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

Xu Shihui; Xu Xiaobin; Lin Weili; Wang Ying; He Xinhe; Zhang Hualong

doi:10.11898/1001-7313.20150303

Vol.26, NO.3, 2015

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Article Navigation > Journal of Applied Meteorological Science > 2015 > 26(3): 280-290

Xu Shihui, Xu Xiaobin, Lin Weili, et al. Increased mixing ratio of surface ozone by nighttime convection process over the North China Plain. J Appl Meteor Sci, 2015, 26(3): 280-290. DOI: 10.11898/1001-7313.20150303.

Citation:

Xu Shihui, Xu Xiaobin, Lin Weili, et al. Increased mixing ratio of surface ozone by nighttime convection process over the North China Plain. J Appl Meteor Sci, 2015, 26(3): 280-290. DOI: 10.11898/1001-7313.20150303.

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

DOI: 10.11898/1001-7313.20150303

1.
Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition, Chinese Academy of Meteorological Sciences, Beijing 100081
2.
CMA Meteorological Observation Center, Beijing 100081
3.
Guangdong Meteorological Observatory, Guangzhou 510080

Received Date: 2014-09-30
Rev Recd Date: 2015-02-10
Publish Date: 2015-05-31

Abstract

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 (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.
- enhanced ozone concentration;
- convection process;
- downdrafts;
- the North China Plain

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References(31)

Figures and Tables

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，NO_x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, NO_x and CO on the days with nighttime convection process

(the grey indicates the period with convection process)

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Fig. 2 Variations of O₃, θ_se 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

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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)

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图 4 2013年8月4日20:00地面 (a) 和850 hPa (b) 高度θ_se(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 θ_se(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)

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图 5 2013年6—9月固城站不同对流过程附近时刻θ_se垂直剖面图

(圆圈表示该对流过程后地面θ_se对应的高度)

Fig. 5 Vertical profile of θ_se over Gucheng at times around four different convection processes during Jun-Sep in 2013

(circles indicate the corresponding θ_se observed on the ground in processes)

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Fig. 6 O₃ profiles observed at Gucheng at dusk on 16 Jun 2013

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Fig. 7 Maxing ratios of O₃ and θ_se observed at Gucheng and in Beijing urban when convection processes occurred on 4 Aug, 15 Aug and 12 Sep in 2013

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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^-9	O₃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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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θ_se/K	后1 h 平均θ_se/K	Δθ_se/K	下沉气流来源高度/hPa
06-25	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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