Potential impact of rainfall on the air-surface exchange of total gaseous mercury from two common urban ground surfaces

The impact of rainfall on total gaseous mercury (TGM) flux from pavement and street dirt surfaces was investigated in an effort to determine the influence of wet weather events on mercury transport in urban watersheds. Street dirt and pavement are common urban ground surfaces that concentrate many substances (eroded soil, leaf and vegetation litter, automobile debris, industrial atmospheric fallout) which can contain elevated mercury concentrations. In this study, the primary analyses included (i) observing the time series flux of TGM from pavement and street dirt following surface wetting and (ii) determining if wet deposition provides a fresh source of mercury that is available for release (emission) when applied to these surfaces. Application of de-ionized water (DI) and rainwater both induced an immediate 65% increase in TGM emission from pavement (from 0.5 to 1.4 ng m−2 h−1 [based on averages]). For street dirt, an immediate 70% increase in emission was induced following DI water application (from 3.0 to 9.0 ng m−2 h−1 [based on averages]) and an immediate 30% increase in emission following rainwater application (from 4.5 to 6.5 ng m−2 h−1 [based on averages]). Both surfaces showed continuous elevated release of TGM following the initial water application stage. There was a decrease in emission as the pavement surface dried. Despite the difference in immediate TGM emission from street dirt using both solutions, statistical evaluation indicated there was no prolonged difference. This suggests that mercury in rainwater was not available for re-emission when applied to these surfaces, at least for the time frame studied (2 h after water application). Therefore, it is likely that the elevated TGM emission following water application resulted primarily from pre-existing mercury. Removal of pre-existing mercury by water application followed a zero order process for both surfaces; however, removal rates were much different for each surface (k = 0.26 ng m−2 min−1 for street dirt; k = 0.03 ng m−2 min−1 for pavement). Results from laboratory surface washing experiments revealed only 0.1% of all available surface-bound mercury on pavement was removed by surface emission 90 min after a simulated light rainfall event (0.13 cm of rainfall).

Research highlights
► Wetting the surfaces of pavement and street dirt increased total gaseous mercury (TGM) emission.
► Enhanced TGM emission following simulated rainfall is a result of pre-existing surface-bound mercury.
► The quantity of mercury that is emitted to the atmosphere from pavement after wetting is much smaller than the mercury load that is delivered by rainfall.
► Removal rates of pre-existing surface-bound mercury are highly different for pavement and street dirt.