Formic and Acetic Acids in the Boundary Layer Over the North Atlantic Ocean

Gaseous formic acid (HCOOH) and acetic acid (CH _3COOH), abbreviated as HFo _{rm g} and HAc_ {rm g}, respectively, are significant contributors to the acidity of precipitation on a global scale. They are reactive in the atmosphere in both liquid and gas phases, and are important in the trace-gas chemistry of the remote marine atmosphere. This work describes the first reliable measurements of HFo_{rm g} and HAc_{rm g} in the lower atmospheric boundary layer over the North Atlantic Ocean. Gas-phase organic acid concentrations were measured (60 minute samples every third hour) using an aqueous mist -chamber sampler on a cruise from Iceland to Barbados in the summer of 1988. Particulate- and aqueous-phase acids were also collected. HFo_{rm g} and HAc_{rm g} concentrations exhibited diurnal cycles, ranging from minima near the detection limit (cong2 nmol m ^{-3} for either acid) during the night to maxima of 2.0-9.0 nmol m^{ -3} of either acid at mid-day. Maximum concentrations of both acids increased with decreasing latitude. Continental influences (inordinately high HFo_{ rm g} and HAc_{rm g} concentrations) were not apparent, except when samples were collected in the lee of islands in the Azores. A marine photochemical source of these species is implied by effective residence times <=q 12 hours. The source of HFo_{ rm g} appears to be the heterogeneous conversion of gaseous formaldehyde (CH_2 O) to aqueous HFo within clouds. The uptake of acetaldehyde (CH_3CHO) by cloud water is too slow to account for the observed rate of change of the concentrations of HAc_{rm g} by the same mechanism. The source of HAc _{rm g} remains unknown. Downward entrainment of HFo_{rm g} and HAc_{rm g} from the upper atmosphere may have been an important source of the acids to the lower boundary layer during the early morning hours of some studies. Dry gaseous deposition onto the sea surface was the largest measurable atmospheric removal process for either acid. The net daytime depositional fluxes of HFo _{rm g} and HAc _{rm g} ranged from 600-3000 nmol m^{-2} day ^{-1}, and 100-1500 nmol m ^{-2} day^{ -1} respectively, and were independent of latitude. The carboxylic acid gases may contribute a significant fraction of the organic carbon cycling within the marine atmosphere when compared to the air-sea fluxes of CH _4 or CO. (Abstract shortened with permission of author.).