Retrieval accuracy of HCHO vertical column density from ground-based direct-sun measurement and first HCHO column measurement using Pandora

In the present study, we investigate the effects of signal to noise (SNR), slit function (FWHM), and aerosol optical depth (AOD) on the accuracy of formaldehyde (HCHO) vertical column density (HCHO_VCD) using the ground-based direct-sun synthetic radiance based on differential optical absorption spectroscopy (DOAS). We found that the effect of SNR on HCHO retrieval accuracy is larger than those of FWHM and AOD. When SNR = 650 (1300), FWHM = 0.6, and AOD = 0.2, the absolute percentage difference (APD) between the true HCHO_VCD values and those retrieved ranges from 54 (30%) to 5% (1%) for the HCHO_VCD of 5.0 × 10¹⁵ and 1.1 × 10¹⁷ molecules cm^-2, respectively. Interestingly, the maximum AOD effect on the HCHO accuracy was found for the HCHO_VCD of 3.0 × 10¹⁶ molecules cm^-2. In addition, we carried out the first ground-based direct-sun measurements in the ultraviolet (UV) wavelength range to retrieve the HCHO_VCD using Pandora in Seoul. The HCHO_VCD was low at 12:00 p.m. local time (LT) in all seasons, whereas it was high in the morning (10:00 a.m. LT) and late afternoon (4:00 p.m. LT), except in winter. The maximum HCHO_VCD values were 2.68 × 10¹⁶, 3.19 × 10¹⁶, 2.00 × 10¹⁶, and 1.63 × 10¹⁶ molecules cm^-2 at 10:00 a.m. LT in spring, 10:00 a.m. LT in summer, 1:00 p.m. LT in autumn, and 9:00 a.m. LT in winter, respectively. The minimum values of Pandora HCHO_VCD were 1.63 × 10¹⁶, 2.23 × 10¹⁶, 1.26 × 10¹⁶, and 0.82 × 10¹⁶ molecules cm^-2 at around 1:45 p.m. LT in spring, summer, autumn, and winter, respectively. This seasonal pattern of high values in summer and low values in winter implies that photo-oxidation plays an important role in HCHO production. The correlation coefficient (R) between the monthly HCHO_VCD values from Pandora and those from the Ozone Monitoring Instrument (OMI) is 0.61, and the slope is 1.25.