Cloud chemistry at the Puy de Dôme: variability and relationships with environmental factors

[摘要] The chemical composition of cloud water was investigated during thewinter-spring months of 2001 and 2002 at the Puy de Dôme station (1465 mabove sea level, 45°46′22′′ N, 2°57′43′′ E) in an effort tocharacterize clouds in the continental free troposphere. Cloud droplets weresampled with single-stage cloud collectors (cut-off diameter approximately 7 µm)and analyzed for inorganic and organic ions, as well as totaldissolved organic carbon. Results show a very large variability in chemicalcomposition and total solute concentration of cloud droplets, ranging from afew mg l^-1 to more than 150 mg l^-1. Samplings can be classified inthree different categories with respect to their total ionic content andrelative chemical composition: background continental (BG, total solutecontent lower than 18 mg l^-1), anthropogenic continental (ANT, totalsolute content from 18 to 50 mg l^-1), and special events (SpE, totalsolute content higher than 50 mg l^-1). The relative chemicalcomposition shows an increase in anthropogenic-derived species(NO₃^-, SO₄^2- and NH₄⁺) from BG to SpE, and adecrease in dissolved organic compounds (ionic and non-ionic) that areassociated with the anthropogenic character of air masses.We observed a high contribution of solute in cloud water derived from thedissolution of gas phase species in all cloud events. This was evident fromlarge solute fractions of nitrate, ammonium and mono-carboxylic acids incloud water, relative to their abundance in the aerosol phase. Thecomparison between droplet and aerosol composition clearly shows the limitedability of organic aerosols to act as cloud condensation nuclei. The strongcontribution of gas-phase species limits the establishment of directrelationships between cloud water solute concentration and LWC that areexpected from nucleation scavenging.

[学科分类] 大气科学