With the accelerated pace of industrialization and urbanization,coal and oil are consumed in large quantities as the main fuels.This has led to an increasingly serious air pollution problem,which has attracted the attention of all countries.Sulfur dioxide(SO2),as one of the main pollutants emitted from coal-fired power generation and industrial production,does harm to the human body,environment,and buildings.SO2 detection is a prerequisite for environmental protection and is very important to improve the regional and even global air pollution.Due to the advantages of long-term,largescale and non-contact observations,remote sensing technology has gradually become an important method for air pollution monitoring.Satellite remote sensing technology has been widely used to estimate SO2 emissions with the development of optical and spectroscopy technology.In this study,we improve the optimal estimation(OE)method to dedicate for the retrieval of SO2 vertical column densities(VCDs)from different space-based spectroscopic measurements.Finally,we successfully detect the SO2 distribution from anthropogenic and natural sources based on satellite observations,and analyze the impact of human activities and volcanic eruptions on SO2 concentration.The low concentration and weak absorption of SO2 in the atmosphere make it difficult to retrieve SO2 VCDs based on satellite observations.In this study,we have performed several critical methods and independently retrieve the Level 2 SO2 VCDs(OE-SO2 product)from the Level 1 spectra observed by Tropospheric Monitoring Instrument(TROPOMI),which is the latest generation of space-based spectroscopies with high spatial resolution.These methods include on-orbit wavelength calibration,radiometric calibration,fitting window selection,and background correction.The OESO2 product is in good agreement with the observations from the ground-based multiaxis differential absorption spectrometer(MAX-DOAS),with the correlation coefficients greater than 0.7 and the mean bias smaller than 13.7%.Furthermore,we evaluate the feasibility of SO2 retrieval based on observations from TROPOMI BAND2 channel.Moreover,this product is also consistent with the ground-based MAX-DOAS observations,and the correlation coefficient is>0.65,with a mean bias smaller than 14%.Based on TROPOMI measurements,we assess the retrieval performance of TROPOMI operational SO2(DOAS-SO2)product and OE-SO2 product,and find that OE-SO2 product has higher retrieval precision and smaller solution error.The comparisons between these two TROPOMI SO2 products and the Ozone Mapping and Profile Suite Nadir Mapper Instrument(OMPS)operational SO2 product,and the ground-based MAX-DO AS product,and the surface in situ product indicate that the OE-SO2 product has better consistency and smaller deviation with the other products.On the contrary,DOAS-SO2 product is obvious overestimated in SO2 VCDs,especially in high SO2 pollution areas.On the whole,by comparing the retrieval algorithms and settings of DOAS-SO2 and OE-SO2 products,radiometric calibration and fitting window selection are essential for SO2 retrieval and might be the key factor in the overestimation in DOAS-SO2 product.The Environmental Monitoring Instrument(EMI),onboard the Gaofen-5 satellite launched in May 2018,is the first Chinese space-borne ultraviolet-visible spectrometer aiming to trace gas detection.We also apply the OE method to EMI measurements and successfully retrieve the distribution of SO2 VCDs(EMI-SO2 product)on a global scale.Although the performance deficiencies in EMI(e.g.,low frequency of observed solar spectrum,significant performance difference between across-track positions)limit the retrieval of SO2 VCDs,we have performed the irradiance reconstruction,measurement error simulation and along-track pixel merging for each across—track position to address these issues.Complying with all mentioned methods,the retrieval precision of the EMI-SO2 product reaches to a considerable level of that of the OESO2 product.What’s more,the EMI-SO2 product has good spatial and temporal consistency with the Indian surface in situ observations and the OE-SO2 product.It helps to cut off the dependence on foreign satellite measurements in trace gas detection.For anthropogenic emissions,COVID-19 provides a natural condition to estimate the impact of anthropogenic activities on SO2 emissions.We apply DOAS-SO2 and OE-SO2 products during the COVID-19 pandemic.For the Beijing-Tianjin-Hebei region,due to the epidemic lockdown,the OE-SO2 product indicated that SO2 VCDs in March 2020 decreased by 51%compared with that in March 2019.In April 2020,due to the resumption of production,the SO2 concentrations returned to a comparable level in April 2019,showing an increase of 50%compared to that in March 2020.However,DOAS-SO2 product observed an unreliable SO2 increase of 19%in the lockdown period than after the lockdown period,which was inconsistent with the actual situation and surface in situ observations.For Maharashtra,India,the lockdown began on March 25,2021.Compared to the SO2 concentrations in March 2019,both OE-SO2 and surface in situ products observed reductions in SO2 concentrations in March 2020,with the value of 36.6%and 72.2%,respectively.However,the DOAS-SO2 product showed that the SO2 VCDs in March 2019 and March 2020 were close,which was inconsistent with the OE-SO2 and surface in situ observations.Overall,OE-SO2 product can better reflect the impact of human activities on SO2 concentrations,while DOAS-SO2 product not only suffers from overestimation problems but also observes a wrong trend of SO2 VCDs.For volcanic emissions,based on TROPOMI and the EMI-2 on the Gaofen-5 02 satellite successfully launched in September 2021,we explore the daily burden,distribution,and evolution of SO2 from the Tonga volcanic eruption from January 14 to 17,2022.From the observations,we further quantify that the total SO2 emission exceeded 900 kt from this volcanic,which is comparable to that from top eruptions in the 21st century,and that from worldwide top anthropogenic SO2 hotspots.If the subsequent eruptions followed the Plinian eruption pattern in 1991,the estimated total SO2 emission is expected to reach over 3000 kt.We find that the observations of the two space-borne spectrometers are consistent and complementary,indicating the utility and effectiveness of satellite observations for monitoring volcanic SO2 emissions. |