华北平原夜间对流天气对地面O_3混合比抬升效应

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

2013年6—9月在河北省固城站观测到多次夜间对流性天气伴随地面O_3混合比快速抬升的过程,并引起次日清晨到中午O_3混合比升高。大多数对流过程中,O_3混合比在半小时内升高至60×10^(-9)~80×10^(-9),同时NO_x等反应性气体混合比下降,θ_(se)值降低,说明下沉气流将高空气团带到地面,造成了O_3混合比的升高。通过再分析资料得到下沉气团基本来源于对流层中下层,这一结论与当地进行的一次飞机观测结果吻合。多数对流过程中固城站和北京城区地面O_3混合比和θ_(se)值有相同的变化趋势和程度。根据观测结果,推测华北地区在夏季和初秋时,对流层中下层存在O_3高值区,混合比约为60×10^(-9)~80×10^(-9)。对流性天气对地面O_3抬升的影响区域与对流系统的影响范围有关,可达到中尺度范围。华北地区光化学污染严重,对流性天气引起的地面O_3混合比抬升程度比较强,对环境的影响值得关注。

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 than30×10^(-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 around60 × 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.