Reactive and particulate mercury in the Asian marine boundary layer

Abstract

The variability of atmospheric mercury in elemental, reactive, and particulate forms has been studied at a remote site (Cape Hedo Observatory, CHO) at Okinawa Island (Japan) March 23 to May 2, 2004, downwind of the major Asian source regions. Under prevailing meteorological conditions, episodes of higher levels of atmospheric mercury and other atmospheric species are observed at CHO. The mean (±1 σ) concentrations of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (PHg) are 2.04 ± 0.38 ng m −3, 4.5 ± 5.4 pg m −3 and 3.0 ± 2.5 pg m −3, respectively. In Asian outflow the combined contribution of RGM and PHg constitutes less than 1% of the GEM in the boundary layer, which indicates that most mercury export in the marine boundary layer is due to the GEM form, and direct outflow of RGM and PHg is very low. While the data from Okinawa suggest minimal export of RGM and PHg, this does not preclude greater export of these species at higher elevations. Based on the correlations of PHg and submicron aerosol mass (SAM), we found a ΔPHg/ΔSAM ratio of 0.20 μg g −1 ( R = 0.58, p < 0.01), which we believe to be characteristic of East Asian industrial aerosols during outflow. A diurnal variation is observed in RGM with a peak near noon. Using a rate constant of 9 × 10 −14 cm 3 molecule −1 s −1 for the OH oxidation rate (Sommar, J., Gardfeldt, K., Stromberg, D., Feng, X., 2001. A kinetic study of the gas phase reaction between the hydroxyl radical and atomic mercury. Atmospheric Environment 35, 3049–3054; Pal, B., Ariya, P.A., 2004a. Gas-phase HO center dot-initiated reactions of elemental mercury: kinetics, product studies, and atmospheric implications. Environmental Science and Technology 38 (21), 5555–5566.) and a typical OH concentration of 1–5 × 10 6 cm −3 would result in RGM production rates of 0.6–3.0 pg m −3 h −1. Although OH may not be the sole oxidant, this is consistent with the observed change in concentration during daytime of 1.4 ± 1.5 pg m −3 h −1. A significant correlation is found between GEM and CO; GEM and SAM; and PHg and SAM. Lower ΔSAM/ΔCO and ΔGEM/ΔCO are observed for transport events with rainfall and for air parcels remaining in the mixed layer. Back trajectory analysis along with the correlation study suggests that the air from China has a higher GEM concentration compared to the air coming from southern Japan.