Organic carbon and non-refractory aerosol over the remote Southeast Pacific: oceanic and combustion sources
1Department of Oceanography, University of Hawaii , Honolulu, HI, USA
2National Center for Aerosol Research, Boulder, CO, USA
3Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
Abstract. Submicron aerosol physical and chemical properties in remote marine air were measured from aircraft over the Southeast Pacific during VOCALS-REx in 2008 and the North Pacific during IMPEX in 2006, and aboard a ship in the Equatorial Pacific in 2009. A High Resolution – Particle Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) measured non-refractory submicron aerosol composition during all campaigns. Sulfate (SO4) and organics (Org), during VOCALS and the cruise show lower absolute values than those reported for previous "clean air" studies. In the marine boundary layer, average concentrations for SO4 were 0.52 μg m−3 for the VOCALS region and 0.85 μg m−3 for the equatorial region while average Org concentrations were 0.10 and 0.07 μg m−3, respectively. Campaign average Org/SO4 ratios were 0.19 (VOCALS) and 0.08 (Equatorial Pacific), while previous studies report "clean marine" Org/SO4 ratios between 0.25 and 0.40, and in some cases as high as 3.5. CO and black carbon (BC) measurements over the Southeast Pacific provided sensitive indicators of pollution, and were used to identify the least polluted air, which had average concentrations of SO4 and Org of 0.14 and 0.01 μg m−3, respectively, with an average Org/SO4 of 0.10. Furthermore, under cleanest MBL conditions, identified by CO below 60 ppbv, we found a robust linear relationship between Org and combustion derived BC concentrations between 2 and 15 ng m−3, suggesting little to no marine source of submicrometer Org to the atmosphere over the Eastern South Pacific. This suggests that identification of Org in clean marine air may require a BC threshold below 4 ng m−3, an order of magnitude lower than has been used in prior studies. Data from IMPEX was constrained to similar clean air criterion, and resulted in an average Org/SO4 ratio of 0.19.