Validation of tropospheric NO2 column density data observed by TROPOMI: Comparison with 4AZ-MAXDOAS at Chiba, Japan

To confirm the reported underestimate in tropospheric nitrogen dioxide (tropNO₂) column data from the latest sensor TROPOspheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor satellite and clarify its causes, we developed ground-based 4-different-azimuth-viewing Multi-Axis Differential Optical Absorption Spectroscopy (4AZ-MAXDOAS) and made continuous observations at Chiba, Japan (35.63ºN, 140.10ºE, 21 m asl) in 2019. The simultaneous tropNO2 observations by 4AZ-MAXDOAS in 4 different azimuth directions enable the evaluation of the horizontal inhomogeneity of tropNO2, which previous studies have considered to be the primary cause of the underestimate. From 4AZ-MAXDOAS observations, we found that differences in tropNO₂ column data among 4 different azimuth directions reached up to 40%, indicating the existence of significant horizontal spatial inhomogeneity in NO₂ around the observation site. Then, we compared 4AZ-MAXDOAS data with coincident TROPOMI data. TROPOMI data showed smaller tropNO₂ column by 53% in annual average compared to 4AZ-MAXDOAS data, confirming the underestimate in TROPOMI data. However, the correlation was not clear between the magnitude of the underestimate and the coefficient of variance in 4 azimuth direction data from 4AZ-MAXDOAS observations. This suggests that the observed underestimate cannot be explained only by the effect of the NO₂ spatial inhomogeneity. The causes for the underestimate should consist of multiple factors, including the effect of the vertical profile of NO₂.