Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

Abstract

Abstract. The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO 2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOSChem. Both model approaches reproduce the diurnal trends in OH and HO 2 . Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8 % at midday at the Cape Verde Atmospheric Observatory), while the global model exhibits a small increase in OH at the Cape Verde Atmospheric Observatory (by 0.6 % at midday) but overall shows a decrease in the global annual mass-weighted mean OH of 4.5 %. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO 2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen-catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model, and overall the global impact of the longer timescale response (reduced primary production due to lower O 3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO 2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole.

Exploring Oxidation in the Remote Free Troposphere: Insights From Atmospheric Tomography (ATom)

“…Third, the uptake of HO 2 and OH on aerosol was added for a few model runs, assuming an uptake coefficient of 0.2 and a loss of HO x to the aerosol particles. Fourth, the halogen chemical mechanism of Stone et al () was added to the photochemistry for a few model runs. BrO measurements were made on ATom‐3 and ATom‐4 and were typically below 0.5 pptv.…”

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

“…Apart from O 3 , N 2 O 5 and inorganic radicals, halogen activation can also be initiated by triplet excited states of lightabsorbing organic compounds (Tinel et al, 2014;Jammoul et al, 2009). Typically referred to as brown carbon (BrC; Laskin et al, 2015), organic compounds absorbing in the UV-A-VIS range are ubiquitously present in atmospheric aerosols.…”

“…The concentration of iodide and bromide in sea spay aerosol particles may reach 10 −6 M (Pechtl et al, 2007;Baker, 2004Baker, , 2005 and 8 × 10 −3 M (Herrmann et al, 2003), respectively. Using the abovementioned concentration and a rate coefficient of the reaction between a typical sensitizer triplet state and iodide of 5×10 9 M −1 s −1 (Tinel et al, 2014), we calculate that iodine activation may reach 2.5×10 −7 M s −1 . In the absence of diffusion limitations, this leads to a short reactive lifetime of the order of seconds for iodide in the aqueous phase.…”

Impact of halogen chemistry on summertime air quality in coastal and continental Europe: application of the CMAQ model and implications for regulation

“…An extremely high level of BrO, ∼ 100 pptv, was observed over the Dead Sea (Matveev et al, 2001;Holla et al, 2015). CMAQ is not able to reproduce such a high level of BrO due to the lower bromide content in typical ocean water (which was used in the present study for the Dead Sea) compared to the exceptionally high bromide content in the Dead Sea (Tas et al, 2006;Sarwar et al, 2015). Global measurements show that the IO levels observed by ground-based campaigns were generally between 0.2 and 2.4 pptv, while those by ship measurement were ∼ 3.5 pptv (Saiz-Lopez and von Glasow, 2012).…”