Atmospheric hydrogen peroxide Does it share a role with ozone in degrading air quality?
Hiroshi Sakngawa Isaac R. Kaplan Wangteng Tsai Ynram Cnhen rlniversiry of California, Los Angeles Los Angeles, CA 90024 Ozone (0,) has long been considered to be the oxidant that determines the air quality of an urban atmosphere (1). Its toxic effect on crops, forest trees, and humans also has been of special concem (lj. During the 1980s. however, atmospheric chemists identified hydrogen a photochemical prodperoxide (H202). uct in air, as another oxidant that may significantly degrade air quality, especially through acid rain formation. H,O, also may damage trees because it harms plant cells (2-7), and therefore might contribute to the decline of forests in Noah America and central Europe (8, 9). In view of the importance of H,O, in acid rain and acid cloud formations and possible harmful effects on vegetation, studies have been performed to understand the chemism of this oxidant in the atmosphere (1,lO). Numerous investigations regarding atmospheric H202have been conducted during the 1980s; nevertheless, the distribution panem of H202and its formation,decomposition, and deposition processes still are not clearly understood. In this article, we summarize previous investigators’ fmdings concerning the levels of H,O, in the gas and aqueous phases at various locations in the world. We also review and evaluate previous studies on factors affecting the formation of gaseous H202Additionally, we review studies on the decomposition and deposition of atmospheric H20,,including its gas-phase and aqueous-phase reactions, to evaluate the role of H20Zas an oxidizer of sulfur dioxide (SO,) in the atmosphere. Finally, we suggest that although H,O, may be the dominant oxidant of atmospheric SO, to sulfate, the limited amount of field data currently available on the t e m m and geographic concentration and distribution of H202binders the identification of major factors affecting the formation of mospheric H,O,.
Atmospheric levels nf H,O, Table 1 presents a summary of previous studies, conducted at various locations in North America, Europe, Brazil,
and Japan, on the field measurements of H202levels in the gas phase, and the rain and cloud phases. All of the reported concentrations of gaseous H,O, are less than 10 parts per billion (ppb) by volume, except for the few reported by Bnfalii et al. (11) and by Kok et al. (12). It is likely that previous measure-
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ments of gaseous H,O, (11, 12) have been distorted by the formation of artifact H202within collectors because of the presence of 0,in ambient air (18. 38, 50-52). In those studies, gaseous H,O, had been determined by one or more of the following methods: measurement of chemiluminescence or fluorescence following aqueous stripping of gaseous H20,from the air (16,17,19, 22,24-26,29-32,53-55); measurement of chemiluminescence or fluorescence following a cryogenic collection of gas-
0013-936w9o/o924-145~02.~/0 @ 1990 American Chemical Sociely
eous H,O, (14.18,33);Colorimetry using pyridine-2,6-dicarboxylicacid and vanadate reagents following the aqueous From the reported values of gasaous HzO, (Table 1). the f stripping of gaseous H,O, (56);ampermars to emeroe: ometry or colorimetry using a ferrous HO , , concentrations are highest in the afternoon and lowest at night (Figure thiocyanate reagent following the col1) (21. 23. 27-29. 33). lection of gaseous H,02 by an annular H202k&ntratdns are highest in the summer and loWest in the winter(22, 25, 28). The seasonal variation of gaseous HO ,, concentration in North denuder (23.57);and tunable diode laAmerica has been studied in Los Angeles (28,at the top of Whitetop M o m ser absorption specwscopy (15).The lain, VA (22),and over the central United States (25). Boatman el 81. (25) detection limits of gaseous H202 usin found that the average concentration of gas8ws HO , , vanes by a factor of these methods range from levels of 10-4 16 (0.348ppb) between winter and summer in the boundary layer atmoppb to l(r' ppb, depending on the samspheres (1450 f 150 m above sea level) over the central United States (Figpling time, air flow rate, and sensitivity ure 2). It is worth noting that studies of H20z levels in ice cores taken from of the analytical method. polar