Retrieval of tropospheric ozone using TANSO-FTS-2 on GOSAT-2: a regional trend analysis of metropolitan area in Japan

<p>The Greenhouse gases Observing SATellite-2 (GOSAT-2, launched on October 29, 2018) has shown immense potential in CO₂ and CH₄ research, as a successor of GOSAT; however, the retrieval of tropospheric ozone is yet to be analyzed. Meanwhile, metropolitan areas are regarded as tropospheric ozone source for emission of ozone precursors (NO_x and VOC) by anthropogenic activities. The extreme surface temperature due to urban heat island effect can potentially modify the averaging kernel (AK) of satellite retrieval; this effect, "thermal contrast", is expected to favor the analysis of ozone in urban areas.</p> <p>In the paper, we aim at providing the first results of 3-year tropospheric ozone retrieval using thermal infrared (TIR) band from the nadir-looking instrument Thermal And Near infrared Sensor for carbon Observation &#8211; Fourier Transform Spectrometer-2 (TANSO-FTS-2), which is onboard GOSAT-2. The retrieval is focused on the urban area of Tokyo, Japan by the targeting mode of FTS-2 with around 40&#215;40 km² domain. In the retrieval process, we basically apply the optimal estimation and the Line-By-Line Radiative Transfer Model (LBLRTM) as the forward model, with the spectroscopic line parameters from HITRAN 2016 database. The climatological mean of the tropospheric ozone in Tokyo is applied as the <em>a priori</em> profile. For validation, we use ozonesonde data in Tsukuba, Japan (36.06&#176; N, 140.13&#176; E) with averaging kernel smoothing method. The results are also compared with ground measurements obtained from Atmospheric Environmental Regional Observation System (AEROS) of Japan. The performance of retrieval is evaluated by the degree of freedom of signal (DOFS), root-mean square error (RMSE) and mean bias (BIAS), in comparison with previous satellite missions such as TES and IASI.</p> <p>Our results present day-to-day and seasonal variation of tropospheric ozone in metropolitan area, while elucidating the urban heat island effect on the sensitivity of satellite retrieval and the characteristics of retrieved ozone episodes in view of meteorological conditions. Our results facilitate the assessment of urban tropospheric ozone trend and lead to better understanding of ozone chemistry using new satellite observations. Global validation and analysis of GOSAT-2 retrieval will be presented in a subsequent paper.</p>