Sources and sinks of hydroxyl radical in sea-salt particles

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Abstract

[1] We have examined the photochemical formation of hydroxyl radical (OH) in aqueous extracts of supermicron sea-salt particulate matter (SS PM) collected from the coast of northern California. All extracts formed OH during illumination, indicating that this process is widespread in sea-salt particles. Scaling extract results to SS PM conditions reveals that in situ rates of OH photoformation are typically 100–1000 μM hr−1 in midlatitude sea-salt particles (summer, midday, 88% relative humidity). These rates are comparable to calculated rates of partitioning of gas phase OH to the particles and are 3–4 orders of magnitude greater than OH photoformation rates in surface seawater. Photolysis of nitrate was a dominant source of OH in the particle extracts, accounting for an average of 59 ± 25% of its formation. The other sources of OH have not been identified, but photolysis of organic compounds derived from seawater is likely important. The lifetimes of OH in the sea-salt particles are of the order of 10−9–10−8 s and are primarily controlled by reactions with unidentified, but probably organic, compounds. Bromide and chloride are also significant sinks of OH, typically accounting for approximately 25% of its loss. The rapid formation and destruction of OH in sea-salt particles likely significantly affects the chemistry of halides, organic compounds, and other reduced species in SS PM. In turn, these particle reactions probably alter the budgets of gases such as ozone and volatile organic compounds in the marine boundary layer.

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