Ozone pollution in the North China Plain spreading into the late-winter haze season

Abstract

Surface ozone is a severe air pollution problem in the North China Plain, which is home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HOx) that can overcome the radical titration caused by high emissions of nitrogen oxides (NOx) from fuel combustion. Ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-h average (MDA8) ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone is driven by HOx radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NOx emissions. Formaldehyde is produced by oxidation of reactive volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an ozone-producing regime in January–February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter–spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NOx emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe ozone pollution events into the winter–spring season.

Harmonized Emissions Component (HEMCO) 3.0 as a versatile emissions component for atmospheric models: application in the GEOS-Chem, NASA GEOS, WRF-GC, CESM2, NOAA GEFS-Aerosol, and NOAA UFS models

“…It serves as a re-gridding and subsetting tool for these data. This has in particular enabled the FlexGrid capability in GEOS-Chem where regional nested domains are selected at runtime and all input data are processed for these domains (Li et al, 2021;Shen et al, 2021).…”

“…In GEOS-Chem 'Classic', HEMCO is used not only for emissions but as a general data broker to read and process all model input data, including meteorological fields and initial conditions. This has enabled in particular the FlexGrid algorithm to run GEOS-Chem on any custom nest selected at runtime (Li et al, 2021). This implementation of HEMCO is also used in the recently developed coupling of GEOS-Chem with the Weather Research and Forecasting (WRF) model (WRF-GC; Lin et al, 2020, Feng et al, 2021.…”

In situ Ozone Production is highly sensitive to Volatile Organic Compounds in the Indian Megacity of Delhi

“…CC BY 4.0 License. (Li et al, 2021). In a NOx-limited regime, reducing NOx emissions is the most efficient approach for reducing O3 production.…”

Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during cold-season haze events

“…Nitrous acid (HONO) is usually considered as the dominant source of OH radical in the early morning (Alicke et al, 2003) and during cold days (Hendrick et al, 2014;Kim et al, 2014;Lu et al, 2019) that is mainly generated from direct traffic emissions and heterogeneous conversion of NO2 on ground and aerosol surfaces (Xue et al, 2020;Liu et al, 2021). In addition, photolysis of carbonyls may also greatly influence HOx (HOx = OH + HO2) and ozone (O3) production at lower NOx concentrations during cold seasons (Edwards et al, 2014;Li et al, 2021).…”

“…For example, Li et al (2021) found that HOx radicals from photolysis of formaldehyde (HCHO) accelerated the ozone upsurge during COVID-19 in Beijing in response to substantial NOx reductions.…”