Abstract
Recently, atmospheric ozone pollution has demonstrated an aggravating tendency in China. To date, most research about atmospheric ozone has been confined near the surface, and an understanding of the vertical ozone structure is limited. During the 2016 G20 conference, strict emission control measures were implemented in Hangzhou, a megacity in the Yangtze River Delta, and its surrounding regions. Here, we monitored the vertical profiles of ozone concentration and aerosol extinction coefficients in the lower troposphere using an ozone lidar, in addition to the vertical column densities (VCDs) of ozone and its precursors in the troposphere through satellite-based remote sensing. The ozone concentrations reached a peak near the top of the boundary layer. During the control period, the aerosol extinction coefficients in the lower lidar layer decreased significantly; however, the ozone concentration fluctuated frequently with two pollution episodes and one clean episode. The sensitivity of ozone production was mostly within VOC-limited or transition regimes, but entered a NOx-limited regime due to a substantial decline of NOx during the clean episode. Temporary measures took no immediate effect on ozone pollution in the boundary layer; instead, meteorological conditions like air mass sources and solar radiation intensities dominated the variations in the ozone concentration.
Highlights
Air pollution has become an increasingly serious environmental problem in China[1,2]
Similar to other significant events held in Beijing, strict pollution control measures were implemented in Hangzhou and its surrounding regions from Aug. 25 to Sep. 6
We retrieve the vertical column densities (VCDs) for tropospheric O3, nitrogen dioxide (NO2) and formaldehyde (HCHO) from Aug. 14 to Sep. 18 based on Ozone Monitoring Instrument (OMI) satellite products
Summary
General signature of O3 and aerosol extinction. Because the field of view between the laser and receiver does not completely overlap, vertical fade zones always exist in lidar observations[23,24,25]. Based on the WRF-Chem results, we further investigated the relationship between O3 and the normalized HCHO and NO2 concentrations in different model layers when the satellite passed over the monitoring site using the same analysis method based on VCDs. The sensitivities always showed a VOC-limited regime under lower HCHO/NO2 ratios and a NOx-limited or transition regime under higher HCHO/NO2 ratios in each model layer (Table S1). To determine the influence of regional transport on the O3 concentration, 1-day air mass back trajectories (BTs) arriving at the lidar site at the bottom (300 m AGL), middle (400 m AGL) and top (500 m AGL) of the lower lidar layer were calculated for every hour using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model Meteorological data from both the Global Data Assimilation System (GDAS) and the WRF-Chem modelling were adopted to drive the HYSPLIT model. Regional transport played a much more important role on O3 in the boundary layer
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