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

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.