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.