Increased Mixing Ratio of Surface Ozone by Nighttime Convection Process over the North China Plain

Surface ozone and other reactive gases are observed at Gucheng (39°08′57″N, 115°44′02″E) in Hebei Province of China from June to September in 2013. There are 10 cases with rapid increases of the mixing ratio of surface ozone, and sharp decreases of the mixing ratios of nitric oxides and carbon monoxide when convection processes occurs at night. The mixing ratio of surface ozone mostly increases from less than 30×10^-9 to 60×10^-9-80×10^-9 within less than 1 hour and stays at a higher level during the night and the next morning than that on undisturbed days. Such phenomenon cannot be explained by photochemical production. The increase rate of surface ozone level is not correlated with wind speed. Therefore, the change in ozone cannot be attributed to horizontal transport of polluted airmass.To understand the phenomenon, meteorological data from Gucheng and from ECMWF reanalysis are analyzed. Surface pseudo-equivalent potential temperature (θ_se) for each case is calculated from the simultaneously measured meteorological data. In all nighttime cases of convection process, the surface θ_se values decrease dramatically within a short time, coinciding with the steep increases of the ozone level and the wind speed. This suggests that the mixing ratio of surface ozone is enhanced by descending air from aloft. The convective process occurs in the warm area ahead of the front in most cases except for once near the cold front. These clearly indicate that convective downdrafts transport air with higher ozone and lower θ_se from upper atmosphere to the surface layer. With the vertical profiles of θ_se values calculated from ECMWF reanalysis data, levels of origins of downdrafts are estimated as from around 500-800 hPa. Vertical profiles of ozone observed using an unmanned aircraft near the station show that ozone mixing ratio over the boundary layer at dusk is higher than 60×10^-9, supporting the view that the increased mixing ratio of surface ozone during and after the nighttime convection process is caused by air descending from the lower to mid free-troposphere. The phenomena with ozone enhancement is also observed at an urban station in Bejiing. In most cases when Gucheng and Beijing urban are impacted by the same convective systems, and ozone and θ_se at both sites show similar trends. All above implies that ozone mixing ratio maintains around 60×10^-9-80×10^-9 in the mid and lower free-troposphere over the North China Plain in summer and early autumn, and ozone increase by convective downdrafts is able to impact a large area of the North China Plain. Compared with other places, convection process causes larger ozone increase, which may exert stronger impact on the atmospheric environment.