Light absorption and the photoformation of hydroxyl radical and singlet oxygen in fog waters

Publication date: September 2017
Source:Atmospheric Environment, Volume 164
Author(s): R. Kaur, C. Anastasio
The atmospheric aqueous-phase is a rich medium for chemical transformations of organic compounds, in part via photooxidants generated within the drops. Here we measure light absorption, photoformation rates and steady-state concentrations of two photooxidants – hydroxyl radical (^•OH) and singlet molecular oxygen (¹O2*) – in 8 illuminated fog waters from Davis, California and Baton Rouge, Louisiana. Mass absorption coefficients for dissolved organic compounds (MACDOC) in the samples are large, with typical values of 10,000–15,000 cm² g-C⁻¹ at 300 nm, and absorption extends to wavelengths as long as 450–600 nm. While nitrite and nitrate together account for an average of only 1% of light absorption, they account for an average of 70% of ^•OH photoproduction. Mean ^•OH photoproduction rates in fogs at the two locations are very similar, with an overall mean of 1.2 (±0.7) μM h⁻¹ under Davis winter sunlight. The mean (±1σ) lifetime of ^•OH is 1.6 (±0.6) μs, likely controlled by dissolved organic compounds. Including calculated gas-to-drop partitioning of ^•OH, the average aqueous concentration of ^•OH is approximately 2 × 10⁻¹⁵ M (midday during Davis winter), with aqueous reactions providing approximately one-third of the hydroxyl radical source. At this concentration, calculated lifetimes of aqueous organics are on the order of 10 h for compounds with ^•OH rate constants of 1 × 10¹⁰ M⁻¹ s⁻¹ or higher (e.g., substituted phenols such as syringol (6.4 h) and guaiacol (8.4 h)), and on the order of 100 h for compounds with rate constants near 1 × 10⁹ M⁻¹ s⁻¹ (e.g., isoprene oxidation products such as glyoxal (152 h), glyoxylic acid (58 h), and pyruvic acid (239 h)). Steady-state concentrations of ¹O2* are approximately 100 times higher than those of ^•OH, in the range of (0.1–3.0) × 10⁻¹³ M. Since ¹O2* is a more selective oxidant than ^•OH, it will only react appreciably with electron-rich species such as dimethyl furan (lifetime of 2.0 h) and substituted polycyclic aromatic hydrocarbons (e.g., 9,10-dimethylbenz[a]anthracene with a lifetime of 0.7 h). Comparing our current Davis samples with Davis fogs collected in the late 1990s shows a decrease in dissolved organic carbon content, similar mass absorption coefficients, lower ^•OH concentrations, but very similar ¹O2* concentrations.



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