94.2 99.8 99.97

4.9 0.01 0.006

0.7 0.15 0.024

0.07 3.37 5.45

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

5.43 6.12 2.59 3.56

Case

2 N 0 2 , and the photochemical destruction of N 0 3 in Reactions 18 and 19. During the evening hours, however, the rates shown in Figure 13, although not insignificant, are much lower than those of Figure 8 in which the N 2 0 5 H2O reaction is included at its maxi­ mum rate. The only significant night­ time sources of H O N 0 2 contained in the simulation mechanism for this condition are the N03-aldehyde Re­ actions 15 and 16. Thus, even in the absence of Reaction 17, theory suggests that relatively large rates of H O N 0 2 generation can occur during the nighttime hours. Cases A, F, G, and Η show maximum nighttime rates of about 8.2%, 3.3%, 1.5%, and 2.2%/h - 1 , respectively. There are no unambiguous labora­ tory and field experiments that could help judge which of the two choices for the value of the rate constant of the N2O5-H2O reaction is more nearly correct. The limited but seemingly reliable data available recently from the nighttime measurements of H O N 0 2 in Claremont, Calif., show that for the heavily polluted urban at­ mosphere corresponding roughly to our case A, some significant source of H O N 0 2 remains during the nighttime hours {28, 29). It must be recognized that the deposition rate of H O N 0 2 at the ground is important (30), and in the present work we have neglected this loss mechanism. The ambient concentrations of H O N 0 2 and nitrate aerosol depend, of course, upon the rates of removal at the ground as well as the rates of genera­ tion within the troposphere. Decreased at night are the strong thermal gradi­ ents, which drive the vertical air mo­ tions during the daytime, increase the contact of the air pollutants with the ground, and so enhance their removal rates. Thus, for a fixed generation rate for HONO2 and height of the polluted layer in the troposphere, ambient levels Environ. Sci. Technol., Vol. 17, No. 9, 1983

437A

FIGURE 11

Theoretical rates of HONO2 generation from the various reaction pathways for the moderately polluted air mass, case F (Table 1) at 50% relative humidity

in Ο

χ g

I

Ε CL

cz ο

m Φ

c a> en CM

Ο

-ζ.

ο χ

"ο ω œ EC

Time (h)

FIGURE 12

Theoretical rates of HONO2 generation from the various reaction pathways for the mildly polluted air mass, case G (Table 1) at 50% relative humidity

CD

Ο

ι—

χ

Τ Έ ε α. ο ω 9 Ο) (Ν

Ο ζ

ο χ

"S φ CC

Time (h)

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Environ. Sci. Technol., Vol. 17, No. 9, 1983

of H O N 0 2 should rise at night. If Reaction 17 were important as as­ sumed in the simulations made in de­ riving Figure 8, we would expect a significant increase in [HONO2] to occur at night. Apparently this is not what is observed in the real tropo­ sphere. Obviously, a proper account of the important air mass motions as well as the chemistry is required to judge properly the ambient levels of H O N 0 2 and nitrate aerosols expected theoret­ ically. We have not attempted to in­ clude these features in the present work. See Russell et al. for a recent example of the development of a model incorporating dry deposition, trans­ port, as well as the chemistry of HONO2 and aerosol nitrate formation in a highly polluted urban troposphere (31). It seems probable to us that the near equality of the sum of H O N 0 2 and aerosol nitrate concentrations mea­ sured by Grosjean during daylight and nighttime hours, coupled with the somewhat lower nighttime ground loss rates anticipated theoretically, favor the second option adopted in our sim­ ulations, namely, that k| 7 is near zero. It appears to us that HONO2 (and nitrate aerosol) generation at night from the NOvaldehyde reactions alone can account qualitatively for the ambient levels of H O N 0 2 and nitrate aerosol observed at night. Grosjean has attributed the nighttime formation to the occurrence of Reactions 13 and 14 followed by the heterogeneous gener­ ation of H O N 0 2 (or nitrate) at night in aqueous aerosols. (See also the dis­ cussions of Peterson and Seinfeld [32], Middleton and Kiang [33], Heikesand Thompson [34], Chameides and Davis [35], and Russell et al. [31].) We feel that the gas-phase N0 3 -aldehyde re­ actions provide an attractive alterna­ tive to the Grosjean suggestion. A proper test of the relevance of the present alternative hypotheses for HONO2 generation mechanisms is not possible with the existing data since many important factors are currently ill-defined. These include the concen­ tration-time record for NH 3 , the temperature-time profile for the air mass (critical for the evaluation of the equilibrium, H O N 0 2 + N H 3 ^ NH4NO3 [aerosol]), the rates of ground deposition of H O N 0 2 , NH4NO3, and their precursors for day and nighttime periods. The Lagrangian model employed by Russell et al. (31) to simulate the 1974 El Monte, Calif., data gave reasonably good fits for HONO2, nitrate aerosol, and N1H3. Based on an expansion of

