Current understanding of the driving mechanisms for spatiotemporal variations of atmospheric speciated mercury: a review

Abstract

Abstract. Atmospheric mercury (Hg) is a global pollutant and thought to be the main source of mercury in oceanic and remote terrestrial systems, where it becomes methylated and bioavailable; hence, atmospheric mercury pollution has global consequences for both human and ecosystem health. Understanding of spatial and temporal variations of atmospheric speciated mercury can advance our knowledge of mercury cycling in various environments. This review summarized spatiotemporal variations of total gaseous mercury or gaseous elemental mercury (TGM/GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) in various environments including oceans, continents, high elevation, the free troposphere, and low to high latitudes. In the marine boundary layer (MBL), the oxidation of GEM was generally thought to drive the diurnal and seasonal variations of TGM/GEM and GOM in most oceanic regions, leading to lower GEM and higher GOM from noon to afternoon and higher GEM during winter and higher GOM during spring–summer. At continental sites, the driving mechanisms of TGM/GEM diurnal patterns included surface and local emissions, boundary layer dynamics, GEM oxidation, and for high-elevation sites mountain–valley winds, while oxidation of GEM and entrainment of free tropospheric air appeared to control the diurnal patterns of GOM. No pronounced diurnal variation was found for Tekran measured PBM at MBL and continental sites. Seasonal variations in TGM/GEM at continental sites were attributed to increased winter combustion and summertime surface emissions, and monsoons in Asia, while those in GOM were controlled by GEM oxidation, free tropospheric transport, anthropogenic emissions, and wet deposition. Increased PBM at continental sites during winter was primarily due to local/regional coal and wood combustion emissions. Long-term TGM measurements from the MBL and continental sites indicated an overall declining trend. Limited measurements suggested TGM/GEM increasing from the Southern Hemisphere (SH) to the Northern Hemisphere (NH) due largely to the vast majority of mercury emissions in the NH, and the latitudinal gradient was insignificant in summer probably as a result of stronger meridional mixing. Aircraft measurements showed no significant vertical variation in GEM over the field campaign regions; however, depletion of GEM was observed in stratospherically influenced air masses. In examining the remaining questions and issues, recommendations for future research needs were provided, and among them is the most imminent need for GOM speciation measurements and fundamental understanding of multiphase redox kinetics.

Impacts of large-scale circulation on urban ambient concentrations of gaseous elemental mercury in New York, USA

“…The median concentration of TGM/GEM in global continental remote areas was 1.6 ng m −3 (180 ppqv), estimated from a large body of measurement studies (Mao et al, 2016), and the background concentration of GEM in the Northern Hemisphere was 1.5-1.7 ng m −3 (168-190 ppqv) (Lindberg et al, 2007). Urban concentrations of GEM/TGM in the US varied between 0.05 and 324 ng m −3 (5.6-36 288 ppqv) (Mao et al, 2016).…”

“…Urban concentrations of GEM/TGM in the US varied between 0.05 and 324 ng m −3 (5.6-36 288 ppqv) (Mao et al, 2016). In comparison, urban concentrations and their temporal variability were larger than rural, remote, and high-elevation concentrations in the Northern Hemisphere (e.g., Kim and Kim, 2001;Feng et al, 2003;Denis et al, 2006;Liu et al, 2007;Peterson et al, 2009;Sprovieri et al, 2010;Zhu et al, 2012;Lan et al, 2012Lan et al, , 2014Chen et al, 2013;Civerolo et al, 2014;Fu et al, 2015;Brown et al, 2015;Mao et al, 2016, and references therein) owing to numerous controlling factors, including anthropogenic and legacy emissions, deposition, meteorology, transport, and atmospheric chemistry (Mao et al, 2016).…”

“…Urban ambient atmospheric TGM/GEM concentrations in Canada on average ranged from 1.7 to 4.5 ng m −3 (190-504 ng m −3 ) (Mao et al, 2016, and references therein). Urban GEM/TGM concentrations in Asia could be an order of magnitude larger than those in the US, Canada, and Europe (Mao et al, 2016, and references therein).…”

Evaluation of CMAQ Coupled With a State‐of‐the‐Art Mercury Chemical Mechanism (CMAQ‐newHg‐Br)

“…Atmosphere Hg speciation is operationally defined as gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate bound mercury (PBM). GEM is relatively inert comprising about 75% of the global atmospheric Hg pool (Gustin et al, ) with lifetime of 0.5–1 year and is considered as the main form of Hg transported over long distances (Driscoll et al, ; Mao et al, ). With relatively short atmospheric lifetimes ranging from hours to weeks, GOM and PBM can be emitted directly or produced from oxidation of GEM (Cole et al, ).…”

A synthesis of research needs for improving the understanding of atmospheric mercury cycling

“…Mercury transformation mechanisms and speciation profiles for Hg formed in and released from flue gases of coal-fired boilers, non-ferrous metal smelters, cement plants, iron and steel plants, waste incinerators and biomass burning were documented in Zhang et al (2016a). Worldwide measurements of Hg spanning the past 4 decades were reviewed in Mao et al (2016) together with the mecha-nisms driving the observed spatiotemporal variations of speciated Hg in various environments including oceans, continents, high elevation, the free troposphere, and low to high latitudes. Current knowledge of Hg dry deposition was reviewed in Wright et al (2016), including dry deposition algorithms used in chemical transport models and at monitoring sites, measurement methods and quantification of dry deposition of oxidized Hg, and Hg in litterfall and throughfall.…”