基于车载多轴差分吸收光谱技术对城市NO2污染分布和排放评估研究

Study of the Urban NO2 Distribution and Emission Assessment B ased on Mobile MAX-DOAS Observations

二氧化氮(NO2)在大气光化学中发挥着重要作用,不仅参与了对流层臭氧(O3)的催化生成,同时也能促进生成二次气溶胶。NO2作为交通运输和工业过程中的重要排放产物,通常也被视为一种评估人为污染源排放的指示物,因此,开展城市NO2分布与排放研究对于城市大气污染管控与治理具有重要意义。2018年1月和2月期间,基于车载多轴差分吸收光谱技术(MAX-DOAS)对衡水市区开展了4次走航观测实验,获取了衡水市区环城路上对流层NO2垂直柱浓度(VCD)的空间分布,其浓度范围为0.89×10^15~56.33×10^15 molecule·cm^-2,均值在22.42×10^15~30.20×10^15 molecule·cm^-2。观测结果表明衡水市NO2污染源主要分布在城外东南方向的工业聚集区,以及市区环城路东部的立交桥路段;而城市西部和北部则较为干净,当风场来自该区域会对污染源区起到一定的清洁作用,可使源区NO2浓度降低20%以上。航测期间进行了站点对比观测,综合两者的观测结果评估了衡水市东部污染区域的相对贡献,其NO2含量比西部洁净区域高出了30.1%~61.9%、贡献值高7.89×10^15~13.32×10^15 molecule·cm^-2。将NO 2城市分布与WRF模式模拟的气象数据相结合,可计算出目标区域的NO2本地排放通量为0.86×10^24 molecule·s^-1,该结果相对较低,一方面说明衡水市区相对于其他研究区域,其NO2污染源并非在集中在市区内部;另一方面是因为本次实验研究区域的面积仅有50 km^2,远小于其他研究的城市区域范围。对于实验测得的衡水市区输出总通量,其中96.16%来源于外部传输,3.84%为本地排放造成,进一步证明了衡水市NO2主要污染源位于城外。通过实验期间衡水市区的后向轨迹气团与OMI卫星的NO2平均结果可看出,衡水市除城市东部和东南部的本地污染源外,也受到了北部的保定、廊坊和西北部的石家庄等地的污染传输影响。总体来看,车载多轴差分吸收光谱技术对于城市NO2等污染气体的源区确认、污染贡献评估和排放通量计算方面有较出色的应用前景。

Nitrogen dioxide(NO 2)plays a vital role in atmospheric photochemistry.It participates in the catalytic formation of tropospheric ozone(O 3)and also contributes to the formation of secondary aerosols.As an important emission product in transportation and industrial processes,NO 2 is usually regarded as a proper indicator of the intensity of the anthropogenic emission.Therefore,research on urban NO2 distribution and emissions is very important for urban air pollution control.During January and February 2018,we conducted 4 times mobile measurements based on Multi-Axis Differential Optical Absorption Spectroscopy(MAX-DOAS)in the outer route of urban Hengshui.Furthermore,the spatial distribution of the tropospheric NO2 vertical columndensity(VCD)ranges from 0.89×10^15~56.33×10^15 molecule·cm^-2,besides the mean value of each measurement is from 22.42×10^15 to 30.20×10^15 molecule·cm^-2.It can be found the NO2 pollution sources in Hengshui are mainly near the external urban factory cluster in the southeast direction and the overpass section in the east of the outer route.The western and northern areas in Hengshui are relatively clean.If the wind comes from this area,it will play a certain cleaning effect on the pollution source area,which can reduce the NO2 concentration in the source area by more than 20%.During the mobile measurements,we also conducted stationary MAX-DOAS observations.Combined with both measurement results,we evaluated the relative contribution of pollution sources in eastern Hengshui.Combined with both measurement results,we evaluated the relative contribution of NO2 pollution sources area(eastern part of Hengshui),which is 30.1%~61.9%higher than the clean area(western part)and contributes more than 7.89×10^15~13.32×10^15 molecule·cm^-2.With the supplementary of meteorological information simulated by the WRF model,we can calculate the NO2 emission flux in urban Hengshui,which is 0.86×10^24 molecules·s^-1.This result is relatively lower than other cities in previous studies,which might be caused by two factors:one is the pollution source of Hengshui is not concentrated in the urban area;the other one is due to the research area is only 50 km^2 in this study,which is much smaller than the urban area of other studies.For the measured total flux of Hengshui,we found 96.16% is from transportation,and 3.84% is caused by city emission,which indicates that the main pollution source of NO2 in Hengshui City is not located in the inner city.Through the mean value of the OMI tropospheric NO2 with the backward trajectories of air mass during this campaign,we found that Hengshui was also affected by pollution transmission from northern regions(such as Baoding,Langfang)and northwest regions(such as Shijiazhuang).In general,the mobile MAX-DOAS has a good application prospect for urban pollution control,such as for finding the pollution source location,estimating the contribution,and calculating the emission flux of urban areas.