the chemical mechanism and transport model of Falls and Seinfeld (36), Falls et al. (37), and McRae et al. (38-40), the model of Russell et al. did not in­ clude the N0 3 -aldehyde reactions but used the N 2 0 5 - H 2 0 reaction rate constant near the high maximum value given by Morris and Niki. It is not clear whether our alternative choice of mechanism would generate data that more closely resemble the field data. We have recently obtained experi­ mental evidence that is relevant to the HONO2 formation mechanism and the nighttime chemistry of these sys­ tems. In preliminary laboratory ex­ periments in cooperation with Chris­ topher A. Cantrell and Donald H. Stedman of the University of Michi­ gan, we prepared dilute mixtures of O3 (about 800 ppb), N 0 2 (about 50 ppb), and CH2O (about 100 ppb) in air at atmospheric pressure. The chemical changes to the gaseous mixture in the dark were followed using an O3 mon­ itor and the H 0 2 - R 0 2 "chemical amplifier" of Cantrell and Stedman (41). As the reactions between N 0 2 and O3 occurred in the dark to form NO3 and N 2 Os, the generation of radicals, presumably H 0 2 , was clearly evident, and the magnitude of the sig­ nals was in qualitative accord with those expected theoretically in this system using the appropriate reaction sequence including Reactions 13, 14, 16, and subsequent reactions. Thus, the incorporation of Reactions 15 and 16 into the mechanism of H O N 0 2 generation in the troposphere has both a theoretical and an experimental basis. We are continuing further work on this system in order to better eval­ uate what appears to be its strong po­ tential for importance in the nighttime chemistry of the troposphere. In other recent work we have simu­ lated the [NO3], [ N 0 2 ] , and [O3]time profiles observed during the nighttime hours in the polluted Los Angeles troposphere (2). Reasonably good matches of the field data were obtained in computer simulations in which the N0 3 -aldehyde Reactions 15 and 16 were included as well as the two alternative mechanism choices made here: ki 7 is either at zero or at its maximum value. These data alone could not prove or disprove the occur­ rence of Reaction 17. In one sense they favored its occurrence indirectly, since the theoretical maximum in the [ N 0 3 ] - t i m e profile was larger than that measured in the field unless Re­ action 17 was included. In other sim­ ulations in which k 17 was set equal to zero, however, the shape of the [N0 3 ]-time profile seemed to match

F I G U R E 13

Theoretical rates of conversion of NO2 to HONO2 for the various polluted air scenarios A-H"

CM

Ο Ζ Ε

g

ο œ

αϊ c ω en OJ

Ο Ο χ

"S ω Ή

CC

Time (h) a

The initial compositions are outlined in Table 1 and the relative humidity is 50%. In these simulations the rate constant ki? for the reaction of N2O5 with H2O was taken as zero. Contrast this figure with the results in Figure 8 for which ki7 was taken at its maximum possible value.

the field data best. Conceivably other presently unrecognized sinks for NO3 may exist so that this evidence does not provide a clear answer to the problem of the correct magnitude of k 17. In summary, we conclude tenta­ tively that the gas-phase generation of H O N 0 2 in the polluted troposphere may occur at rates very much higher than those estimated for S 0 2 . These transformations occur not only during daylight hours but during the night as well. The theoretical rates of gas-phase conversion of N 0 2 to H O N 0 2 are strongly dependent on the magnitude of the rate constant assumed for the N 2 0 5 - H 2 0 Reaction 17. If we choose the maximum value of k|7 as suggested by Atkinson et al., then the theoretical rates of H O N 0 2 generation from N 0 2 for the various scenarios A - H (50% relative humidi­ ty), averaged over a 24-h, cloud-free period in summer, are as follows: case A, 23%; B, 36%; C, 8%; D, 0.3%; E, 0.1%; F, 19%; G, 16%; and H, 8%/h" 1 . For the less polluted air masses, these rates are expected to increase with a rise in relative humidity; thus at 100% relative humidity for case F, 29%; G, 19%; and H, 11%/rr 1 are the 24-h averaged rates in theory. For the al­ ternative choice of k|7 = 0, the theo­ retical rates of H O N 0 2 formation from N 0 2 are somewhat lower. Av­ eraged over a 24-h period as before for cases A, F, G, and H, the rates are

3.5%, 6.7%, 10.5%, and 5.5%/h-', re­ spectively, at 50% relative humidity; the rates are 4.7%, 9.9%, 12.2%, and 7.1%/h - 1 , respectively, at 100% rela­ tive humidity. The theoretical influence of the gas-phase tropospheric chemistry on the liquid-phase processes leading to acid rain. The very rapid conversion of S 0 2 to H 2 S04 that has been observed to occur in tropospheric cloud water (equivalent to gas-phase S 0 2 conver­ sion rates of hundreds of a percent per hour; see References 42-44) and the demonstrated great effectiveness of H 2 0 2 and O3 as reactants for HSOJ in aqueous solutions in laboratory ex­ periments (see References 45-50), strongly suggest that in the presence of clouds the importance of solutionphase reactions leading to acid devel­ opment in the troposphere is very great. The gas-phase chemistry of the polluted atmosphere is expected to be a major source of oxidants required for these solution-phase oxidations of S(IV) in the cloud-containing tropo­ sphere. For example, note the theoretical [O3] vs. time profiles shown in Figure 2 for the various polluted air masses A - H of Table 1. In all but the N O v rich mixtures, we expect significant O3 generation to occur through homoge­ neous gas-phase reactions. In the highly polluted atmosphere of case A, an [O3] m a x of about 280 ppb develops E n v i r o n . S c i . T e c h n o l . , V o l . 17, No. 9, 1983

