Abstract
Abstract. Ground-level ozone (O3) pollution has been steadily getting worse in most parts of eastern China during the past 5 years. The non-linearity of O3 formation with its precursors like nitrogen oxides (NOx= NO + NO2) and volatile organic compounds (VOCs) are complicating effective O3 abatement plans. The diagnosis from space-based observations, i.e. the ratio of formaldehyde (HCHO) columns to tropospheric NO2 columns (HCHO / NO2), has previously been proved to be highly consistent with our current understanding of surface O3 chemistry. HCHO / NO2 ratio thresholds distinguishing O3 formation sensitivity depend on regions and O3 chemistry interactions with aerosol. To shed more light on the current O3 formation sensitivity over China, we have derived HCHO / NO2 ratio thresholds by directly connecting satellite-based HCHO / NO2 observations and ground-based O3 measurements over the major Chinese cities in this study. We find that a VOC-limited regime occurs for HCHO / NO2 < 2.3, and a NOx-limited regime occurs for HCHO / NO2 > 4.2. The HCHO / NO2 between 2.3 and 4.2 reflects the transition between the two regimes. Our method shows that the O3 formation sensitivity tends to be VOC-limited over urban areas and NOx-limited over rural and remote areas in China. We find that there is a shift in some cities from the VOC-limited regime to the transitional regime that is associated with a rapid drop in anthropogenic NOx emissions, owing to the widely applied rigorous emission control strategies between 2016 and 2019. This detected spatial expansion of the transitional regime is supported by rising surface O3 concentrations. The enhanced O3 concentrations in urban areas during the COVID-19 lockdown in China indicate that a protocol with simultaneous anthropogenic NOx emissions and VOC emissions controls is essential for O3 abatement plans.
Highlights
Ground-level ozone (O3) is one of the major air pollutants that has negative impacts on human health and can result in eye and nose irritation, respiratory disease, and lung function impairment (Jerrett et al, 2009; Khaniabadi et al, 2017; Huang et al, 2018)
We first evaluate if satellite-based HCHO and NO2 columns can capture the non-linear O3–NO2–HCHO chemistry shown by the Chemistry Land-surface Atmosphere Soil Slab model (CLASS) model
By directly connecting HCHO columns from Ozone Monitoring Instrument (OMI) observations with ground-based measurements of NO2 and O3 from 360 cities across China during May–October from 2016 to 2019 in Fig. 1b, we find that the satellite-based HCHO columns and ground-based NO2 concentrations can capture non-linear O3 chemistry consistent with the CLASS model results
Summary
Ground-level ozone (O3) is one of the major air pollutants that has negative impacts on human health and can result in eye and nose irritation, respiratory disease, and lung function impairment (Jerrett et al, 2009; Khaniabadi et al, 2017; Huang et al, 2018). Duncan et al (2010) combined models and Ozone Monitoring Instrument (OMI) HCHO and NO2 data to show certain ranges of FNR that can be useful for classifying a region into VOC-limited or NOxlimited regime. An FNR in the range of 1–2 should generally be considered indicative of the transitional regime These FNR thresholds defined by Duncan et al (2010) have been widely used for various regions (Choi and Souri, 2015; Jin and Holloway, 2015; Souri et al, 2017; Jeon et al, 2018) and with different satellite instruments (Choi et al, 2012).
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