SEASONALLY DEPENDENT SIZE DISTRIBUTIONS OF ALIPHATIC AND POLYCYCLIC AROMATIC-HYDROCARBONS IN URBAN AEROSOLS FROM DENSELY POPULATED AREAS

Aerosol samples corresponding to well-defined cold/warm seasons in the atmosphere of Barcelona have been collected and size-fractionated during stable atmospheric conditions. A predominant hydrocarbon pattern derived from gasoline and diesel vehicular traffic emissions has been identified in most particle fractions. In quantitative terms, these hydrocarbons exhibit a predominant occurrence in the submicron fractions, particularly the <0.5-mum fraction, which is likely characteristic of early aging stages after gas-phase emission of combustion residues to the atmosphere. This predominant occurrence contrasts with the bimodal log-normal distribution of particles found in the city. The seasonal fluctuations of these hydrocarbons reflect a strong decrease in the warm period. The changes in fine particulate (<1.5 mum) content of n-alkanes and polycyclic aromatic hydrocarbons (PAH) show that these fluctuations are due to atmospheric dilution and, in the case of PAH, photochemical degradation. Another seasonal effect is related to the hydrocarbon content in the individual particulate fractions, showing a preferential association of n-alkanes with molecular weight (MW) >254 and PAH with MW >202 for the 0.56-0.96- and <0.5-mum size fractions in the cold and warm periods, respectively. The qualitative and quantitative composition of these hydrocarbon distributions is described, and the observed changes are discussed in terms of particle-size dependency and seasonal effects.