439A

during the first day, while in case B, 250 ppb forms. For the successively less polluted air masses of cases F and G, somewhat less O3 builds up: 110 and 50 ppb, respectively. Excluding the NO. x -rich mixtures D and E, common only to slowly reacting stack gas plumes, note that the highest O3 gen­ eration results from the air masses with the highest levels of pollutants. Of course, this amount depends on the hydrocarbon-ΝO v ratio in the mix­ ture as well. Although the solubility of O3 in cloud water is low and the tropospheric O3 levels are relatively low, the theoretical equivalent rates of gas-phase S 0 2 conversion in cloud water with pH values greater than 5 are expected to be greater than 100%/h-'. However, the HSOJ-O3 reaction in cloud water at pH values of less than 4 becomes an unimportant acid generating pathway (45, 47) be­ cause SO2 decreases in solubility and the inverse dependence of the rate constant for the O3-HSO3" reaction on [ H + ] both slow the rate of the reac­ tion. Recent research has shown clearly that hydrogen peroxide should be the dominant oxidant for H S O j in com­ monly occurring acidic cloud water with pH values of less than 4.5. Ho­ mogeneous gas-phase reactions in the troposphere generate this oxidant as well. The H2O2 arises from reactions involving the hydroperoxy and the hydrated hydroperoxy radicals: 2 H 0 2 ^ H,02 + 0 7

(20)

H02 + H20 ^ H20-H02

(21)

Η02·Η20 + H02 — H202 + 0 2 + H20

(22)

The hydrate Η 2 0·ΗΟ"2 represents only a few percent of the total HO2 concentration in the troposphere even at high relative humidities ( / / ) . However, the inequality of the rate

constants, k22 » k2o, ensures that its role in H2O2 formation is significant. The theoretical [H 2 02]-time profiles for the various polluted air scenarios A - H (at 50% relative humidity) are plotted in Figure 4. Note that for the highly polluted air mass A, little H2O2 appears until the evening hours. In this case, the relatively rapid removal of H 0 2 radicals by NO ( H 0 2 + NO — H O 4- NO2) restricts the rapid oc­ currence of Reactions 20 and 22 until the [NO] is lowered by conversion to NO2 and other products. This occurs late in the day and during the night­ time hours as the reaction, O3 4- N O — 0 2 4- N 0 2 , converts N O to N 0 2 very effectively. At this time, NO3 develops through the NO2-O3 Reac­ tion 13, and H 0 2 is generated by the NO3-CH2O Reaction 16. In theory, as a consequence of these and other reactions, H2O2 generation is expected to occur even at night in cases A and F. In all other cases con­ sidered here, this nighttime H 2 0 2 de­ velopment is unimportant. From Fig­ ure 4 we can see that H2O2 generation is most favored in the air masses of highest hydrocarbon-to-nitrogen-oxide molar ratio (cases Β and C). In case B, enough H2O2 forms in one day to oxi­ dize up to 20 ppb of SO2 impurity, provided that cloud water or rain water is available in the atmosphere to pro­ mote this reaction. One should be aware that the amount of H2O2 formed in a given air mass is in theory a strong function of the relative humidity of that air mass. This effect arises from the difference in rate constants for Reactions 20 and 22 coupled with the fact that the fraction of HO2 in the form of the hy­ drated radical, H2O-HO2, increases as the humidity rises. One anticipates that when all other conditions remain the same (such as impurity levels and sunlight intensity), the amount of hy­ drogen peroxide that forms should in­

crease with an increase in relative hu­ midity. In fact, roughly twice as much H2O2 should form at 100% as that formed at 50% relative humidity (conditions of Figure 4). For example, after the two-day simulation in case A, the [ H 2 0 2 ] rises from 18 ppb at 50% to 34 ppb at 100% relative humidity. For case F, 3.4 ppb forms at 50% and 8.8 ppb at 100%; in case G, 2.7 ppb forms at 50% and 4.3 ppb at 100% relative humidity. In theory, the [H2O2] gen­ erated in the atmosphere reflects not only the type and degree of impurities present, but it also is a strong function of the gaseous water content of the air. The solubility of H2O2 in water is so great that a very large fraction of the gaseous supply in the troposphere is transferred to the liquid phase as clouds appear. Under these circum­ stances, the oxidation of HSOJ by the H2O2 occurs rapidly in cloud water. In some air masses, we expect little H2O2 to develop. If new S 0 2 is injected into H202-poor air masses along their tra­ jectories, the H S O j oxidation in clouds may be limited by the amount of oxidant in these cases. Recent evi­ dence suggests that other liquid phase reactions may generate H2O2 in the troposphere as well; the nature of these and their significance are ill-defined at present (51, 52). However, it is clear from the present computer simulations of gas-phase chemistry that a large share of the total oxidant, 0 3 and H2O2, required to oxidize SO2 ( H S O J ) in cloud water, can be pro­ vided from homogeneous gas-phase reactions that are expected to occur in the polluted troposphere, particularly in very humid air. Other potential oxidants for H S O j in cloud water are expected to be gen­ erated in the complex chemistry of the polluted troposphere. Free radicals formed in the gas-phase processes such

