Global tropospheric halogen (Cl, Br, I) chemistry and its impact on oxidants

Abstract

Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transport model and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneous chemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model to include mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixing ratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison to surface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytime measurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very large missing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a global mean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global mean tropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane, 8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %, NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozone simulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.

Earth system modeling of mercury using CESM2: part 1. atmospheric model CAM6-Chem/Hg v1.0

“…As the debromination of sea salt aerosol is not included in CAM6-Chem, the model may underestimate the concentrations of Br in the MBL. Indeed, the mean tropospheric Br and BrO concentrations are 0.03 and 0.13 ppt respectively, which are lower than 0.08 and 0.48 ppt in Schmidt et al (2016) but are more consistent with 0.03 and 0.19 ppt simulated by Wang et al (2021). The vertical distribution of zonal mean Br and BrO are shown in Fig.…”

N<sub>2</sub>O<sub>5</sub> uptake onto saline mineral dust: a potential missing source of tropospheric ClNO<sub>2</sub> in inland China

“…We use GEOS-Chem (version 12.9.3) to quantify the effects of ClNO 2 formation due to heterogeneous reaction of N 2 O 5 with saline dust in China. The model, which includes a detailed representation of coupled ozone-NO x -VOC-aerosol-halogen chemistry (Wang et al, 2021), is driven by MERRA2 (the Modern-Era Retrospective Analysis for Research and Applications, Version 2) assimilated meteorological fields from the NASA Global Modeling and Assimilation Office (GMAO) with a native horizontal resolution of 0.25 • × 0.3125 • and 72 vertical levels from the surface to the mesosphere. Our simulation was conducted over East Asia (10 • S-55 • N, 60-150 • E) at the native resolution with dynamical boundary conditions from a 4 • × 5 • global simulation.…”

Global Impact of Lightning‐Produced Oxidants

“…The ozonesonde observations are obtained from the World Ozone and Ultraviolet Data Center (WOUDC, http://www.woudc.org), with data only from Electrochemical Concentration Cell and no WOUDC-suggested correction factors applied (X. Wang et al, 2021). We show that the base run tends to underestimate ozone in free troposphere by 3-10 ppbv in Northern Hemisphere (90°-60°N, 60°-30°N, and 30°N-Eq).…”

“…Figure S4 in Supporting Information shows comparison on annual mean vertical profile of ozone mixing ratios in six zonal bands between ozonesonde observations and three model simulations (base run, H10_O10, and H100_O100) for the year of 2016. The ozonesonde observations are obtained from the World Ozone and Ultraviolet Data Center (WOUDC, http://www.woudc.org/), with data only from Electrochemical Concentration Cell and no WOUDC‐suggested correction factors applied (X. Wang et al., 2021). We show that the base run tends to underestimate ozone in free troposphere by 3–10 ppbv in Northern Hemisphere (90°–60°N, 60°–30°N, and 30°N–Eq).…”