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We present an improved TROPOspheric Monitoring Instrument (TROPOMI) retrieval of formaldehyde (HCHO) over China. The new retrieval optimizes the slant column density (SCD) retrieval and air mass factor (AMF) calculation for TROPOMI observations of HCHO over China. Retrieval of HCHO differential SCDs (DSCDs) is improved using the basic optical differential spectroscopy (BOAS) technique resulting in lower noise and smaller random error, while AMFs are improved with a priori HCHO profiles from a higher resolution regional chemistry transport model. Compared to the operational product, the new TROPOMI HCHO retrieval shows better agreement with ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Beijing. The improvements are mainly related to the AMF calculation with more precise a priori profiles in winter. Using more precise a priori profiles in general reduces HCHO vertical column densities (VCDs) by 52.37 % (± 27.09 %) in winter. Considering the aerosol effect in AMF calculation reduces the operational product by 11.46 % (± 1.48 %) and our retrieval by 17.61 % (± 1.92 %) in winter. The improved and operational HCHO are also used to investigate the spatial–temporal characteristics of HCHO over China. The result shows that both improved and operational HCHO VCDs reach maximum in summer and minimum in winter. High HCHO VCDs mainly located over populated areas, i.e., Sichuan Basin and central and eastern China, indicate a significant contribution of anthropogenic emissions. The hotspots are more obvious on the map of the improved HCHO retrieval than the operational product. The result indicates that the improved TROPOMI HCHO retrieval is more suitable for the analysis of regional- and city-scale pollution in China. [ABSTRACT FROM AUTHOR]

Satellite remote sensing is an important technique providing long-term and large-scale information of formaldehyde (HCHO), which plays a crucial role in atmospheric chemistry. Low signal-to-noise ratio and poor stability of the Environmental Trace Gases Monitoring Instrument (EMI) On board Gaofen-5 satellite, the first Chinese space-borne spectrometer, make HCHO retrieval extremely difficult. Here we firstly retrieved HCHO vertical column densities (VCDs) from EMI through in-flight spectral calibration, retrieval setting optimization and stripe correction. Retrieved EMI HCHO VCDs correlate well with those measured by Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) with normalize mean bias (NMB) below 25%. EMI HCHO VCDs are comparable with those observed by Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI). This study reveals that HCHO can be observed successfully by algorithm optimization despite of poor performance of space-borne spectrometer. The retrieved EMI HCHO VCDs are applied to locate emission sources of volatile organic compounds (VOCs). • First retrieval of HCHO from Environmental Trace Gases Monitoring Instrument (EMI). • Algorithm optimization is proposed suitable for EMI HCHO retrieval. • HCHO vertical column density (VCD) correlates well with ground-based measurements. • EMI HCHO measurements are improved to a comparable level with OMI measurements. • HCHO hotspots and source of volatile organic compounds are captured successfully by EMI. [ABSTRACT FROM AUTHOR]

Recently, the occurrence of fog and haze over China has increased. The retrieval of trace gases from the multi-axis differential optical absorption spectroscopy (MAX-DOAS) is challenging under these conditions. In this study, various reported retrieval settings for formaldehyde (HCHO) and sulfur dioxide (SO2) are compared to evaluate the performance of these settings under different meteorological conditions (clear day, haze, and fog). The dataset from 1st December 2019 to 31st March 2020 over Nanjing, China, is used in this study. The results indicated that for HCHO, the optimal settings were in the 324.5–359 nm wavelength window with a polynomial order of five. At these settings, the fitting and root mean squared (RMS) errors for column density were considerably improved for haze and fog conditions, and the differential slant column densities (DSCDs) showed more accurate values compared to the DSCDs between 336.5 and 359 nm. For SO2, the optimal settings for retrieval were found to be at 307–328 nm with a polynomial order of five. Here, root mean square (RMS) and fitting errors were significantly lower under all conditions. The observed HCHO and SO2 vertical column densities were significantly lower on fog days compared to clear days, reflecting a decreased chemical production of HCHO and aqueous phase oxidation of SO2 in fog droplets. [ABSTRACT FROM AUTHOR]

