Measurement of dry deposition and air-water exchange of polycyclic aromatic hydrocarbons with the water surface sampler

Recent work has shown that dry deposition and gas exchange of semivolatile organic compounds in the Great Lakes is important. However, there is no gen erally accepted method for collecting particulate dry deposition or gas exc hange. in this work the Water Surface Sampler (WSS) was modified and used i n combination with dry deposition plates to measure the particulate dry dep osition and gas exchange of PAHs. Simultaneous dry deposition and ambient a ir samples were collected between June and October 1995 in Chicago, II. Tot al (particulate + gas) fluxes were measured with the WSS, while particulate fluxes were measured with dry deposition plates. Average Sigma(14)-PAH and particulate Sigma(14)-PAH fluxes were 360 +/- 146 mu g/m(2) d and 144 +/- 60 mu g/m(2) d, respectively. Gas-phase fluxes, determined by subtracting t he dry deposition plate fluxes (particulate) from WSS fluxes (particulate gas), averaged 170 +/- 72 mu g/m(2) d. The measured particulate dry deposi tion and gas exchange fluxes were substantially higher than those previousl y reported for the Great Lakes and nonurban areas, probably due to the sign ificantly higher ambient PAH concentrations found in Chicago. Particulate p hase overall dry deposition velocities calculated using the dry de position fluxes and ambient concentrations averaged 6.7 +/- 2.8 cm/s. This value is higher than values estimated using dry deposition models; however, it is i n good agreement with values determined using similar techniques.