Origin of oxidized mercury in the summertime free troposphere over the southeastern US

Abstract

Abstract. We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and 15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1.49 ± 0.16 ng m−3, model: 1.51 ± 0.08 ng m−3), as well as the vertical profile of THg. The majority (65 %) of observations of oxidized mercury (Hg(II)) were below the instrument's detection limit (detection limit per flight: 58–228 pg m−3), consistent with model-calculated Hg(II) concentrations of 0–196 pg m−3. However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212 ± 112 pg m−3, model: 67 ± 44 pg m−3). The highest Hg(II) concentrations, 300–680 pg m−3, were observed in dry (RH < 35 %) and clean air masses during two flights over Texas at 5–7 km altitude and off the North Carolina coast at 1–3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg(0) + Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg(0) + Br, result in 1.5–2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8–1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II).

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

“…A combination of ETMEP and CARIBIC observations over Germany resulted in a uniform TM and GEM distribution in the free troposphere during summer and TM concentrations close to those measured at ground level were found on six overflights of the CARIBIC aircraft in April, June, and September. A similar vertical dis- tribution was found in North America during winter (Brooks et al, 2014) and summer Gratz et al, 2015;Shah et al, 2016). In none of these cases a substantial TM gradient was found inside the free troposphere and the GEM / TM ratio was in the range of 0.95-0.99 in the upper free troposphere, which is a ratio typically found inside the PBL.…”

“…While gaseous elemental mercury (GEM) makes up the vast majority of total atmospheric mercury near the surface (Sprovieri et al, 2016 this issue), recent aircraft-based observations have indicated that there is significant oxidation of mercury occurring in the free troposphere (FT) (Brooks et al, 2014;Lyman and Jaffe, 2012;Jaffe et al, 2014;Gratz et al, 2015;Shah et al, 2016). However, apart from GEM no individual mercury compound has been identified so far and the atmospheric oxidized mercury is an unknown mixture of mercury bound to Br, Cl, OH, O, and NO 2 compounds .…”

“…Furthermore, there were only two OM measurements that indicate the decline above 6000 m and it would also be possible that this peak extended further upwards and was due to a deep stratospheric intrusion. Looking at the correlation coefficient, it can be seen that model (Shah et al, 2016;Gratz et al, 2016). The error bars indicate the 66 % quantile range of the observations in each altitude; the sample size for each altitude is indicated on the y axis of the legend.…”

“…Both were later analyzed in the laboratory. In addition, 19 flights were flown in June and July 2013 mostly over the southeastern USA at altitudes between 500 and 7000 m during the NOMADSS (Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks) campaign Shah et al, 2016). Here, oxidized mercury was calculated based on a differential method using two Tekran 2537B instruments, one of which was equipped with GOM trap (quartz wool or ion-exchange membrane) using the University of Washington Detector for Oxidized Hg Species (DOHGS) (Lyman and Jaffe, 2012;Ambrose et al, 2015).…”

Sensitivity model study of regional mercury dispersion in the atmosphere

“…In BRCHEM1 the underestimation is evident (ratio BRCHEM1 / observations 0.77) while in BRCHEM2 the model results and observations are closer (ratio 0.90). Thus, increasing the Br concentration gives model results closer to observations, as shown also in Shah et al (2016), who tripled the Br and BrO concentrations in GEOS-CHEM model (with respect to Parrella et al, 2012, where an underestimation of 30 % was found for BrO concentrations compared to the global mean obtained by satellite observations) to model the NOMADSS field campaign (Gratz et al, 2015). Differently to the Shah et al (2016) study, RGM reduction is not implemented here; for the purposes of this paper it was necessary to standardise the experiments as far as possible.…”