A Large Burst of Hydroxyl Radical Formation in Cloud Water: Contributions of Particulate Matter Components

Hydroxyl radical (OH) reactions in cloud water play key roles in secondary organic aerosol formation and DMS oxidation. We collected aerosol samples (PM4) at an urban receptor site in Claremont, California during summer and at a site in Fresno, California with substantial biomass burning aerosol during winter. Filters were extracted in small quantities of water at pH 3.5, simulating cloud water formation. Samples were analyzed for particle mass, OH generation in the presence of near UV light, soluble trace metals, soluble Fe(II) and Fe(III), quinones and biomass burning aerosol (BBA) content. OH formation was characterized by an initial spike in formation lasting for 1-3 minutes with a formation rate of ~ (0.2 - 1)×10^-8 M×s^-1, followed by a second much slower phase of (0.1 - 10)×10^-11 M×s^-1. This newly discovered initial burst of OH formation will be the dominant source of OH in cloud and fog drops under many conditions. For Claremont, Soluble speciated iron was only 22 ± 7 % of the soluble Fe_ICP and was approximately evenly divided between Fe(II)_fzn and Fe(III)_fzn. For Fresno, Fe_fzn was mostly in its reduced form. For Claremont, the highest concentrations of Fe_fzn came from the urban area; high Fe_ICP came both from city and mountains and the mountains were the dominant source of Cu. OH formation activity for the Claremont samples was strongly correlated with mass and soluble (ICP) Fe and Cu but not with ferrozine iron. Fresno samples containing biomass burning aerosol produced far more OH than other samples, and a multivariate linear regression showed that a combination iron, manganese and BBA were the most significant contributors to OH formation, with r² > 0.9.