Abstract
Formaldehyde (HCHO) is one of the most abundant hydrocarbons in the atmosphere. Its absorption features in the 320–360 nm range allow its concentration in the atmosphere to be retrieved from space. There are two versions of HCHO datasets derived from the Ozone Monitoring Instrument (OMI)—one provided by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) and one provided by the National Aeronautics and Space Administration (NASA)—referred to as OMI-BIRA and OMI-NASA, respectively. We conducted daily comparisons of OMI-BIRA and multi-axis differential optical absorption spectrometry (MAX-DOAS), OMI-NASA and MAX-DOAS, and OMI-BIRA and OMI-NASA and monthly comparisons of OMI-BIRA and MAX-DOAS and OMI-NASA and MAX-DOAS. Daily comparisons showed a strong impact of effective cloud fraction (eCF), and correlations were better for eCF < 0.1 than for eCF < 0.3. By contrast, the monthly and multi-year monthly mean values yielded correlations of R2 = 0.60 and R2 = 0.95, respectively, for OMI-BIRA and MAX-DOAS, and R2 = 0.45 and R2 = 0.78 for OMI-NASA and MAX-DOAS, respectively. Therefore, use of the monthly mean HCHO datasets is strongly recommended. We conducted a sensitivity test for HCHO air mass factor (AMF) calculations with respect to the HCHO profile, the aerosol extinction coefficient (AEC), the HCHO profile–AEC combination, the aerosol optical depth (AOD), and the single scattering albedo (SSA) to explicitly account for the aerosol optical effects on the HCHO AMF. We found that the combination of AEC and HCHO profiles can account for 23–39% of the HCHO AMF variation. Furthermore, a high load of absorptive aerosols can exert a considerable effect (−53%) on the AMF. Finally, we used the HCHO monthly mean profiles from Goddard Earth Observing System coupled to Chemistry (GEOS-Chem), seasonal mean AECs from Cloud-Aerosol LIDAR with Orthogonal Polarization (CALIOP) and monthly climatologies of AOD and SSA from the OMAERUV (OMI level-2 near UV aerosol data product) dataset at Xianghe station to determine the aerosol correction. The results reveal that aerosols can account for +6.37% to +20.7% of the HCHO monthly change. However, the changes are greatest in winter and are weaker in summer and autumn, indicating that the aerosol correction is more applicable under high-AAOD conditions and that there may be other reasons for the significant underestimation between satellite and MAX-DOAS observations.
Highlights
In recent years, with air pollution control, PM2.5 has declined significantly in China
The changes in effective cloud fraction (eCF) can change the correlation between the satellite data and the MAX-DOAS observations when the upper limit of eCF changes from 0.3 to 0.2 for all seasons except winter; less variation is observed for stricter cloud screening for Figure A1 to Figure A3 show daily comparisons between Ozone Monitoring Instrument (OMI)-Belgian Institute for Space Aeronomy (BIRA) and MAX-DOAS, OMI-National Aeronautics and Space Administration (NASA)
With respect to the aerosol effect on HCHO air mass factor (AMF), our results indicate that the combination of the HCHO profile and the aerosol extinction coefficient (AEC) profile can make a considerable difference under pollution
Summary
With air pollution control, PM2.5 has declined significantly in China (http://kjs. mep.gov.cn/). Previous validation results in Xianghe show that the regression line between satellite observation and multi-axis differential optical absorption spectrometry (MAX-DOAS) strongly depends on the parameter settings when calculating the air mass factor (AMF). Because the aerosol parameters (extinction coefficient profile, aerosol optical depth (AOD), single scattering albedo (SSA), asymmetry factor, etc.) are complex, it is difficult to quantify them using the conventional look-up table method and, the effects of aerosols on the HCHO AMF have not been explicitly considered Both HCHO retrieval algorithms implicitly exploit aerosol information in cloud information. We validate daily and monthly averages of the HCHO data derived from OMI by BIRA and NASA using the MAX-DOAS observations in Xianghe.
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