Authentic cloud water samples absorb ultraviolet radiation and thereby initiate formation of peroxides (primarily hydrogen peroxide (H2O2)). This aqueous phase photoformation of H2O2 is a widespread phenomenon in the terrestrial troposphere; midday equinox‐normalized H2O2 photoformation rates ranged up to 3.0 μM/h for 36 different cloud water samples collected from sites in New York, North Carolina, Ontario, Virginia, and Washington. By comparison, previously published field studies have shown that over the eastern United States and Canada, approximately 100% of winter cloud water samples, 58–100% of spring and fall samples, and 3–7% of summer samples had peroxide concentrations less than 5 μM.. Previously published model estimates of aqueous phase H2O2 formation rates in cloud drops from gas‐to‐drop transfer of H2O2 and HO2• range from 1–10 μM/h for all but the most pristine regions of the troposphere. Based on a comparison of these published field and modeling results with the measured aqueous phase H2O2 photoformation rates reported here, we conclude that aqueous phase photochemistry is a significant and in some cases probably dominant source of H2O2 to tropospheric cloud drops. Theory predicts and experiments confirm that the initial rate of aqueous phase H2O2 photoformation is linearly dependent on solar actinic flux. The chromophores responsible for aqueous phase peroxide photoformation have not been identified, but there is evidence that organic chromophores are responsible for the H2O2 photoformation reported here.

An investigation was carried out to identify some of the oxidants formed during the sunlight photolysis of authentic cloud and fog water samples from North Carolina, Virginia, New York, Washington, and Oregon. Constituents (chromophores) present in the atmospheric water samples absorb solar ultraviolet radiation, and thereby initiate formation of peroxyl radicals (RO2· and HO2·), and singlet molecular oxygen, O2(1Δg). In the sunlit cloud and fog waters studied, (1) photostationary state total concentrations of unspeciated peroxyl radicals ranged from 1 to 30 nanomolar (nM), and (2) photostationary state concentrations and aqueous‐phase photochemical formation rates of O2(1Δg) ranged from 27 to 1100 femtomolar (fM) and 5 to 200 nanomolar/second (nM/s), respectively. The chromophores responsible for initiating these photochemical reactions have not yet been identified. Although these studies contain uncertainties, we conclude that aqueous‐phase photochemical reactions are a significant, and in some cases dominant, source of peroxyl radicals and O2(1Δg) in tropospheric cloud and fog drops. This represents a heretofore unrecognized source of these oxidants in atmospheric water drops and the first discussion and report of their detection in authentic condensed phases of the atmosphere.

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Evidence is presented for the photochemical formation of singlet molecular oxygen (1O2) in air-saturated aqueous solutions of several sunscreen active ingredients using sunlight-range illumination. This is of significance because (1) 1O2 is known to be cytotoxic, and (2) there have been several reports of toxic effects associated with the use of some sunscreens; most notably, with p-aminobenzoic acid (PABA). Illuminated aqueous solutions of PABA, 2-ethylhexyl p-(dimethylamino)benzate (ODPABA), 2-hydroxy-4-methoxybenzophenone (BZ3), 2,2'-dihydroxy-4-methoxybenzophenone (BZ8), 2-ethylhexyl 2-cyano-3,3-diphenylacrylate (OCR), 2-ethylhexyl p-methoxycinnamate (OMC), and 2-ethylhexyl salicylate (OCS) were evaluated individually for 1O2 formation. Furfuryl alcohol (FFA), a well-known chemical trap for 1O2, was added to each of the aqueous sunscreen solutions. The FFA was consumed when solutions of PABA, ODPABA, OMC, and OCR were illuminated, but no loss of FFA other than by direct photolysis occurred in solutions of BZ3, BZ8, or OCS. There was also no significant loss of FFA in any of these solutions kept in the dark. Further evidence for the formation of 1O2 in illuminated aqueous sunscreen solutions is provided by the results of experiments in which individual solutions containing sunscreen active ingredients and FFA that were diluted with D2O exhibited an increased rate of FFA consumption while the addition of azide ion (N3-) reduced the rate of FFA consumption. Continuous sunlight-range illumination of aqueous PABA solutions produced significantly higher steady-state concentrations of 1O2 than in solutions containing any of the other sunscreen active ingredients evaluated. The substituted benzophenone compounds (BZ3 and BZ8) and the salicylate-based compound (OCS) not only appear to produce no 1O2, but they also appear to produce no other reactive oxidant species that are capable of consuming FFA. This indicates that BZ3, BZ8, and OCS may be peferable, from the standpoint of toxic oxidant formation, for use as sunscreen active ingredients when compared to the other compounds evaluated in this study.

Gas-to-drop partitioning of hydrogen peroxide and its precursor, the hydroperoxyl radical (HO2.), has been considered the predominant or sole source of hydrogen peroxide in atmospheric water drops. However, atmospheric water can absorb solar ultraviolet radiation, which initiates the photoformation of peroxides (primarily hydrogen peroxide). Measurements of peroxide photoformation rates in authentic atmospheric water samples demonstrate that aqueous-phase photochemical reactions are a significant, and in some cases dominant, source of hydrogen peroxide to cloud and fog drops. This additional source could significantly change the current understanding, and hence, the models, of sulfuric acid deposition because hydrogen peroxide is the limiting reagent in the dominant pathway for the oxidation of sulfur dioxide to sulfuric acid in the troposphere over eastern North America.