regions, which presumably reflect histori atmospheric levels of HP02, It should be noted that the quantitative have shown that concentrations of H20z vary seasonally, and that the highagreement of the values of gaseous est concentrations occur during the summer (58, 59). H20? determined by various methodolo* "Latitudedependency" of gaseous HO , , has been detected by three akgies IS not satisfactory. For example, re, , levels over the continental United States craft-based observations of HO sults of two method comparison studies (17, 19, 3Q in which the highest concentrationswere m e w e d in the south. of gaseous H202. conducted at North Heikes et al. ( 1 7 ) observed that concentrationsof H202near the surface and Carolina and California in 1986,indicatat cloud tops increased with decreasing l a t i d e over the eastern United ed that the differences in the H202valStates. During the winter of 1987. Van V a h et at. ( 1 9 ) determined that the ues, which were determined by the variconcentration of gasews HO , , from north to south over the ambel United ous methods, are significant (20,Zl).In States increases by 0.04-0.05 ppb per degree of latitude (Figure 3). In June the comparison study conducted in Cali, , deuease from lower 1987, they also observed that concentrations of HO fornia, the difference in the H202values (40O N ) to higher (44O N ) latitudes over the northeastern United States (30). varied by a factor of two (Zl),whereas It should be noted, however, that in anomer aircratkbased measurement of in the comparison study conducted in the levels of H,O, over the central United States. these latitudinal variations North Carolina the difference was much lower (within f30% difference) (20). These variations in the performance of different analytical methodologies sug- ides before it was analyzed (46,61).The Reaction 16) and on solid surfaces (i.e., gest that further improvement in H,O, quantitative agreement between the per- particulates) (13,18,65-71). The signifsampling and analysis methods is ur- oxyoxalate chemiluminescent and fluo- icance of these formation processes for gently needed. rescent methods for the determination of the atmospheric production of H202 is Because no significant emissions of H202is excellent (62),despite some in- not yet clearly understood, and this area H202from natural and anthropogenic terferences in the luminol chemilumi- deserves further attention. sources have been reported, it is be- nescence technique (47,63). In addition to H202: organic peroxlieved that H,O, is formed by photoThe H202concentrations reported for ides can be generated m the gas phase chemical reactions in the atmosphere cloud droplets also have been deter- by chemical reactions, including the re[dominated by a coupling of two hy- mined by chemiluminescent or fluores- action of a hydroperoxyl radical with an droperoxyl (HO,) radicals (Reaction 8 cent methods following the collection of organic peroxyl radical (Table 2, Reacin Table 2)l (1,10).In light of the val- cloud droplets by samplings from air- tion 10) (1,72).Currently, limited inforues reported in Table 1, photochemical craft (26. 39, 40, 45, 46, 49) or by mation is available regarding the ahnoactivity may largely determine the diur- ground-based collections on mountains spheric levels of organic peroxides. nal, seasonal, and latitudinal variations at high elevations (22, 47, 48). Mea- Total concentrations of organic peroxobserved (see box). sured concentrations of cloud H202 ides in rain and cloud waters, which As reported in previous studies, con- ranged from sub-pM levels to levels of were determined by the enzyme fluorescenmtions of H202 in rainwater were huntlreds of pM (0.01-247 pM). Con- cence technique (47),were much lower found to be within a range of 0.01centrations of H202are highest in the than those of H202 (39,42).Kelly et al. 199 pM (Table 1). Levels of H202 are summer (22, 39,47). (39)reported that the concenhation of observed to be high in the summer and The major source of aqueous-phase total organic peroxides accounted for low in the winter (35,3842). The H202 H202 (e&, rain, cloud, and fog) is be- less than 5% of the total peroxides levels also are high in the aftemoon and lieved to be gaseous H202. because (H202 + total organic peroxides) in low at night (33,38,41,42,44).Con- H202 is highly