Characteristics of atmospheric Total Gaseous Mercury (TGM) observed in urban Nanjing, China

摘要

Long-term continuous measurements of total gaseous mercury (TGM = gaseouselemental mercury (GEM) + reactive gaseous mercury (RGM)) were conductedsimultaneously along with meteorological variables and a suite of tracegases at an urban site in Nanjing, China from 18 January to 31 December2011. Measurements were conducted using a high resolution mercury vaporanalyzer (Tekran 2537B) with 5-min time resolution. The averageconcentration of TGM was 7.9 ± 7.0 ng m?3 with a range of0.8–180 ng m?3 over the study period. TGM concentrations followed a typicallognormal pattern dominated by a range of 3–7 ng m?3, which wassignificantly higher than the continental background values(~1.5 ng m?3) in Northern Hemisphere. The mean seasonal TGM concentrationsdecreased in the following order: summer, spring, fall, and winter. Thisseasonal pattern was quite different from measurements at most other sitesaround the world. We attributed high monthly average concentrations to there-volatilization of deposited mercury during the warm season due to hightemperatures and greater solar radiation. Previous modeling studiessuggested that Nanjing and the surrounding region have the largest Chinesenatural emissions during the summer. Positive correlations betweentemperature, solar radiation, and TGM concentration combined with nocorrelation between CO and TGM in summer provide a strong indication thatnatural sources are important in Nanjing while most sharp peaks were causedby anthropogenic sources. TGM concentrations in Nanjing exhibited anoticeable diurnal pattern with a sharp increase after sunrise and peak ofgreater than 8 ng m?3 during 7–10 a.m. local time. Further,seasonally averaged diurnal cycles of TGM exhibited considerably differentpatterns with the largest variation in spring and insignificant fluctuationsin winter. Using HYSPLIT backwards trajectories from six clusters, it wasindicated that the highest TGM concentrations, 11.9 ng m?3, was derivedfrom local air masses. The cleanest air masses, with an average TGMconcentration of 4.7 and 5.9 ng m?3, were advected from the north viafast transport facilitated by sweeping synoptic flows.