Analysis of IASI tropospheric O 3 data over the Arctic during POLARCAT campaigns in 2008

Ozone data retrieved in the Arctic region from infrared radiance spectra recorded by the Infrared Atmospheric Sounding Interferometer (IASI) on board the MetOp-A European satellite are presented. They are compared with in situ and lidar observations obtained during a series of aircraft measurement campaigns as part of the International Polar Year (IPY) POLARCAT activities in spring and summer 2008. Different air masses were sampled during the campaigns including clean air, polluted plumes originating from anthropogenic sources, forest fire plumes from the three northern continents, and stratospheric-influenced air masses. The comparison between IASI O 3 [0-8 km], [0-12 km] partial columns and profiles with collocated aircraft observations is achieved by taking into account the different sensitivity and geometry of the sounding instruments. A detailed analysis is provided and the agreement is discussed in terms of information content and surface properties at the location of the observations. Overall, IASI O 3 profiles are found to be in relatively good agreement in the free troposphere with smoothed in situ and lidar profiles with differences less than 40% (25% over the sea for both seasons) and 10%, respectively. The correlation between IASI O 3 retrieved partial columns and the smoothed aircraft partial columns is good with DC-8 in situ data in spring over North American forest fire regions (r = 0.68), and over Greenland with ATR-42 lidar measurements in summer (r = 0.67). Correlations with other data are less significant highlighting the difficulty with which IASI is able to capture O 3 variability in the Arctic upper troposphere and lower stratosphere (UTLS) with sufficient precision as noted in comparison with the [0-12 km] partial columns. However the [0-8 km] partial columns show good results with IASI which displays a negative bias (maximum of 26% over snow) compared to columns derived from in situ measurements. Despite these difficulties in the Arctic UTLS, this work also shows that IASI can be used to study particular cases where stratospheric intrusions are present using a O 3 /CO ratio diagnostic.