TABLE 3

Theoretical concentrations of some minor oxidizing compounds at late afternoon of first day (5:20 P.M.) Concentration ( p p b ) Case3

CH3COO2NO2

H02N02

1Cr 5 10~ 2 1Cr 2 1Cr 10

Χ ΙΟ-3

9.4 7.7 4.2 X 1 0 _ 1 5.5 X 1er 4

Ε F

8

2.8 Χ 1(Τ 4.5 Χ Ι Ο " 3

5

1.5 X 1Cr 7.9 X 1 0 _ 1

5.7 X 1Cr 2.2 X 1(T 4

2.0 1.2 5.2 1.3 9.7 8.1

G Η

2.5 Χ Ι Ο " 4 2.6 Χ Ι Ο - 5

2.4 Χ 10~ 2 1.2 Χ 10~ 3

7.2 Χ Ι Ο - 4 2.4 X 1Cr 4

7.1 X 10~ 2 1.2 X 10~ 1

5.6 3.4 2.9 6.5

X X Χ X

a

In all cases the relative humidity is 5 0 % . " RO2H represents all alkyl hydroperoxides other than CH 3 0 2 H.

Environ. Sci. Technol., Vol. 17, No. 9, 1983

5.9 3.5 1.2 2.2

X X X X

13

R02Hfi

CH3O2H

10~ 2 1CT2 ΙΟ-3 10~ 7

A Β C D

440A

CH3COO2H

X X X Χ

10~ 1 10~ 8 10~ 1 1 10-3

5.9 3.0 2.4 6.9 2.0 1.4

Χ ΙΟ-3

Χ ΙΟ-8 X 10~ 1 0 X 10~ 2

4.7 X 10~ 2 1.7 X 1 0 - 2

as N 0 3 , HO, H 0 2 , and R 0 2 may be captured effectively by cloud droplets, and in theory these can lead readily to H2SO4 generation through their oxi­ dation of HSOJ in the droplet (34, 35). Gas-phase reactions lead to other molecular products that are potentially good oxidants for H S O j in cloud water. Among these are peroxynitric acid (HO2NO2), methyl hydroperox­ ide (CH3O2H), peroxyacetic acid (CH3COO2H), and other higher mo­ lecular weight organic hydroperoxides (RO2H). Most of the hydroperoxides are formed in chain termination reac­ tions involving reactions of alkylperoxy and acylperoxy radicals with HO2 radicals, so the anticipated theoretical amounts of these are not large. We can compare in Table 3 the es­ timated concentrations of the com­ pounds for late afternoon of the first day of the various simulations (5:20 P.M.). Some of these products (HO2NO2 and CH3O2H) should be sufficiently soluble in water and strong enough oxidizing agents in solution so that they could contribute to H S O j oxidation and acid generation in cloud water. However, our results support the view that H2O2 and O3 are proba­ bly the dominant highly oxidizing products formed in gas-phase reactions in the troposphere, and they may be the most significant reactants leading to HSOJ oxidation in cloud water. Peroxyacetylnitrate (PAN) is a substantial product of gas-phase tropospheric chemistry. Its solubility in H 2 0 appears to be relatively low as evidenced by the observation of Penkett (53). He noted that its ambi­ ent concentration does not seem to be altered significantly with the appear­ ance of rain. Like H 2 0 2 and O3 vapor,

the ability of PAN and that of its dis­ sociation product CH3COO2 to oxi­ dize SO2 in the gas phase is very low. It is, however, a long-lasting source of CH3COO2 radicals even during the nighttime hours. If clouds are present in the atmosphere, these radicals may be captured effectively by cloud water droplets, and they, like H2O2 and O3 in aqueous solution, may be trans­ formed from poor oxidants in the gas phase to good oxidants for SO2 ( H S O j ) in cloud water. The theoretical concentration-time profiles for PAN are shown in Figure 3 for the different air parcels ( A - H ) considered here. In theory, [PAN] should show a positive dependence on relative humidity; this is seen in case A, in which the largest concentrations of PAN are expected at 100% relative humidity. These simulations predict another previously unexpected result: We see that [PAN] is predicted to grow during the nighttime hours for the highly polluted air mass A. In theory, this arises from the creation of CH3CO radicals through the N O 3 CH3CHO Reaction 15 followed by:

