OH, HO2 and RO2 Radical and OH Reactivity Observations during the Summertime in Beijing: High In-Situ Ozone Production and Evidence of a Missing OH Source

Lisa Whalleya,b), Eloise Slatera), Robert Woodward-Masseya), Chunxiang Yea), Leigh Crilleyc), Louisa Kramerc), William Blossc), Tuan Vuc), James Hopkinsd,e) Rachel Dunmored), Jacqueline Hamiltond), James Leed,e), Freya Squiresd) Alistair Lewisd,e) and Dwayne Heard a,b)

a) School of Chemistry, University of Leeds, Leeds, UK

b) National Centre for Atmospheric Science, School of Chemistry, University of Leeds, Leeds, UK

c) School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK

d) Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK

e) National Centre for Atmospheric Science, Department of Chemistry, University of York, York, UK

Despite substantial reductions in primary emissions of pollutants in China over the past decade, concentrations of the secondary pollutant, ozone, still frequently exceed air quality threshold limits in urban areas during the summertime.

We will present measurements of OH, HO2 and RO2 radicals and OH reactivity made in central Beijing at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, close to the North 4th ring road in May and June 2017 which formed the summer phase of ‘An Integrated Study of AIR Pollution PROcesses’.

Elevated levels of O3 (>100 ppbv) were regularly observed. NO concentrations were elevated during the morning but often decreased to below the instrument limit of detection during the afternoon hours when the ozone concentrations peaked. Biogenic emissions influenced the chemistry at the site, with several ppbv of isoprene measured during the afternoons.

The OH measurements were made using the FAGE technique, equipped with an inlet pre injector (IPI) which provides an alternative method to determine the instrument background signal by injecting a scavenger to remove ambient OH and ensures an artefact-free OH measurement. Elevated levels of OH were observed, with a mean peak OH concentration of 1.2×107 molecule cm-3 at noon; but with OH concentrations reaching up to 2.5×107 molecule cm-3 on some days. Mean peak HO2 concentrations of ~3×108 molecule cm-3 and total RO2 of ~1.2×109 molecule cm-3 were recorded, with maximum concentrations of 1.0×109 molecule cm-3 and 4×109 molecule cm-3 observed for HO2 and RO2 respectively, suggesting significant in situ ozone production.

A comparison of the artefact-free OH observations with steady state calculations, constrained to the total OH reactivity measurement and known OH precursors that were measured alongside OH, highlights a significant missing daytime OH source under low [NO], with the steady state OH concentrations approximately a factor of two lower than the OH concentrations observed at noon. The magnitude of this missing OH source is similar to the unexplained OH concentrations reported from other studies in China under low NO conditions as well as studies made in rainforests, implying that the uncertainty in the oxidation chemistry determined from observations in forested regions is relevant to the chemistry in urban regions also.