Interactions of HOx with aerosols; Nitric acid-HONO-NOx cycling via particulate nitrate photolysis

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• Daniel Moon and Dr Chunxiang Ye, University of Leeds
• Monday 24 October 2016, 14:15-15:15
• Pfizer Lecture Theatre, Department of Chemistry.

If you have a question about this talk, please contact Peter Gallimore.

Heterogeneous production and loss of HOx by airborne TiO2 particles and implications for climate change mitigation strategies and Heterogeneous loss of HO2 from Cu-Fe redox coupling in inorganic salt aerosols – Daniel Moon

The seminar presents findings from two laboratory studies that both focus on heterogeneous reactions between HOx and aerosols. The first study investigates the heterogeneous reaction between airborne sub-micron TiO2 particles and HO2 radicals using an aerosol flow tube and the FAGE (fluorescence assay by gas expansion) technique. Experiments performed in dark conditions at the most stratospherically relevant conditions within this study (RH = 11.1%) determined γHO2 = (2.08 ± 0.11) × 10-2. A positive dependence of γHO2 with RH was observed which showed a correlation between γHO2 and the number of monolayers of water adsorbed on the particle surface. Modelling of stratospheric chemistry (TOMCAT) showed that uptake of HO2 by injected TiO2 aerosols would have a negligible effect to stratospheric concentrations of HO2 and O3. However, when experiments were illuminated with near-UV light (365 nm) significant HO2 production was observed. This is the first time HO2 radicals from aerosols have been directly observed. The concentrations were dependent on light flux, RH and total particle surface area. Work is still under-going to show its effect to stratospheric chemistry.

Secondly, conventionally the product of HO2 uptake by aerosols was thought to be H2O2 , however, Mao (2013) proposed an alternative reaction mechanism where a fast electron transfer reaction between Cu(I) and Fe(III) occurs resulting in the production of H2O . This has important implications to atmospheric modelling as H2O2 is a far less efficient sink of HOx than H2O affecting predicted tropospheric concentrations of OH, O3, CO and other species. This study investigates the heterogeneous reaction between HO2 and (NH4)2SO4 aerosols doped with varying ratios of Cu(II) and Fe(II) ions totalling in concentrations of 10-3 and 10-4 M in the atomiser solution using an aerosol flow tube and FAGE . Results from the study show a significant enhancement of the uptake coefficient from what would be expected if the two ions were reacting in isolation as the Cu(II)/Fe(II) mole fraction increases from 0 to 0.15 suggesting likely interaction between the two transition metal ions. Work is currently underway using the KM-SUB model to predict the uptake coefficient with the varying ratio of Cu(II) and Fe(II) ions using conventional reaction mechanisms and the reaction mechanism proposed by Mao to interpret results.

Field, laboratory and model evidences on cycling chemistry of particulate nitrate-HONO-NOx – Chunxiang Ye

Nitrogen oxides are essential to the formation of secondary atmospheric aerosol and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than in observations. A ‘renoxification’ process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign in the North Atlantic Ocean and find evidences for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Master Chemical Mechanism model calculations suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions.

This talk is part of the Centre for Atmospheric Science seminars, Chemistry Dept. series.

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