IASI nitrous oxide (N 2 O) retrievals: validation and application totransport studies at daily time scales

The aim of this paper is to present a method to retrieve nitrous oxide (N 2 O) vertical profiles from the Infrared Atmospheric Sounding Interferometer (IASI) onboard the MetOp platform. We retrieved N 2 O 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 N 2 O_B1 and N 2 O_B2, respectively) are sensitive to the mid-to-upper troposphere with a maximum of sensitivity at around 309 hPa. The degrees of freedom for N 2 O_B1 and N 2 O_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 N 2 O_B1 and N 2 O_B2, respectively. However, the impact of H 2 O contamination on N 2 O_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 N 2 O especially during the Asian summer monsoon. Over the mid-latitude regions, both variations of N 2 O_B1 and N 2 O_B2 at 309 hPa are influenced by the stratospheric N 2 O-depleted air because of the relative coarse shape of the averaging kernel. The analysis of N 2 O_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, N 2 O_B1 and N 2 O_B2 offer an unprecedented possibility to study global upper tropospheric N 2 O on a daily basis.