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Origin of oxidized mercury in the summertime free troposphere over the southeastern US
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 ngm(-3), model: 1.51 +/- 0.08 ngm(-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 pgm(-3)), consistent with model-calculated Hg(II) concentrations of 0-196 pgm(-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 pgm-3, model: 67 +/- 44 pgm(-3)). The high-est Hg(II) concentrations, 300-680 pgm(-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)
BrO and inferred Bry profiles over the western Pacific: Relevance of inorganic bromine sources and a Bry minimum in the aged tropical tropopause layer
We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box model to infer total gas-phase inorganic bromine (Br) over the tropical western Pacific Ocean (tWPO) during the CONTRAST field campaign (January-February 2014). The observed BrO and inferred Br profiles peak in the marine boundary layer (MBL), suggesting the need for a bromine source from sea-salt aerosol (SSA), in addition to organic bromine (CBr). Both profiles are found to be C-shaped with local maxima in the upper free troposphere (FT). The median tropospheric BrO vertical column density (VCD) was measured as 1.6×1013 molec cm-2, compared to model predictions of 0.9×1013 molec cm-2 in GEOS-Chem (CBr but no SSA source), 0.4×1013 molec cm-2 in CAM-Chem (CBr and SSA), and 2.1×1013 molec cm-2 in GEOS-Chem (CBr and SSA). Neither global model fully captures the C-shape of the Br profile. A local Br maximum of 3.6 ppt (2.9-4.4 ppt; 95 % confidence interval, CI) is inferred between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective tropical tropopause layer (TTL) to the aged TTL, gas-phase Br decreases from the convective TTL to the aged TTL. Analysis of gas-phase Br against multiple tracers (CFC-11, H2O-O3 ratio, and potential temperature) reveals a Br minimum of 2.7 ppt (2.3-3.1 ppt; 95 % CI) in the aged TTL, which agrees closely with a stratospheric injection of 2.6 ± 0.6 ppt of inorganic Br (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Br increases to 6.3 ppt (5.6-7.0 ppt; 95 % CI) in the stratospheric >middleworld> and 6.9 ppt (6.5-7.3 ppt; 95 % CI) in the stratospheric >overworld>. The local Br minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-Chem, and suggests a more complex partitioning of gas-phase and aerosol Br species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA-derived gas-phase Br) are needed to explain the gas-phase Br budget in the upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere-Lower Stratosphere aerosols. The total Br budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas-phase Br species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. Such simultaneous measurements are needed to (1) quantify SSA-derived Br in the upper FT, (2) test Br partitioning, and possibly explain the gas-phase Br minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Br to the lower stratosphere.Peer Reviewe