G and H has been minimized in these simulations because its initial value was assumed to be zero. Since the lifetime of PAN is significant for many tropospheric conditions, it is more likely that these air masses contain some residual PAN formed in the more highly polluted atmospheres from which these air masses developed through atmospheric dispersion pro­ cesses. Nitric acid can in theory be gener­ ated in another interesting process. NO3 and N 2 0 5 formed in the gas phase may be captured by fog, cloud water, or rainwater in which HONO2 may be generated effectively. This conclusion, favored and suggested by others and supported by us as well, is based upon the very short lifetime of NO3 (as monitored by its visible ab­ sorption [662 nm] in the troposphere during the evening hours when fog or cloud water is present [54]) and seemingly sound theoretical consid­ erations derived from model calcula­ tions (34, 35). The gas-phase generation of other acids in addition to H 2 S 0 4 and HON0 2 . In theory, several inorganic and organic acids should be formed in the polluted troposphere. Thus, small amounts of H 0 2 N 0 2 and H O N O are anticipated in addition to formic acid (HCO2H) and the higher molecular weight acids ( R C 0 2 H ) . From the present simulations, estimates of the concentrations of each of these acids as well as H 2 S 0 4 and H O N 0 2 have been made. In Table 4 the theoretical amount of total acid formed during one day and the fraction of the total that is made up of each of these acids are shown for the various polluted air mass cases A - H .

C H 3 C O + 0 2 — CH3COO2 CH3COO2 + N 0 2 — CH3C002N02 The relatively high concentrations and reasonably high stability of PAN should allow its long-range transport along with S 0 2 . It may ultimately participate in the oxidation of HSO3" in cloud water through the CH3COO2 radicals, which it provides on disso­ ciation and which are captured by the cloud droplets. The [PAN] in the rel­ atively clean tropospheric air masses

TABLE 4

Case

Relative humidity (% )

A A

10 50

A Β

[SO 2 ]0

Fraction of the total acid HONO

HO 2 NO 2

HC02H

RC02H

HI

Theoretical amoijnts of toteil acids formed and the fraction of each acid present in late afternoon of first day (5:20 P.M.) H2SO4

HONO2

0.6 0.6

0.103 0.055

0.685 0.736

0.0007 0.0008

0.0006 0.0005

0.106 0.103

0.105 0.104

56.7 73.6

100 50

0.6 6.0

0.046 0.145

0.789 0.106

0.0010 0.0013

0.0004 0.0000

0.082 0.375

0.081 0.373

93.7 26.6

C D

50 50

60.0 0.06

0.090 0.012

0.005 0.841

0.0002 0.0000

0.0000 0.0038

0.453 0.072

0.452 0.071

17.9 3.71

Ε F

50 50

0.006 0.6

0.002 0.085

0.953 0.829

0.0000 0.0005

0.0047 0.0019

0.020 0.042

0.020 0.042

1.27 9.84

F G

100 50

0.6 0.6

0.106 0.114

0.832 0.828

0.0001 0.0000

0.0019 0.0024

0.032 0.026

0.031 0.026

11.7 1.22

G H H

100 50 100

0.6 0.6 0.6

0.126 0.088 0.097

0.827 0.823 0.833

0.0002 0.0003 0.0002

0.0016 0.0030 0.0020

0.023 0.043 0.034

0.023 0.043 0.034

[NOx]o

1.28 0.083 0.098

Environ. Sci. Technol., Vol. 17, No. 9, 1983

441A

The theoretical fraction of total acid that is composed of organic acids is relatively large. For the typical highly polluted air mass (case A), about 20% of the total acids are formic acid and the higher molecular weight organic acids. As the starting mixture of the polluted air mass of composition A is diluted by factors of 0.1, 0.01, and 0.001 respectively, the organic acid fraction falls to 8% and 5% and then rises to 9% of the total acids present. Formic acid at concentrations of about one-tenth of those of formaldehyde has been identified spectroscopically in the polluted urban atmosphere (55). Both formic and acetic acids have been found among the acidic components of rain (56). The gas-phase reactions considered here as well as liquid-phase reactions may contribute to the total organic acid input to acid rain. Thus, gaseous oxidizing agents (such as HO and HO2) transferred to cloud water can in theory react with dissolved C H 2 0 (CH 2 (OH) 2 ) and act as an appreciable additional source of or­ ganic acids in clouds (57). It is ap­ parent that the organic acid compo­ nent of acid rain may not be insignifi­ cant. It is interesting to compare the fractions of H C 0 2 H derived from the three different reactions that we have included in our mechanism (see Table 5). The first two involve Reactions 23 and 24 of the Criegee intermediate CH2O2, while the third results from Reaction 25 in the reaction sequence that follows H 0 2 radical addition to CH20: CH202 — Η002Η+ - ^ 1 HC02H (23) -*• (Other products) CH202 + H 2 0 -* HC02H + H 2 0 (24) CH20 + H02 ^ H02CH20 *=* O2CH2OH 0 2 C H 2 O H + N O (or R 0 2 ) — O C H 2 O H + N 0 2 (or RO + 0 2 ) OCH2OH + O, — HC02H + H 0 2 (25) The vibrationally rich C H 2 O Î product of the O3-1 -alkene reactions rearranges to an excited H C 0 2 H t molecule. Reaction 23 represents the fate of that fraction of the excited formic acid, which is relaxed to stable, lower vibrational levels by collision and hence does not dissociate into other products. Reaction 24 involves the H20-catalyzed rearrangement of CH202(//). 442A

