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

Abstract

Long-term continuous measurements of total gaseous mercury (TGM = gaseous elemental mercury (GEM) + reactive gaseous mercury (RGM)) were conducted simultaneously along with meteorological variables and a suite of trace gases at an urban site in Nanjing, China from 18 January to 31 December 2011. Measurements were conducted using a high resolution mercury vapor analyzer (Tekran 2537B) with 5-min time resolution. The average concentration of TGM was 7.9 ± 7.0 ng m⁻³ with a range of 0.8–180 ng m⁻³ over the study period. TGM concentrations followed a typical lognormal pattern dominated by a range of 3–7 ng m⁻³, which was significantly higher than the continental background values (~1.5 ng m⁻³) in Northern Hemisphere. The mean seasonal TGM concentrations decreased in the following order: summer, spring, fall, and winter. This seasonal pattern was quite different from measurements at most other sites around the world. We attributed high monthly average concentrations to the re-volatilization of deposited mercury during the warm season due to high temperatures and greater solar radiation. Previous modeling studies suggested that Nanjing and the surrounding region have the largest Chinese natural emissions during the summer. Positive correlations between temperature, solar radiation, and TGM concentration combined with no correlation between CO and TGM in summer provide a strong indication that natural sources are important in Nanjing while most sharp peaks were caused by anthropogenic sources. TGM concentrations in Nanjing exhibited a noticeable diurnal pattern with a sharp increase after sunrise and peak of greater than 8 ng m⁻³ during 7–10 a.m. local time. Further, seasonally averaged diurnal cycles of TGM exhibited considerably different patterns with the largest variation in spring and insignificant fluctuations in winter. Using HYSPLIT backwards trajectories from six clusters, it was indicated that the highest TGM concentrations, 11.9 ng m⁻³, was derived from local air masses. The cleanest air masses, with an average TGM concentration of 4.7 and 5.9 ng m⁻³, were advected from the north via fast transport facilitated by sweeping synoptic flows.