In this paper, we present the Hefei EMI Cloud Retrieval Algorithm (HECORA), which uses information from the O2-O2 absorption band around 477 nm to retrieve effective cloud fraction and effective cloud pressure from satellite observations. The retrieved cloud information intends to improve the atmospheric trace gas products based on the Environment Monitoring Instrument (EMI) spectrometer. The HECORA method builds on OMCLDO2 and presents some evolutions. The Vector Linearized Discrete Ordinate Radiative Transfer (VLIDORT) model has been used to produce the Top of the Atmosphere (TOA) reflectance Look-up Tables (LUT) as a function of the cloud fraction and cloud pressure. Applying the Differential Optical Absorption Spectroscopy (DOAS) technique to the synthetic reflectance LUT, the reflectance spectra can be associated with O2-O2 geometrical vertical column densities (VCDgeo) and continuum reflectance. This is the core of the retrieval method, since there is a one-to-one relationship between O2-O2 VCDgeo and continuum reflectance, on the one hand, and effective cloud fraction and effective cloud pressure, on the other hand, for a given illumination and observing geometry and given surface height and surface albedo. We first used the VLIDORT synthetic spectra to verify the HECORA algorithm and obtained good results in both the Lambertian cloud model and the scattering cloud model. Secondly, HECORA is applied to OMI and TROPOMI and compared with OMCLDO2, FRESCO+, and OCRA/ROCINN cloud products. Later, the cloud pressure results from TROPOMI observations obtained using HECORA and FRESCO+ are compared with the CALIOP Cloud Layer product. HECORA is closer to the CALIOP results under low cloud conditions, while FRESCO+ is closer to high clouds due to the higher sensitivity of the O2 A-band to cloud vertical information. Finally, HECORA is applied to the TROPOMI NO2 retrieval. Validation of the tropospheric NO2 VCD with ground-based MAX-DOAS measurements shows that choosing HECORA cloud products to correct for photon path variations on the TROPOMI tropospheric NO2 VCD retrievals has better performance than using FRESCO+ under low cloud conditions. In conclusion, this paper shows that the HECORA cloud products are in good agreement with the well-established cloud products and that they are suitable for correcting the effect of cloud in trace gas retrievals. Therefore, HECORA has the potential to be applied to EMI. [ABSTRACT FROM AUTHOR]

In this study, ship-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed over the sea area between China and Korea during winter 2018 and spring 2019. Though the Differential Optical Absorption Spectroscopy (DOAS) technique analysis we obtain the spatial distribution of Vertical Column Densities (VCDs) of nitrogen dioxide (NO2), sulfur dioxide (SO2), and formaldehyde (HCHO) along the cruise of ship-based observation. In addition, the ship-based MAX-DOAS trace gases VCDs were compared with satellite observations of Ozone Monitoring Instrument (OMI) and achieved a good agreement between two data sets. Besides the trace gases spatial distribution information, combine with the meteorological condition, we have also quantified the contributions of external transmission and local emission of pollutants. Meanwhile, the vertical profiles of these trace gases are achieved from the measured Differential Slant Column Densities (DSCDs) at different elevation angles using optimal estimation method. The retrieved profiles displayed the typical vertical distribution characteristics, which exhibits the low concentrations of trace gases in clean area of the marine boundary layer far from coast while the high value was detected close to the ports or even large ships. This study provided further understanding of the main air pollutants in the marine boundary layer of the sea area between China and Korea and also benefited to quantify the pollutants transport between the China and Korea continental region [ABSTRACT FROM AUTHOR]

In this study, the spatiotemporal distribution of the NO 2 and SO 2 Vertical Columns Densities (VCDs) in the North China Plain (NCP) region was measured by long-distance mobile measurements using a mobile Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument. The mobile observations were performed in both summer (July 2017) and winter (January and February 2018) and the total driving mileage exceeded 3000 km. The concentrations of NO 2 and SO 2 in different seasons and areas were significantly different. During winter, serious NO 2 and SO 2 pollution was observed in northern Anhui Province, central Shandong Province, and the Beijing-Tianjin-Hebei Region. The evolution and transportation processes of three typical heavy pollution cases were discussed in detail. Combined with the Weather Research and Forecasting with Chemistry model simulated wind field information, the NO 2 transportation flux from northern Jiangsu Province to northern Anhui Province is quantified as 7.12 kg s−1. Finally, we estimated the NO 2 and SO 2 emissions from the Dezhou and Hengshui power plants by plume cross section scanning and encircled observations, respectively. The NO 2 and SO 2 emission fluxes of the Dezhou Power Plant are 0.79 kg s−1 and 1.11 kg s−1, respectively, while the NO 2 and SO 2 emission fluxes of the Hengshui Power Plant are 0.12 kg s−1 and 0.36 kg s−1, respectively. This study quantitatively analyzed the transportation of atmospheric pollutants and power plant emissions, which is helpful for understanding the occurrence and evolution of pollution and is also useful for the government to introduce policies to protect and control the atmospheric environment. • Large-scale and long-distance mobile MAX-DOAS measurements were performed. • The spatiotemporal patterns of the NO 2 and SO 2 VCDs in NCP was measured. • A transportation calculation method was used to estimate the pollutant transportation. • Pollutant emission reduction measures significantly reduce emissions of power plants. [ABSTRACT FROM AUTHOR]