Environ. Sci. Technol., Vol. 17, No. 9, 1983

TABLE 5

Relative importance of formation pathways for formic acid in simulated polluted tropospheric air masses Fraction formed by path shown b

[HC0 2 H], ppb c

Case»

F24*

F23 e

F25'

A Β

0.896 0.907

0.087 0.088

0.017 0.006

7.60 9.99

C D

0.911 0.911

0.087 0.088

0.002 0.001

8.12 0.266

Ε F

0.912 0.893

0.088 0.080

0.000 0.028

0.0262 0.411

G H

0.820 0.890

0.073 0.079

0.108 0.032

0.0317 0.00361

a

In ail cases the relative humidity is 5 0 % . " Calculated for 12:40 P.M. c Concentrations at 5:20 P.M.

Note that in Table 5 the major source of HCO2H, according to our mechanism choice, is the Criegee intermediate derived from the O 3 - I alkene reactions. The fraction that comes from the H 0 2 - C H 2 0 reaction is small; it has a maximum of about 11% for the relatively clean air mass, case G. The higher organic acids (represented solely by CH3CO2H in the simplified reaction scheme used here) are derived entirely from the reaction of the Criegee intermediates (CH3CHO2 and the higher homologues) that form in the reaction of O3 with the alkenes larger than C2H4. The rates of Reaction 24 and its analogues for the higher molecular weight Criegee intermediates are dependent upon the relative humidity of the air mass because H 2 0 is a reactant here. One can discern from the data in Table 4 that somewhat larger amounts of the organic acids are formed at the higher relative humidities for the case of air mass A. This in turn lowers the extent of S 0 2 oxidation by the Criegee intermediates at high humidities as discussed earlier. Summary The present computer simulations of the tropospheric chemistry strongly suggest that gas-phase generation of acids in polluted tropospheric air masses can be important. This is particularly true for clear summertime conditions for which acid generation in cloud water is unimportant. We conclude that for typical impurity levels encountered in the long-range transport of air pollutants (cases F - H ) , the average rate of gas-phase oxidation of S 0 2 to H2SO4 is in the range of about 13-24% per 24-h period. This of course depends on the composition of the

d

CH 2 0 2 + H 2 0 — HC0 2 H + H 2 0 (24) · CH 2 0 2 — HC0 2 H (23) ' HOCH2Q + 0 2 - * HCQ2H + HQ 2 (25)

particular air mass (relative humidity, 50%). The rates increase with rising relative humidity. The theoretical conversion rates for N 0 2 to HONO2 are much higher than those for S 0 2 and depend on the choice of rate constant for the N2O5-H2O Reaction 17. With k 17 at its maximum value, the rates, averaged over a 24-h period, are 1 6 - 1 9 % / h - 1 for a moderately polluted troposphere (50% relative humidity). The alternative hypothesis of ki7 = 0 leads to 24-h averaged rates for N 0 2 to H O N 0 2 conversion of from 6% to 11%/h - 1 for the moderately polluted air masses (cases F-H). Theory suggests that the rate of N 0 2 oxidation may be enhanced significantly by gas-phase conversion processes involving NO3 and N2O5 that continue to occur during the nighttime hours. When clouds are abundant in the troposphere, the very fast in-cloud conversion of S 0 2 to H 2 S 0 4 through Ο3, H2O2, and possibly other oxidants must contribute significantly and, in fact, dominate the mechanisms of acid gen­ eration. But it is important to observe that these solution-phase processes de­ pend at least in part on the gas-phase chemistry that generates the required oxidants (such as H2O2 and O3) for the HSO3" in the cloud chemistry. The model calculations further suggest that from 5% to 20% of the total acids generated in gas-phase re­ actions in the troposphere may be composed of organic acids. Other liq­ uid-phase reactions may add to the organic acid input. We conclude from the present study that in order to simulate any reason­ able semblance of the true effects of the many variables that control acid generation in the troposphere, it may be necessary to incorporate rather

