哨兵5号欧洲业务二氧化硫产品在中国的准确性评估

Evaluation of the accuracy of the Sentinel-5 Precursor operational SO2 products over China

2017年11月欧洲空间局发射的哨兵5号搭载了新一代光谱仪(对流层大气监测仪,TROPOMI),可实现包括二氧化硫(SO2)在内的对流层大气成分高空间分辨率全球监测.欧洲空间局提供的SO2数据(TROPOMI欧洲业务SO2产品)对中国大气污染监测有重要支撑作用,然而数据有效性尚需验证.通过对比地基多轴差分吸收光谱仪数据(MAX-DOAS)和生态环境部中国环境监测总站(国控站点)数据,发现该数据明显高估中国北方SO2浓度.相对于地基MAX-DOAS站点高估61%~140%,且相关性小于0.5,相对于国控站点平均高估约54.6%.进一步研究表明,反演算法是造成显著性差异的原因.为了检验TROPOMI高光谱数据的适用性,自主研发的最优化估计方法也用于处理光谱数据,反演得到的SO2柱总量(中国科学技术大学(中国科大)SO2产品)与地基MAX-DOAS的偏差大幅降低(-4.8%~22%),相关性显著提高(0.72~0.89).与国控站点数据的平均偏差更小,为-25.8%.TROPOMI欧洲业务SO2产品相对于中国科大SO2产品偏高41.3%,并且标示了更大范围的重污染区域,与中国SO2大幅减排的事实相悖.而中国科大SO2产品大大提高TROPOMI结果在中国应用的准确性和价值.

The Sentinel-5Precursor(S-5P)was launched into low earth orbit with a single payload TROPOspheric Monitoring Instrument(TROPOMI)on 13 October,2017.TROPOMI is able to provide atmospheric composition observations including sulfur dioxide(SO2)daily and globally.It has a higher spatial resolution,specifically with a ground resolution of 7 km×3.5 km in the nadir,than previous satellite instruments such as Ozone Monitoring Instrument(OMI,13 km×24 km)and Ozone Mapping and Profiler Suite(OMPS,50 km×50 km).The three types of S-5P operational SO2 products(offline(OFFL),Reprocessing(RPRO)and Near Real Time(NRTI))are on basis of the differential optical absorption spectroscopy(DOAS)algorithm,and available from the S-5P Pre-Operations Data Hub.To evaluate these data,the results of OFFL are compared to the measurements obtained by ground-based Multi-Axis DOAS(MAX-DOAS)and China National Environmental Monitoring Center(CNEMC).We find that the S-5P operational SO2 products significantly overestimate SO2 concentrations over Northern China.Compared with four ground-based MAX-DOAS measurements in terms of the weekly average SO2 vertical column densities(VCDs),the S-5P operational SO2 products exhibit an overestimation of 61%-140%,with the correlation coefficient less than 0.5.The a priori SO2 profiles in S-5P operational products are from daily Tracer Model(TM5)simulations.SO2 surface concentrations are converted by the S-5P operational SO2 products and a priori SO2 profiles used in the retrievals.The correlation coefficient between the SO2 surface concentrations measured by CNEMC over Northern China and those from the S-5P operational products is higher than 0.5,and the S-5P operational products at these CNEMC sites show an overestimation of about 54.6%on average.In this study,the SO2 products(TROPOMI USTC SO2 products)are retrieved by the University of Science and Technology of China(USTC)with the optimal estimation algorithm(OE)from TROPOMI measurements,with the a priori SO2 profiles from monthly average GEOS-Chem simulations.The mean biases between the TROPOMI USTC SO2 products and the ground-based MAX-DOAS measurements are reduced significantly(-4.8%-22%),together with an improvement in the correlation coefficients(0.72-0.89).TROPOMI USTC SO2 surface concentrations are transformed by the TROPOMI USTC SO2 products and a priori SO2 profiles in the retrievals.Based on the evaluation statistics of the S-5P operational SO2 products,a smaller mean bias of-25.8%is found between the TROPOMI USTC SO2 surface concentrations and CNEMC measurements over Northern China.Compared to the TROPOMI USTC SO2 products in the heavily polluted area(33°-45°N,110°-122°E),the S-5P operational SO2 products have an overestimation of around 41.3%,with a correlation coefficient of 0.73.In addition,S-5P operational SO2 products show a larger area of heavy pollution than TROPOMI USTC SO2 products.Additionally,the sensitivity analysis indicates many influential factors of the SO2 retrievals using OE algorithm,such as radiative calibration,fitting window and a priori SO2 profiles.In the fitting window of 310.5-326.0 nm,the SO2 columns retrieved by the a priori profiles from monthly GEOS-Chem simulations show better agreement with the MAX-DOAS measurements than those from daily TM5 simulations.A possible reason is that the emission inventory used in GEOS-Chem is based on a Chinese power sector database for 31 provinces supported by CMEP(Chinese Ministry of Environmental Protection),resulting in a better performance of GEOS-Chem than that of TM5 over China in simulations.And the overestimation of the S-5P operational SO2 products over Northern China may be attributed to the fitting window used.