Abstract
Compared with other regions in China, air pollution on the North China Plain (NCP) is serious. Fine particle pollution has been studied in-depth, but there is less research on long-term troposphere ozone (O3) variation. This study focuses on the summer interannual tropospheric O3 variation on the NCP and its influential factors. Our analysis relies on satellite observations (O3, nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and formaldehyde (HCHO), determined as vertical column density of the troposphere) and dynamical processes (El Niño-Southern Oscillation (ENSO), potential vorticity (PV), the quasibiennial oscillation (QBO), and East Asian summer monsoon index (EASMI)). Our results show the vertical column density of tropospheric O3 has a transition from the increasing trend to decreasing trend during the summer of 2005–2016. The summer series of tropospheric O3 show two distinct phases: the first phase (2005–2011), with an average growth rate of 0.55 ± 0.20 DU/yr, and a second phase (2012–2016), with an average reduction rate of 0.16 ± 0.23 DU/yr. The tropospheric NO2 column in the NCP also has a transition from the increasing trend to decreasing trend during the summer of 2005–2016. Tropospheric NO2 and CO column concentrations obtained from satellite observations indicate that emission reductions might be the main cause of the tropospheric O3 decrease. Particularly, the reduction of nitrogen oxides (NOx) is more significant, and NO2 decreased by (0.45 ± 0.11) × 1015 molec·cm−2 per year in summer since 2012. However, tropospheric column HCHO shows an increase of 0.05 × 1015 molec·cm−2 per year during the whole period of 2005 to 2016. An O3-NOx-VOC sensitivity experiment in the NCP showed that the O3 is still in a NOx-saturated state in some heavily polluted cities, although the NOx emissions are decreasing overall. In addition to the chemical reactions, atmospheric dynamic processes also have an effect on tropospheric O3. Finally, we built a model to analyze the contributions of chemical processes and dynamic processes to the tropospheric O3 column in the NCP. For the chemical process variables, 69.73% of the observed trend of tropospheric O3 could be explained by the NO2 tropospheric column. Therefore, the reduction of tropospheric O3 since 2012 is associated with the reduction of NOx. For the dynamical process variables, ENSO, PV, and EASMI can explain 60.64% of the observed trend of tropospheric O3. This result indicates that the atmospheric circulation of the western Pacific Ocean in summer has a significant impact on the interannual trends of tropospheric O3 in the NCP. It is also found that chemical processes had a more important impact on interannual tropospheric O3 than dynamic processes, although the dynamic processes cannot be neglected.
Highlights
Atmospheric ozone (O3) is mostly found within the stratosphere, and tropospheric O3 is approximately one-tenth of the atmospheric column O3 [1]
Tropospheric O3 is the entire column in the troposphere [29, 30]. e validation is from Ozone Monitoring Instrument (OMI)/Microwave Limb Sounder (MLS) and ozonesonde monthly data. e correlation coe cient R between OMI/MLS tropospheric O3 and WOUDC ozonesonde tropospheric O3 is 0.936 for ozonesonde station locations lying between 25°S and 50°N, and the Root Mean Square Error (RMSE) is 3.18 ppbv. e deviation is smaller in the lower latitudes. is result indicates closely similar signatures for seasonal cycles and spatial variability from the comparisons of OMI/MLS tropospheric O3 between the climatology and other data products [44]
Because of the increase in O3 pollution incidents in North China that occurs during most summers, we focus on the situation of the summer period. e summer time series (Figure 2) have shown that the trend of tropospheric O3 is increasing with an average rate of 0.36 ± 0.09 DU/yr over 2005–2016
Summary
Atmospheric ozone (O3) is mostly found within the stratosphere, and tropospheric O3 is approximately one-tenth of the atmospheric column O3 [1]. O3 long-term variations are investigated based on atmospheric compositions obtained from satellite observations. Satellite data have been used to study air pollutants [20, 21]. Atmospheric environmental satellite loads have nadir and limb scan modes. Tropospheric Emission Spectrometer (TES) operates in a combination of limb and nadir mode. E Ozone Monitoring Instrument (OMI), Measurement of Pollution in the Troposphere (MOPITT), and Total Ozone Monitoring Spectrometer (TOMS) are nadir instruments and provide total vertical column (O3, SO2, NO2, HCHO, CO, and CH4). Satellite data of column density for SO2, NO2, and CO are often used to study air pollution directly.
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