detailed chemical reaction schemes into the atmospheric transport and transformation models under devel­ opment. In their sensitivity analysis of the kinetics of acid rain models, Golomb et al. identified the rates of oxi­ dation of the acid precursors SO2 and N O x as the most important factors in determining rain acidity (58). They concluded that accurate modeling must include time and space variability of the oxidation rate constants. It seems clear to us that the time is past when an assumed linear, fixed trans­ formation rate for S 0 2 and the com­ plete neglect of all atmospheric chemistry, including the NO2 con­ version to HONO2, can be considered to be an adequate representation of the chemistry of acid development in acid rain models. The success of control strategies built upon such unrealistic, nonscientific models is unpredictable at best. References (1) Calvert, J. G.;Stockwell,W. R. "Deviations from the O2-NO-NO-2 photostationary state in tropospheric chemistry"; Can. J. Chem.. in press. (2) Stockwell, W. R.; Calvert, J. G. "The Mechanism of NO3 and H O N O Formation in the Nighttime Chemistry of the Urban Atmosphere"; unpublished work. (3) Cox, R. Α.; Penkett, S. A. Nature 1971, 230, 321-22. (4) Cox, R. Α.; Penkett, S. A. Nature 1971, 229,486-88. (5) Cox, R. Α.; Penkett, S. A. J. Chem. Soc, Faraday Trans. I 1972,6*, 1735-53. (6) Cox, R. A. J. Photochem., 1974/1975, 3, 291-304. (7) Calvert, J. G.; McQuigg, R. D. Int. J. Chem. Kinet. Symp. 1975, / , 113-54. (8) Davis, D. D.; Klauber, G., Int. J. Chem. Kinet. Symp. 1975, / , 543-56. (9) Atkinson, R.; Perry, R. Α.; Pitts, J. N., Jr. J. Chem. Phys. 1976, 65, 306-10. (10) Castleman, A. W., Jr.; Tang, I . N . J. Photochem. 1976/1977,0,349-54. (11) Calvert, J . G . ; S u , F.; Bottenheim, J. W.; Strausz, O . P . Atmos. Environ. 1978, 12. 197-226. (12) Davis, D. D.; Ravishankara, A. R.; Fischer, S. Geophys. Res. Lett. 1979, 6. 113-16. (13) Harris, G. W.; Atkinson, R.; Pitts, J. N., Jr. Chem. Phys. Lett. 1980, 69, 378-82. (14) Friend, J. P.; Barnes, R. Α.; Vasta, R. M. J. Phys. Chem. 1980, 84, 2423-26. ( 15) Su, F.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1980, 84, 239-46. (16) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1981, 85, 1024-27. ( 17) Calvert, J. G.; Stockwell, W. R. In "Acid Precipitation: S 0 2 , NO, NO2 Oxidation Mechanisms: Atmospheric Considerations"; Calvert, J. G., Ed.; Ann Arbor Science Pub­ lishers: Ann Arbor, Mich., in press; Chapter 1. (18) Stockwell, W. R.; Calvert, J. G. "The mechanism of the HO-SO2 reaction"; Atmos. Environ., in press. (19) Durham, J. L.; Demerjian, K. L.; Barnes, H. M.; Seinfeld, J.; Grosjean, D.; Freedman, S. "Sulfur and Nitrogen Chemistry in Long Range Transport Models in Work Group 2," Atmospheric Sciences and Analysis, 1982, Final Report. Technical Basis, Atmospheric

Science Review, Subgroup Report No. 2F-A, under the Memorandum of Intent of Transboundary Air Pollution, Canada and U.S., Aug. 5. 1980. (20) Benson, S. W. Chem. Rer. 1978, 78. 23-35. (21) Kan, C. S.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1981, 85, 11 26-32. (22) Morris, E. D„ Jr.: Niki, H. J. Phys. Chem. 1974,75, 1337-38. (23) Morris, E. D., Jr.; Niki, H. J. Phys. Chem. 1973,77, 1929-32. (24) Atkinson, R.; Lloyd, A. C ; Winges. L. Atmos. Environ. 1982, 16. 1341-55. (25) Altshuler, A. P. Atmos. Environ. 1979, 13, 1653-61. (26) Husar, R. B., Patterson, D. E.; Husar, J. D.; Gillani, N. V.; Wilson, W. E., Jr. Atmos. Environ. 1978, 12, 549-68. (27) Meagher, J. F.; Bailey, E. M.; Luria, M. J. Geophys. Res. 1983, 88. 1525-27. (28) Shelter, R. E.; Stedman, D. H.; West, D.H.J. Air Pollut. Control Assoc. 1983, 33, 212-14. (29) Grosjean, D. Environ. Sci. Techno!. 1983, 17, 13-19. (30) Huebert, B. J. "Measurements of the Dry-Deposition Flux of Nitric Acid Vapor to Grasslands and Forest," Presented at the 4th International Conference on Precipitation Scavenging, Dry Depositions and Resuspension, Nov. 29 Dec. 3, 1982; Santa Monica, Calif. (31) Russell, A. G.; McRae, G. J.; Cass, G. R. "Acid Deposition of Photochemical Oxidation Products—A Study Using a Lagrangian Trajectory Model"; to appear in the 13th In­ ternational Technical Meeting on Air Pollu­ tion Modeling and Its Application; France, Aug. 24-27, 1982. (32) Peterson, T. W.; Seinfeld, J. H. In "Ni­ trogenous Air Pollutants: Chemical and Bi­ ological Implications"; Grosjean, D., Ed.; Ann Arbor Science Publishers: Ann Arbor, Mich., 1979; pp. 259-68. (33) Middleton, P.; Kiang, C. S. In "Nitroge­ nous Air Pollutants: Chemical and Biological Implications"; Grosjean, D., Ed.; Ann Arbor Science Publishers: Ann Arbor. Mich., 1979; pp. 269-88. (34) Heikes, B. G.; Thompson, A. M. EOS Trans., Amer. Geophys. Union 1981, 62, 884. (35) Chameides, W. L.; Davis, D. D. J. Geo­ phys. Res. 1982, 87, 4863-77. (36) Falls, A. H.; Seinfeld, J. H. Environ. Sci. Technol. 1978, 12, 1398-1406. (37) Fall, A. H.; McRae, G. J.; Seinfeld, J. H. Int. J. Chem. Kinet. 1979, / / , 1137-62. (38) McRae, G. J.; Russell, A. G. In "Acid Deposition: Wet and Dry"; Hicks. B. B., Ed.; Ann Arbor Science Publishers: Ann Arbor, Mich., in press. (39) McRae, G J.; Goodin, W. R.; Seinfeld, J. H. Atmos. Environ. 1982, 16, 679-96. (40) McRae, G. J.; Goodin, W. R.; Seinfeld, J. H. J. Comp. Phys. 1982, 45, 1-42. (41) Cantrell, C. Α.; Stedman, D. H. Geophys. Res. Lett. 1982, 9, 846 49. (42) Hegg, D. Α.; Hobbs, P. V. Atmos. Envi­ ron. 1981, 15, 1597-1604. (43) Lazrus, A. L.; Haagenson, P. L.; Kok, G. L.; Huebert, B. J.; Kreitzberg, C. W.; Likens, G. E.; Mohnen, V. Α., Wilson, W. E.; Winchester, J. W. Atmos. Environ., in press. (44) Scott, B. C. Atmos. Environ. 1982, 16, 1735-52. (45) Penkett, S. Α.; Jones, M. R.; Brice, Κ. Α.; Egglcton, A.E.J. Atmos. Environ. 1979, 13. 123-37. (46) Kunen, S. M.; Lazrus, A. L.; Kok, G. L.; Heikes, B. G. J. Geophys. Res., in press. (47) Martin, R. " S 0 2 , NO, N 0 2 Oxidation Mechanisms; Atmospheric Considerations"; Calvert, J. G., Ed.; Ann Arbor Science Pub­ lishers: Ann Arbor, Mich., in press; Chapter 2. (48) Hoffman, M. R.; Jacob, D. J. In " S 0 2 ,

