Abstract
Wintertime urban air pollution in many global megacities is characterised by episodic rapid increase in particulate matter concentrations associated with elevated relative humidity - so-called haze episodes, which have become characteristic of cities such as Beijing. Atmospheric chemistry within haze combines gas- and condensed-phase chemical processes, leading to the growth in secondary species such as sulphate aerosols. Here, we integrate observations of reactive gas phase species (HONO, OH, NOx) and time-resolved aerosol composition, to explore observational constraints on the mechanisms responsible for sulphate growth during the onset of haze events. We show that HONO abundance is dominated by established fast gas-phase photochemistry, but the consideration of the additional formation potentially associated with condensed-phase oxidation of S species by aqueous NO2 leading to NO2- production and hence HONO release, improves agreement between observed and calculated gas-phase HONO levels. This conclusion is highly dependent upon aerosol pH, ionic strength and particularly the parameterisation employed for S(iv) oxidation kinetics, for which an upper limit is derived.
Highlights
Atmospheric chemical processing leads to the removal of primary pollutants and determines the rate of production of secondary species, including ozone and many components of aerosol particles
PM is formed of a complex chemical mixture including both directly emitted and chemically produced components; understanding the relationship between these underpins the accurate identi cation of direct and indirect PM sources, formulating the most effective and efficient clean air policy. This challenge is emphasised by the air pollution climatology of Beijing and the North China Plain, where seasonal haze events characterised by episodic PM2.5 excursions to 500 mg mÀ3 and above, impact the health and wellbeing of around 400 million people
The rate of formation of sulphate aerosol from SO2, driven by chemical oxidation, is observed to increase as PM2.5 levels rise, and recent studies into haze formation have found that the rate of production of sulphate aerosol is signi cantly accelerated in megacities such as Beijing.[1]
Summary
Atmospheric chemical processing leads to the removal of primary pollutants and determines the rate of production of secondary species, including ozone and many components of aerosol particles. We analyse in situ observations collected during the UK–Chinese Air Pollution and Human Health (APHH) campaign in central Beijing during haze episodes in November–December 2016, which constrain the gas-phase production and removal of HONO, and the observed rate of formation of sulphate.
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