NO<sub>2</sub> observations over the western Pacific and Indian Ocean by MAX-DOAS on <i>Kaiyo</i>, a Japanese research vessel

Abstract

Abstract. Nitrogen dioxide (NO 2 ) profile retrievals were performed by ship-borne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) using a compact/lowpower 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 NO 2 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 NO 2 volume mixing ratio (VMR) data for the 0-1 km layer after applying our vertical profile retrieval method. In contrast, NO 2 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 NO 2 DSCD for 425-450 nm shows a negative correlation with water vapor (H 2 O) DSCD, the poor fitting appears to be due primarily to interference by H 2 O. We analyzed a 338-370 nm fitting window, which is free from H 2 O, and found good agreement between NO 2 VMRs retrieved from 460-490 nm and 338-370 nm, even at low latitudes, at NO 2 VMRs higher than ∼ 0.2 ppbv. The results indicate that the background value of NO 2 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.

First quantitative bias estimates for tropospheric NO&lt;sub&gt;2&lt;/sub&gt; columns retrieved from SCIAMACHY, OMI, and GOME-2 using a common standard

“…The observations at Tai'an, Mangshan, and Rudong were made as part of intensive observation campaigns for a limited time period of about 1 month for each site ( Table 1). The instrumentation and retrieval algorithm used for all the sites have been described in detail elsewhere (e.g., Irie et al, 2008Irie et al, , 2009Irie et al, , 2011Takashima et al, , 2012. The retrieval utilizes absorption features by NO 2 and the oxygen dimer (O 4 ) at 460-490 nm.…”

Surveillance of SO&lt;sub&gt;2&lt;/sub&gt; and NO&lt;sub&gt;2&lt;/sub&gt; from ship emissions by MAX-DOAS measurements and implication to compliance of fuel sulfur content

“…As an expanded apparatus, the multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements are high sensitivity to aerosols and trace gases in the lower troposphere by observing scattered sunlight under different viewing angles closed to the horizontal and the zenith directions (Hönninger et al, 2004;Ma et al, 2013;Sinreich et al, 2005;Wang et al, 2014). Due to its portability, MAX-DOAS instrument can be carried on the ship to observe the vertical column densities (VCDs) of NO2 and SO2 along the cruise, during which high levels of pollutants were found close to the busy port and dense lanes (Hong et al, 2018;Schreier et al, 2015;Takashima et al, 2012;Tan et al, 2018). In addition, MAX-DOAS has been successfully employed for monitoring shipping emissions directly.…”

Author's reply to comments from reviewer 2

“…Cross sections 1 NO 2 at 298 K (Vandaele et al, 1996), I 0 -correction using 1 × 10 17 molec cm −2 2 NO 2 at 220 K orthogonalized to 298 K within 425-490 nm 3 NO 2 at 220 K orthogonalized to 298 K within 411-445 nm 4 O 3 at 223 K (Bogumil et al, 2003)…”

“…MAX-DOAS measurements were therefore taken in remote regions for the investigation of background concentrations and in many locations for the validation of satellite observations (e.g. Takashima et al, 2012;Peters et al, 2012).…”