Year‐round records of gas and particulate formic and acetic acids in the boundary layer at Dumont d'Urville, coastal Antarctica

Abstract

Multiple year‐round levels of acetate and formate in gas and aerosol phases were investigated at Dumont d'Urville (DDU, a coastal Antarctic site) by using mist chamber and aerosol filter sampling. Formate and acetate aerosol levels range from <0.5 ppt in winter to 3 ppt in summer. With corresponding gas phase levels of more than a hundred of pptv, formic and acetic acids are mainly (99%) present in the gas phase, representing the 2 major acidic gases before inorganic species (HCl, HNO3 and SO2) there. Mixing ratios of formic acid are minimal from May to August (70 pptv) and increase regularly toward November–February months when levels reach ∼200 pptv. Mixing ratios of acetic acid exhibit a more well‐marked seasonal cycle with values remaining close to 70 pptv from April to October and strongly increase during November–February months (mean value of 400 pptv). These seasonal changes suggest that the 2 carboxylic acids mainly originate from biogenic emissions of the Antarctic ocean whose variations follow the annual cycle of sea ice extent and solar radiation via photochemical production of alkenes from dissolved organic carbon released by phytoplankton. In summer, acetic acid levels show daily variations with maxima at noon and minima at night whereas formic acid levels peaks later in the afternoon. These dial variations in summer suggest that carboxylic acids are rapidly produced during the day and lost at night due to dry deposition on wet surface. It is suggested that the reactions of peroxy acetyl radical produced from propene with HO2 and CH3O2 in these poor NOx environments represent in summer the dominant chemical mechanisms producing acetic acid whereas ozone‐alkene reactions remain of minor importance at that season. Neither ozone‐alkene reactions nor aqueous phase HCHO oxidation can explain the summer levels of formic acid. In winter the long range transport of alkenes emitted at more temperate oceanic regions and reactions with ozone could account for the observed level of formic acid and possibly of acetic acid.