NO2 observations over the western Pacific and Indian Ocean by MAX-DOAS on Kaiyo, a Japanese research vessel
Abstract. Nitrogen dioxide (NO2) profile retrievals were performed by ship-borne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) using a compact/low-power spectrometer on the Japanese research vessel Kaiyo during two ocean cruises around Japan and Japan–Bali (Indonesia)–Indian Ocean. DOAS analysis using a 425–450 nm fitting window revealed a clear land–ocean contrast in NO2 differential slant column densities (DSCDs) but poor fitting results and negative values, especially at low elevation angles at low latitudes (<~20° N). The poor fitting resulted in sparse NO2 volume mixing ratio (VMR) data for the 0–1 km layer after applying our vertical profile retrieval method. In contrast, NO2 VMRs retrieved using fitting results from 460–490 nm are positive even at low latitudes, while they are reasonably similar to those obtained from 425–450 nm at mid-latitudes. Because NO2 DSCD for 425–450 nm shows a negative correlation with water vapor (H2O) DSCD, the poor fitting appears to be due primarily to interference by H2O. We analyzed a 338–370 nm fitting window, which is free from H2O, and found good agreement between NO2 VMRs retrieved from 460–490 nm and 338–370 nm, even at low latitudes, at NO2 VMRs higher than ~0.2 ppbv. The results indicate that the background value of NO2 VMR over the western Pacific and Indian Ocean during the cruises was less than ~0.2 ppbv, with occasional enhancement to levels of ~0.2–0.4 ppbv.