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 HO2 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 GEOS-Chem. Both model approaches reproduce the diurnal trends in OH and HO2. 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 HO2 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 O3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO2 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.
Highlights
Halogen chemistry in the troposphere influences budgets of O3, HOx (OH and HO2) and NOx (NO and NO2) (von Glasow et al, 2004; Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Schmidt et al, 2016; Sherwen et al, 2016a, b), affects the oxidation state of atmospheric mercury (Holmes et al, 2006, 2010), and impacts aerosol formation
The introduction of halogen chemistry, using differential optical absorption spectroscopy (DOAS) measurements of bromine monoxide (BrO) and iodine monoxide (IO) (Saiz-Lopez et al, 2006) to constrain the model, increased the modelled OH concentrations by up to 15 % and decreased HO2 by up to 30 % owing to reactions of HO2 with XO radicals to form HOX which subsequently photolysed to X + OH (Sommariva et al, 2006)
There is little difference in the radical budgets between SOS1 and SOS2. This box modelling study is consistent with previous studies (Kanaya et al, 2002, 2007; Bloss et al, 2005a; Sommariva et al, 2007; Whalley et al, 2010; Mahajan et al, 2010a; Stone et al, 2012) in that it implies that halogen chemistry is likely to increase the OH concentration of the marine boundary layer as it enhances the HO2 to OH conversion through the production of HOBr and HOI
Summary
Halogen chemistry in the troposphere influences budgets of O3, HOx (OH and HO2) and NOx (NO and NO2) (von Glasow et al, 2004; Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Schmidt et al, 2016; Sherwen et al, 2016a, b), affects the oxidation state of atmospheric mercury (Holmes et al, 2006, 2010), and impacts aerosol formation Observationally constrained box model simulations suggest that halogens in the troposphere will increase OH concentrations, primarily because of a change in the HO2 to OH ratio occurring as a result of reactions of halogen oxides (XO) with HO2 to produce a hypohalous acid (HOX) which photolyses to give an OH radical and a halogen atom (Kanaya et al, 2002, 2007; Bloss et al, 2005a; Sommariva et al, 2006, 2007; Whalley et al, 2010). The GEOS-Chem simulations, which incorporate chlorine, bromine and iodine chemistry, show a reduction in global tropospheric ozone concentration of 18.6 %, compared to simulations with no halogen chemistry, a reduction in the global mean OH of 8.2 % to a concentration of 1.28 × 106 cm−3 and a resulting increase in global methane lifetime of 10.8 % to 8.28 years (Sherwen et al, 2016b). We evaluate the impact of halogens on the concentrations of oxidants in the two modelling frameworks and consider the impact of halogen chemistry on global oxidising capacity
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