Empirical Studies of Oxalic Acid Aerosol in Summer Cloud Water and fine PM in the Northeastern U.S

Organic compounds are one of the least well-understood components of atmospheric particulate matter. Of special interest are highly polar organic compounds (HPOC), which we define as those soluble in methanol or having a polarity index of up to 6.6 on the Snyder scale. HPOC are a ubiquitous component of PM and as a class are likely to act as cloud condensation nuclei. Several recent studies (field, lab and model simulations) have suggested the importance of photochemical conversion of isoprene to oxalic acid in the atmosphere. However, cloud water modeling studies have shown insufficient emissions of isoprene fully account for observed concentrations. Recent multiyear studies of cloud water from Whiteface Mountain in upstate New York showed anion imbalance for cloud water, which could be accounted for by organic acids such as oxalic acid. Organic compounds as a whole are known to be a significant mass fraction in cloud droplets. In light of the charge and mass balance problems and our own observations, we suggest that oxalic acid may be formed within the atmospheric water and liquid interface. This production mechanism has not been extensively investigated as a significant pathway. We present and compare concentrations of HPOC, and oxalic acid in particular, from two sampling campaigns: a fine particular matter (PM2.5) collection campaign in and around the New York City metro area, and cloud water samples from upstate New York. The PM summer samples were collected in 2002 and 2006 in urban and rural locations. The cloud water samples were collected in summer 2010 from the top of Whiteface Mountain. Both sample sets were analyzed with PFBHA and BSTFA derivatization and gas chromatography/mass spectrometry. From our quantitative results, we explore the possibility for an additional mechanism of oxalic acid formation from redox processes at the liquid water and vapor phase interface. We discuss the possibility of gaseous CO2 the precursor for oxalic acid and oxalate ion based on the correlation of oxalic acid with local CO2 measurements and the differences in oxalic acid ambient mass concentrations between urban and rural sites. This redox processing of atmospheric CO2 would account for a new removal mechanism within the global biogeochemical carbon cycle.