Temporal variation and long-range transport of gaseous elemental mercury (GEM) over a coastal site of East China

Continuous measurements of gaseous elemental mercury (GEM) play a key role in identifying mercury sources and its behavior in the atmosphere. In order to investigate the characteristics of GEM during the year when Minamata Convention on Mercury has entered into force in China, concentrations of GEM along with other pollutants were continuously measured at a coastal site in Ningbo, China. Hourly mean concentration of GEM at the study site ranged from 0.64 to 13.58 ng m^-3 and showed an annual mean of 2.44 ± 0.95 ng m^-3. The atmospheric GEM showed obvious seasonal variations, with the highest seasonal average concentration in winter (2.62 ± 1.05 ng m^-3) and the lowest value in summer (2.26 ± 0.78 ng m^-3). The monthly variation of GEM/TGM ratios ranged from 72.6% to 98.0% and clear inverse trends for monthly GEM/TGM and O₃ were found during the entire period. Higher O₃ concentration promote the photochemical oxidation of GEM into divalent mercury. Additionally, a high GEM episode that lasted >3 days was analyzed, and it showing high correlation with PM_2.5 and SO₂. The dilution of marine airflow and enhanced oxidation of GEM over sea are important for GEM depletion at the coastal site. Smoothed concentration weighted trajectory (CWT) analysis revealed that the GEM at the study site was mainly impacted by anthropogenic emissions from coastal provinces (Fujian, Jiangsu and Zhejiang) and inland provinces (Jiangxi, Anhui and Hubei). Coordinated inter-regional control on pollutant emissions is essential for the mercury reduction in the study area.