A General Scavenging Rate Constant for Reaction of Hydroxyl Radical with Organic Carbon in Atmospheric Waters

Hydroxyl radical (OH) is an important oxidant in atmospheric aqueous phases such as cloud and fog drops and water-containing aerosol particles. We find that numerical models nearly always overestimate aqueous hydroxyl radical concentrations because they overpredict its rate of formation and, more significantly, underpredict its sinks. To address this latter point, we examined OH sinks in atmospheric drops and aqueous particles using both new samples and an analysis of published data. Although the molecular composition of organic carbon, the dominant sink of OH, is extremely complex and poorly constrained, this sink behaves very similarly in different atmospheric waters and even in surface waters. Thus, the sink for aqueous OH can be estimated as the concentration of dissolved organic carbon multiplied by a general scavenging rate constant [<i>k</i>_C,OH = (3.8 ± 1.9) × 10⁸ L (mol C)⁻¹ s^–1], a simple process that should significantly improve estimates of OH concentrations in atmospheric drops and aqueous particles

Transboundary Secondary Organic Aerosol in Western Japan Indicated by the δ¹³C of Water-Soluble Organic Carbon and the <i>m</i>/<i>z</i> 44 Signal in Organic Aerosol Mass Spectra

The stable carbon isotope ratio (δ¹³C) of low-volatile water-soluble organic carbon (LV-WSOC) was measured in filter samples of total suspended particulate matter, collected every 24 h in the winter of 2010 at an urban site and two rural sites in western Japan. Concentrations of the major chemical species in fine aerosol (<1.0 μm) were also measured in real time by aerosol mass spectrometers. The oxidation state of organic aerosol was evaluated using <i>f</i>₄₄; i.e., the proportion of the signal at <i>m</i>/<i>z</i> 44 (CO₂⁺ ions from the carboxyl group) to the sum of all <i>m</i>/<i>z</i> signals in the organic mass spectra. A strong correlation between LV-WSOC and <i>m</i>/<i>z</i> 44 concentrations was observed, which suggested that LV-WSOC was likely to be associated with carboxylic acids in fine aerosol. Plots of δ¹³C of LV-WSOC versus <i>f</i>₄₄ showed random variation at the urban site and systematic trends at the rural sites. The systematic trends qualitatively agreed with a simple binary mixture model of secondary organic aerosol with background LV-WSOC with an <i>f</i>₄₄ of ∼0.08 and δ¹³C of −17‰ or higher. Comparison with reference values suggested that the source of background LV-WSOC was likely to be primary emissions associated with C₄ plants