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Increased Mixing Ratio of Surface Ozone by Nighttime Convection Process over the North China Plain
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摘要: 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O3混合比快速抬升的过程，并引起次日清晨到中午O3混合比升高。大多数对流过程中，O3混合比在半小时内升高至60×10-9～80×10-9，同时NOx等反应性气体混合比下降，θse值降低，说明下沉气流将高空气团带到地面，造成了O3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O3混合比和θse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O3高值区，混合比约为60×10-9～80×10-9。对流性天气对地面O3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O3混合比抬升程度比较强，对环境的影响值得关注。Abstract: 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 (θse) for each case is calculated from the simultaneously measured meteorological data. In all nighttime cases of convection process, the surface θse 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 θse from upper atmosphere to the surface layer. With the vertical profiles of θse 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 θse 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.
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图 1 2013年6—9月固城站平常日O3混合比平均日变化 (虚线) 及部分对流过程发生日O3，NOx和CO小时均值变化
(图中灰色阴影为每个对流过程的大致时段)
Fig. 1 Average diurnal variation of O3(dashed line) at Gucheng for normal days during Jun-Sep in 2013, and diurnal variations of O3, NOx and CO on the days with nighttime convection process
(the grey indicates the period with convection process)
图 2 2013年6月25日 (a)、7月2日 (b)、8月4日 (c) 与9月12日 (d) 固城站对流过程中O3，θse和风速时间变化
Fig. 2 Variations of O3, θse and wind speed before, during and after convection processes on 25 Jun (a), 2 Jul (b), 4 Aug (c) and 12 Sep (d) in 2013
图 3 2013年8月4日20:00北京大兴雷达PPI回波图 (仰角：1.5°，距离：240 km)(a) 以及ω沿115.5°E垂直剖面图 (单位：Pa·s-1)(b)
Fig. 3 Radar echoes received at Daxing, Beijing at 2000 BT 4 Aug 2013(elevation:1.5°, range:240 km)(a) and vertical profile of ω along 115.5°E (unit:Pa·s-1)(b)
图 4 2013年8月4日20:00地面 (a) 和850 hPa (b) 高度θse(填色) 和风场 (矢量) 分布
(圆圈为雷达回波大致范围，三角形为固城站位置)
Fig. 4 θse(the shaded) and wind (vector) at surface (a) and 850 hPa (b) at 2000 BT 4 Aug 2013
(the circle indicates the range of radar echo, the filled triangle indicates the location of Gucheng)
图 5 2013年6—9月固城站不同对流过程附近时刻θse垂直剖面图
(圆圈表示该对流过程后地面θse对应的高度)
Fig. 5 Vertical profile of θse over Gucheng at times around four different convection processes during Jun-Sep in 2013
(circles indicate the corresponding θse observed on the ground in processes)
图 6 2013年9月16日飞机观测到的O3混合比垂直廓线
Fig. 6 O3 profiles observed at Gucheng at dusk on 16 Jun 2013
图 7 2013年8月4日、8月15日与9月12日固城站与北京城区O3混合比和θse变化图
Fig. 7 Maxing ratios of O3 and θse observed at Gucheng and in Beijing urban when convection processes occurred on 4 Aug, 15 Aug and 12 Sep in 2013
表 1 2013年6—9月固城站对流过程中O3混合比抬升时段与变化特征
Table 1 List of convection processes over Gucheng during Jun-Sep in 2013 and respective changes of O3 level
日期 O3迅速
升高时段抬升前1 h
最小值/10-9抬升后1 h
最大值/10-9O3混合比
变化/10-9最大瞬时
风速/(m·s-1)06-25 18:10—18:20 16.5 72.4 55.9 16.4 07-02 01:50—02:10 2.9 44.3 41.4 12.8 07-31 00:50—01:20 0.6 64.1 63.5 9.8 08-03 00:30—01:30 15.2 68.3 53.1 12.2 08-04 20:00—20:40 29.7 77.1 47.4 20.0 08-07 02:50—03:30 2.6 66.7 64.1 14.7 08-14 00:00—00:10 10.1 67.3 57.2 14.0 08-15 00:00—00:20 2.6 72.6 70.0 14.3 09-12 22:00—22:20 3.0 53.0 50.0 17.9 09-13 21:20—22:10 2.2 47.2 45.0 13.0 
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表 2 2013年6—9月固城站对流过程中下沉气流的特征和来源高度
Table 2 Some features of airmasses from high altitudes descending to Gucheng during June-September in 2013
日期 O3抬升后1 h
最大值/10-9前1 h
平均θse/K后1 h
平均θse/KΔθse/K 下沉气流
来源高度/hPa06-25 72.4 345.3 330.2 -15.1 600～700 07-02 44.3 351.2 340.6 -11.2 500以上 07-31 64.1 355.4 336.3 -19.1 700～800 08-03 68.3 348.4 332.6 -15.8 700～800 08-04 77.1 372.2 333.4 -38.8 600～700 08-07 66.7 368.7 347.1 -21.6 700～800 08-14 67.3 361.9 340.5 -21.4 700～800 08-15 72.6 363.2 345.8 -17.4 700～800 09-12 53.0 334.1 321.3 -12.8 600～700 09-13 47.2 336.2 323.8 -12.4 500～600
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