NO, NO2 Oxidation Mechanisms; Atmo­ spheric Considerations"; Calvert, J. G., Ed. Ann Arbor Science Publishers: Ann Arbor, Mich., in press; Chapter 3. (49) Schwartz, S. E. In " S 0 2 , NO, N 0 2 Oxi­ dation Mechanisms; Atmospheric Consider­ ations"; Calvert, J. G., Ed.; Ann Arbor Science Publishers: Ann Arbor, Mich., in press; Chapter 4. (50) Brock, J. R.; Durham, J. L. In " S 0 2 , NO, NO2 Oxidation Mechanisms; Atmospheric Considerations"; Calvert, J. G., Ed.; Ann Arbor Science Publishers: Ann Arbor, Mich., in press; Chapter 5. (51) Zika, R. G.; Saltzman, E. S. Geophys. Res. Lett. 1982,9,231-34. (52) Heikes, B. G.; Lazrus, A. L.; Kok, G. L.; Kunen, S. M.; Ganrud, B. W.; Gitlin, S. N.; Sperry, P. D. J. Geophvs. Res. 1982, 87, 3045-51. (53) Penkett, S.A. AERE, Harwell, U.K., private communication to the authors. (54) Piatt, U.; Perner, D; Schroder, J.; Kcsslcr, C ; Toennissen, A. J. Geophys. Res. 1981,86, 11965-70. (55) Tuazon, E. C ; Graham, R. Α.; Winer, A. M.; Easton, R. R.; Pitts, J. N. Jr. Atmos. Environ. 1978, 12, 865-75. (56) Galloway, J. N.; Likens, G. E.; Kccnc, W. C ; Miller, J. M. J. Geophys. Res. 1982, 87, 8771-86. (57) Chameides, W. L.; the authors are grateful to Dr. Chameides for discussions of his and Dr. D. D. Davis's recent theoretical work re­ lated to free radical transfer from the gas phase to cloud droplets, which they find can be important in the oxidation of formaldehyde in clouds. (58) Golomb, D.; Batterman, S.; Gruhl, J.; and Labys, W. Atmos. Environ. 1983, 17, 64553.

Jack G. Calvert (I. ) is α senior scientist at the National Center for Atmospheric Re­ search, Boulder, Colo. He is head of the precipitation chemistry, reactive gases and aerosols section in the atmospheric chemistry and aeronomy division. He and his research group have been active in the laboratory study of the photochemistry and the thermal reactions of reactive transients involved in the transformation of common tropospheric trace components including S02, NOx, the hydrocarbons, and their oxidation products. He was the recipient of the American Chemical So­ ciety Award for Creative Advances in the Environmental Sciences and Technology in 1982. William R. Stockwell (r. ) is α postdoctoral fellow in the advanced study program at the National Center for Atmospheric Re­ search. He is actively engaged in labora­ tory studies of the tropospheric chemistry qfS02and NOK-containing systems using Fourier transform infrared spectroscopy and in the development of chemical models for the computer simulation of the com­ plex chemistry of the polluted tropo­ sphere. He has a special interest in the chemistry related to acid rain generation and in the modeling of these complex processes.

Environ. Sci. Technol., Vol. 17, No. 9, 1983

443A