IASI nitrous oxide (N₂O) retrievals: validation and application to transport studies at daily time scales

Abstract. The aim of this paper is to present a method to retrieve nitrous oxide (N₂O) vertical profiles from the Infrared Atmospheric Sounding Interferometer (IASI) onboard the MetOp platform. We retrieved N₂O profiles using IASI clear sky radiances in 2 spectral bands: B1 and B2 centered at ∼ 1280 cm⁻¹ and ∼ 2220 cm⁻¹, respectively. Both retrievals in B1 and B2 (hereafter referred to as N₂O_B1 and N₂O_B2, respectively) are sensitive to the mid-to-upper troposphere with a maximum of sensitivity at around 309 hPa. The degrees of freedom for N₂O_B1 and N₂O_B2 are 1.38 and 0.93, respectively. We validated the retrievals using the High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO). The comparisons between HIPPO and the two retrieved datasets show relatively low standard deviation errors around 1.5 % (∼ 4.8 ppbv) and 1.0 % (∼ 3.2 ppbv) for N₂O_B1 and N₂O_B2, respectively. However, the impact of H₂O contamination on N₂O_B1 due to its strong absorption bands in B1 significantly degrades the quality of the retrievals in tropical regions. We analysed the scientific consistency of the retrievals at 309 hPa with a focus on the long-range transport of N₂O especially during the Asian summer monsoon. Over the mid-latitude regions, both variations of N₂O_B1 and N₂O_B2 at 309 hPa are influenced by the stratospheric N₂O-depleted air because of the relative coarse shape of the averaging kernel. The analysis of N₂O_B2 using results from backtrajectories exhibits the capacity of these retrievals to capture long-range transport of air masses from Asia to northern Africa via the summer monsoon anticyclone on a daily basis. Thus, N₂O_B1 and N₂O_B2 offer an unprecedented possibility to study global upper tropospheric N₂O on a daily basis.