Chemical reactivity and long-range transport potential of polycyclic aromatic hydrocarbons – a review

Polycyclic aromatic hydrocarbons (PAH) are of considerable concern due to their well-recognised toxicity and especially due to the carcinogenic hazard which they present. PAH are semi-volatile and therefore partition between vapour and condensed phases in the atmosphere and both the vapour and particulate forms undergo chemical reactions. This article briefly reviews the current understanding of vapour-particle partitioning of PAH and the PAH deposition processes, and in greater detail, their chemical reactions. PAH are reactive towards a number of atmospheric oxidants, most notably the hydroxyl radical, ozone, the nitrate radical (NO3) and nitrogen dioxide. Rate coefficient data are reviewed for reactions of vapour for lower molecular weight PAH with these species as well as for heterogeneous reactions of the higher molecular weight compounds. Whereas the data for reactions of the 2-3-ring PAH vapour is quite extensive and generally consistent, such data is mostly lacking for the 4-ring PAHs and the heterogeneous rate data (5 and more rings), which are, dependent on the substrate type and reaction conditions, are less comprehensive. The atmospheric reactions of PAH lead to the formation of oxy and nitro derivatives, here reviewed, too. Finally, the capacity of PAH for long range transport and the results of numerical model studies are described. Research needs are identified.