Urban airshed/watershed impacts on the terrestrial flux and atmospheric speciation of mercury.

摘要

This research shows that mercury flux varies considerably over diurnal and seasonal change for paved, grass, and bare soil surfaces. Overall, soil flux was 23x the flux from grass and 320x the flux from pavement. Grass and pavement surfaces were shown to be significant mercury sinks (deposition as high as -2.31 ng/m2-hr). However, grass and pavement became more important to emission during the winter when soil mercury emission decreased. Positive relationships were found between soil mercury flux and solar radiation, surface moisture, evapotranspiration, surface and ambient temperature; a negative relationship existed between soil flux and relative humidity. Grass and pavement surface flux magnitude showed little change with seasons and no significant relationship with seasonal change in meteorological parameters. However, semi-controlled experiments suggest that surface characteristics play a major role in mercury flux from grass. Artificial irrigation of street runon and paved surfaces produced an increase in flux (1 ng/m2-hr for pavement; 6 ng/m 2-hr for street runon) immediately following irrigation. This increase resulted from pre-existing mercury forms on these surfaces. Spatial mercury flux studies showed that significant spatial difference in mercury flux may be present in urban areas as a result of local changes in soil type and landuse.; This research also shows that atmospheric mercury speciation can be significantly different depending on landuse and may vary considerably across airsheds over regional scales. By comparing two airsheds that differed significantly in landuse while having similar local meteorological conditions, we suggest that typical urban airshed impacts account for 17% of the time series variance for all mercury species in an urban setting (Tuscaloosa). Tuscaloosa's concentrations were, on average, 2x the background northern hemisphere concentration for gaseous elemental mercury and 4x for greater for reactive gaseous and fine particulate mercury. Statistical analyses show that meteorology affects the time series variation of atmospheric mercury species in different ways. Wind speed was the meteorological parameter with the greatest impact on time series variation of all atmospheric mercury species. When modeling the time series variation of atmospheric mercury based on the dynamics of meteorology, it is not advised to lump atmospheric mercury species.