Tropospheric and stratospheric NO retrieved from ground-based FTIR measurements

Abstract. Nitric oxide (NO) is a key active trace gas in the atmosphere, which contributes to form “bad” ozone (O3) in the troposphere and to the destruction of “good” O3 in the stratosphere. In this study, we present the NO retrieval from ground-based Fourier-transform infrared (FTIR) solar absorption spectrometry measurements at a polluted site (Xianghe, China) and a background site (Maido, Reunion Island). The Degree Of Freedom (DOF) of the NO retrieval is 2.3 ± 0.4 (1σ) at Xianghe and 1.3 ± 0.1 at Maido. The high NO mole fraction near the surface at Xianghe allows us to derive tropospheric and stratospheric NO partial columns separately, albeit the tropospheric column is almost not able to be retrieved in summer (June–August) because of the high water vapor abundance. At Maido, the NO retrieval is only sensitive to the stratosphere. The FTIR measurements at Maido show that the stratospheric NO partial column increases from the early morning to about 14:00 local time and starts decreasing thereafter. The stratospheric NO partial column is large in summer as compared to winter at both sites, and the seasonal variation of the FTIR stratospheric NO partial columns is consistent with that observed by the co-located Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite measurements. We observe a good correlation between the carbon monoxide (CO) and NO daily partial columns in the troposphere observed by the FTIR measurements at Xianghe with a correlation coefficient of 0.70, because both species have similar anthropogenic sources.