SIZE DISTRIBUTIONS OF POLYCYCLIC AROMATIC HYDROCARBONS—GAS/PARTICLE PARTITIONING TO URBAN AEROSOLS

PAH size distributions were measured at Mumbai (formerly Bombay), India, to examine PAH partitioning to multimodal urban aerosols. Bimodal size distributions were obtained with a predominance of non-volatile PAH species in the fine mode and semi-volatile PAH species in the coarse mode. We develop size-resolved PAH gas/particle partition coefficients, based on adsorption and absorption theory, and analyse measurements in terms of estimated PAH size distributions in typical urban airsheds dominated by primary and secondary aerosol constituents. Adsorption explains PAH presence in the nuclei and accumulation modes where the aerosol surface area predominates (primary emissions), while absorption explains their predominance in the accumulation mode where absorbing organic matter is available (secondary aerosol constituents in a smoggy airshed). Volatilisation of semi-volatile PAH from nuclei mode aerosols, at enhanced rates (Kelvin effect), and their absorptive partitioning to accumulation mode aerosols, enhanced in a smoggy airshed, would explain previous measurements of semi-volatile PAH predominance in the accumulation mode in Los Angeles. The absence of a predominant PAH accumulation mode in Mumbai measurements is likely from the absence of significant photochemistry and aerosol organic matter. Equilibrium adsorption and absorption are unable to explain coarse particle PAH predominance which would occur by dry deposition scavenging of gaseous and nuclei-mode PAH by soil particles which undergo cyclic re-suspension and deposition in an urban airshed. Coarse particle morphology and adsorption affinity of PAH to mineral particles must be examined to better understand PAH predominance in coarse mode aerosols.