NO2 variations over the remote ocean observed by MAX-DOAS on a Japanese research vessel

By ship-borne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) using compact/low power spectrometer, nitrogen dioxide (NO₂) profile retrieval on a Japanese research vessel, Kaiyo, was conducted during two ocean cruises over Japan and Japan-Bali (Indonesia) -Indian Ocean. The 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. This has resulted in sparse NO₂ volume mixing ratio (VMR) data (below 1 km) after applying our vertical profile retrieval method. In contrast, NO₂ VMRs retrieved using fitting results from 460-490 nm are positive even at low-latitude, while they are very similar to those from 425-450 nm at mid-latitude. We also 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-latitude, at NO₂ VMRs higher than ~0.2 ppbv. On the basis of these results, we characterize the NO₂ VMR over the western Pacific and Indian Ocean as being generally below ~0.2 ppbv with occasional enhancements of > ~0.2-0.4 ppbv.