soluble in water. For ex- cloudwater samples collected over the centrations of H202 in rainwater have ample, a ppb level of gaseous H20,can eastern United States. Lee et al. (42) been determined by one or more of the equilibrate with pM levels of Hz02 in could not detect organic peroxides withfollowing methods: luminol chemilumi- water droplets, as calculated from the in the uncertainty of the analysis (i1%) nescence or peroxyoxalate chemilumi- Henry's law constant for H202 (7.4 x in rainwater collected at Long Island, nescence (35,36,38,40,41);enzyme 104 M atm-l at 25 "C) (64).Concentra- NY. Hellpointner and Gab (61)deterfluorometry using p-hydroxyphenylace- tions of aqueous H20, at pM levels can mined the concentrations of individual tic acid or scopoletin (22,32,37,39,42,be found in rain and clouds. A trend in organic peroxides in air and precipita44,60);and ampemmetry using a plati- the diurnal and seasonal variations of tion samples using a fluorescence technum reactor (43).Because organic per- aqueous H202 similar to that of gas nique following separation of individual oxides interfere with the determination phase H202(33,35,3842,44) partially organic peroxides by HPLC. Their reof H202 (47),either the enzymatic or supports the claim that aqueous H202 sults indicated that concentrations of the high performance liquid chromatog- may originate mostly from gas-phase methyl hydroperoxide (CH,OOH) raphy (HPLC) technique was used to HZO2 It also has been shown that H202 and hydroxymethyl hydroperoxide separate H202from the organic perox- forms in the aqueous phase (Table 2, (HOCH,OOH) in the air collected Envimn. Sci. Technol., Vol. 24. No. 10, lSS0 1453
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tions and emissions of nitrogen oxides (NO,), volatile organic compounds (VOC) and carbon monoxide (CO), and meteorological parameters such as solar radiation, temperature, and water vapor PMhWYS Relorenen content. Sas phase reaaons 10,7577 Modeling studies. Compared with the I 1 NO, f hv +OcP) + NO air quality modeling of ozone, the phoM tochemical modeling of H202is a new2 o(3~ + )o2-+ o8 3 0, + hv -+ O('D) + Oz er initiative which emerged as recently 4 O('D) + HO , -+ 2 OH as the early 1980s. when acid rain prob5 OH+CO+2HOz+C08 02 lems captured worldwide attention. Therefore, only a few modeling studies 4 investigating factors affecting the for6 HCHO+hv-+2HO,+CO mation of H202have been conducted, as 4 7 NOa + HCHo -+ HNOs + H02+ CO summarized in Table 3. Results of these 8 HOz + HOz + HzO, + Oz modeling studies all indicate that the 9 HOz. HO , + H q -+ H& + O2 + H20 generation of gaseous HZ02is affected 10 . HO, + RO--AROUH + 0 by the presence of atmospheric poUutants such as NO,, VOC, and CO, and by + 0 4 CO + 0 12 HOz meteorological parameters such as solar radiation, temperature, and water vapor content. Brief descriptions of the influence of these factors on the hydrogen peroxide level are given below. 4 67.74.60.89 A high concentration of NO, tends to inhibit the formation of H202 by scay enging free radical species from the an (75, 76). Calvert and Stockwell (75) demonstrated that in a very polluted (NOX-rich)air, little H202 would appear until evening, because the rapid reaction of HO, radicals with nitric oxide (NO) (Table 2, Reaction 11) restricts the formation of H,O, until the concentration around Munich, Federal Republic of 221 M am-' and 468 M am-', respec- of NO is lowered by conversion to NO, Germany, were 40-179 parts per trillion tively (64).The thud-order rate con- and to other products such as nitric acid (ppt) by volume and 4-15 ppt, respec- stants (k) of peroxides with S(1V) (HNO,). Stockwell (77) showed that tively, whereas concentrations of H,O, (d[peroxidel/dt) = k IH'1 . lperoxidel HO, levels are inversely related to NO .. were in the range of 453-1471 ppt. [SfV)] (Table 2, Reaction 17) are 1.7 x levels in polluted air and that the proIn rainwaters, the concentrations of 10 M-'sK' for CH,OOH (at 23 "0and duction rate of H,O, is extremely sensiCH,OOH, HOCH,OOH and l-hydroxy- 3.5 x 10' w2S-I 5 in the droplets. He calculated that the oxidation rate of S(IV) by H,O, can be higher than loo%/% at low pH ( 4 1 , whereas the oxidation rates by 0, and 0,at low pH are much lower than those by H202 For example, the oxidation rates by 0,and 0,at pH 4.0 are
