Seasonal and diurnal variations of atmospheric mercury across the US determined from AMNet monitoring data

Abstract. Speciated atmospheric mercury observations collected over the period from 2008 to 2010 at the Environmental Protection Agency and National Atmospheric Deposition Program Atmospheric Mercury Network sites (AMNet) were analyzed for its spatial, seasonal, and diurnal characteristics across the US Median values of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) at 11 different AMNet sites ranged from 148–226 ppqv (1.32–2.02 ng m−3), 0.05–1.4 ppqv (0.47–12.4 pg m−3) and 0.18–1.5 ppqv (1.61–13.7 pg m−3), respectively. Common characteristics of these sites were the similar median levels of GEM as well as its seasonality, with the highest mixing ratios occurring in winter and spring and the lowest in fall. However, discernible differences in monthly average GEM were as large as 30 ppqv, which may be caused by sporadic influence from local emission sources. The largest diurnal variation amplitude of GEM occurred in the summer. Seven rural sites displayed similar GEM summer diurnal patterns, in that the lowest levels appeared in the early morning, and then the GEM mixing ratio increased after sunrise and reached its maxima at noon or in the early afternoon. However, sites in Utah (UT96, UT97) and New York (NY95) showed a distinctly different pattern, with the lowest mixing ratios appearing in the afternoon and the highest mixing ratios at night. Unlike GEM, GOM exhibited higher mixing ratios in spring and summer. The largest diurnal variation amplitude of GOM occurred in spring for most AMNet sites. GOM diurnal minima appeared before sunrise and maxima appeared in the afternoon, and the variation in magnitude for all seasons at most monitoring sites fell in the range of 0 to 2 ppqv, except the Utah sites (up to 5 ppqv). The increased GOM mixing ratio in the afternoon indicated a photochemically driven oxidation of GEM resulting in GOM formation. PBM exhibited diurnal fluctuations in summertime instead of wintertime, although the PBM mixing ratio in summer was not as high as in winter. The summertime PBM diurnal pattern displayed a daily maximum in the early afternoon and lower mixing ratios at night, implying photochemical production of PBM in summer. The marine sea salt aerosol uptake of GEM and GOM was not apparent in the PBM data collected at coastal sites, with PBM being higher at inland sites.