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

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

Atmospheric chemistry: Wider role for airborne chlorine

Unexpected chlorine chemistry in the lowest part of the atmosphere can affect the cycling of nitrogen oxides and the production of ozone, and reduce the lifetime of the greenhouse gas methane

Radicals in the marine boundary layer during NEAQS 2004: A model study of day-time and night-time sources and sinks

This paper describes a modelling study of several HOx and NOx species (OH, HO2, organic peroxy radicals, NO 3 and N2O5) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V Brown as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO3] and [N 2O5] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30g€"50%, on average. The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO3 and N 2O5 on fog droplets and on aerosol, dry deposition of NO3 and N2O5, gas-phase hydrolysis of N 2O5 and reactions of NO3 with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N2O5 via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N2O5 (I3N 2O5)>1×10-2, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO3 gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS)

Ozone production in remote oceanic and industrial areas derived from ship based measurements of peroxy radicals during TexAQS 2006

During the Texas Air Quality Study II (TexAQS 2006) campaign, a PEroxy Radical Chemical Amplifier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO2+SRO2). Day-time mixing ratios of HO2+SRO2 between 25 and 110 ppt were observed throughout the study area - the Houston/Galveston region and the Gulf coast of the U.S. - and analyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess radical sources and sinks in the region. The measurements of HO 2+SRO2 were used to calculate the in-situ net photochemical formation of ozone. Measured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted industrial areas. The results are consistent with previous studies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochemical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were analyzed in relation to local wind direction and VOC reactivity to understand the relationship between ozone formation and local VOC emissions. The measurements of HO 2+SRO2 made during the R/V Brown TexAQS 2006 cruise indicate that ozone formation is NOx-limited in the Houston/Galveston region and influenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photooxidation products, such as formaldehyde

High levels of nitryl chloride in the polluted subtropical marine boundary layer

The cycling of halogen compounds in the lower atmosphere is poorly understood. It is known that halogens such as chlorine, bromine and iodine are converted from halides, which are relatively inert, to reactive radicals. These reactive radicals can affect ozone production and destruction, aerosol formation and the lifetimes of important trace gases such as methane, mercury and naturally occurring sulphur compounds. However, the processes by which halides are converted to reactive halogens are uncertain. Here, we report atmospheric measurements of nitryl chloride, an active halogen, along the southeast coastline of the United States and near Houston, Texas. We show that the main source of nitryl chloride is the night-time reaction of dinitrogen pentoxide with chloride-containing aerosol. The levels observed are much greater than earlier estimates based on numerical models and are sufficiently large to affect oxidant photochemistry in areas where nitrogen oxides and aerosol chloride sources coexist, such as urban areas and ship engine exhaust plumes