Evidence for strong, widespread chlorine radical chemistry associated with pollution outflow from continental Asia

Abstract

The chlorine radical is a potent atmospheric oxidant, capable of perturbing tropospheric oxidative cycles normally controlled by the hydroxyl radical. Significantly faster reaction rates allow chlorine radicals to expedite oxidation of hydrocarbons, including methane, and in polluted environments, to enhance ozone production. Here we present evidence, from the CARIBIC airborne dataset, for extensive chlorine radical chemistry associated with Asian pollution outflow, from airborne observations made over the Malaysian Peninsula in winter. This region is known for persistent convection that regularly delivers surface air to higher altitudes and serves as a major transport pathway into the stratosphere. Oxidant ratios inferred from hydrocarbon relationships show that chlorine radicals were regionally more important than hydroxyl radicals for alkane oxidation and were also important for methane and alkene oxidation (>10%). Our observations reveal pollution-related chlorine chemistry that is both widespread and recurrent, and has implications for tropospheric oxidizing capacity, stratospheric composition and ozone chemistry.

Non-methane hydrocarbon (C<sub>2</sub>–C<sub>8</sub>) sources and sinks around the Arabian Peninsula

“…The atmospheric ratio of i-butane to n-butane has been frequently reported to be between 0.3 and 0.8 for a wide range of environmental conditions (e.g. Baker et al, 2016;Rossabi and Helmig, 2018). For the AQABA campaign, the isomeric ratio was on average 0.63±0.44 (median = 0.56), with 82.4 % of the samples between 0.3 and 0.8.…”

“…: 1), and average (930 : 1) OH-to-Cl ratios derived from the EMAC model. used method to determine whether Cl chemistry has occurred is the examination of VOC ratios and inter-relationships (Jobson et al, 1994;Rudolf et al, 1997;Baker et al, 2016). Figure 13 shows the correlation between an OH-reactive pair (i-butane to propane) and a Cl-reactive pair (i-butane to nbutane).…”

“…This variability-lifetime relationship has been applied in characterizing the impact of chemistry at various locations (Jobson et al, 1999;Williams et al, 2000Williams et al, , 2001; Bartenbach et al, 2007;Helmig et al, 2008). In addition, the presence of specific oxidants can be detected by the use of NMHC oxidative pairs (Parrish et al, 2007;Baker et al, 2016;Young et al, 2014). This is particularly important for Cl radicals as there is no instrumentation available for direct Cl measurements, although measurements of ClNO 2 indicate a Cl source to be present.…”

Tropospheric OH and stratospheric OH and Cl concentrations determined from CH4, CH3Cl, and SF6 measurements

“…For example, the reaction with chlorine radicals (Cl) with CH 4 has been shown to be significant in Asian pollution outflow. 30 The aforementioned paper reported the presence of radicals in the ratio [Cl]:[OH] of 9~16 Cl:10 3 OH, and since the reaction rate of CH 4 with Cl is~32 times faster than that with OH (for CH 3 Cl it is only ten times faster), 31 an apparent overestimation of OH eff by about 30-50% can ensue. However, such high Cl radical concentrations have only been seen under very specific conditions (lofted coastal pollution outflow) and so this cannot explain the OH eff difference in the global tropospheric data.…”

An aircraft gas chromatograph–mass spectrometer System for Organic Fast Identification Analysis (SOFIA): design, performance and a case study of Asian monsoon pollution outflow

“…Halogenated organic compounds are capable of destroying both tropospheric and stratospheric ozone (Molina and Rowland, 1974;Read et al, 2008;Saiz-Lopez et al, 2012;Wang et al, 2015), and thus altering the oxidative capacity of the atmosphere (Parrella et al, 2012;Baker et al, 2016), with prominent examples being the chlorofluorocarbons (CFCs) and the hydrochlorofluorocarbons (HCFCs). Their industrial replacements, i.e.…”