Semivolatile behaviour of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley, RSA)

Abstract

In this study aerosol samples from the South African savannah were analyzed for their polar organic constituents. Samples were collected with a front/back-up filter tandem system of quartz fibre filters (dual filter strategy). In all samples (n = 15) dicarboxylic acids and a variety of phthalates, aldehydes and monocarboxylic acids were observed. Oxalic acid was the dominating compound with an average amount of 79.2 ng m (-3) on the front filter and 11.3 ng m (-3) on the back-up filter. The presence of significant concentrations of dicarboxylic acids on the back-up filter was rather unexpected. There are two possible sources to explain the presence of individual compounds on the back-up filter - particle penetration through the front filter or adsorption of compound parts from the gas phase. Interpretation of the data indicates that the dicarboxylic acid concentrations on the back-up filters appear to be caused by the absorption of gaseous organic species. Dicarboxylic acids semivolatilic behaviour is evident with these results. This conclusion refutes the commonly held view that dicarboxylic acids in the atmosphere were associated with the aerosol phase only. Additionally, it was found that the distribution of dicarboxylic acids between the gas and particle phase in the atmosphere is not only dependent on their vapour pressures. The actual gas phase concentration appears to be more determined by the chemical properties of the particles than by pure physical influences. Surprisingly, malonic acid exhibits an anomaly, as it does not show a semi volatile tendency.