Improving the Retrieval of Tropospheric Ozone Profiles from the Ozone Monitoring Instrument
Mielonen, T.; de Haan, J. F.; Veefkind, J. P.
In order to understand the ozone related physical and chemical processes in the atmosphere, global measurements of ozone profiles are essential. Consequently, the total ozone column and ozone profiles have been monitored with spaceborne instruments since the late 1970s. The latest addition to this line of instruments is the Ozone Monitoring Instrument (OMI) on-board Aura satellite which was launched in 2004. We have assessed the sensitivity of the operational OMI ozone profile retrieval (OMO3PR, Kroon et al., 2011) algorithm to a-priori assumptions and studied how to modify the algorithm to improve the retrieval of tropospheric ozone. We studied the effect of stray light correction, surface albedo assumptions and a-priori ozone profiles on the retrieved ozone profiles. We found that stray light correction has a significant effect on the retrieved ozone profile but mainly at high altitudes. Surface albedo assumptions, on the other hand, have the largest impact at the lowest layers. Selection of an ozone profile climatology which is used as a-priori information affects the retrievals at all altitudes. However, the properties of the a-priori covariance matrix have a larger effect on the retrieved profiles than the a-priori ozone profiles. After testing the sensitivity of the retrieval algorithm to these parameters we decided to modify the algorithm by changing the a-priori ozone climatology from LLM to TpO3 (Sofieva et al., 2013), by calculating the a-priori covariance matrix from the climatological values (with minimum stardard deviation of 0.1, correlation legth of 6 km and separated troposhere and stratosphere) and by assuming that the surface albedo is linearly dependent on wavelenght in the UV2 channel. We found that, on average, these modification do not change tropospheric O3 levels significantly (typically less than 1%) but the spatial variation increases. In addition, the degrees of freedom in the troposphere double on average due to the new a-priori covariance matrix. However, the degrees of freedom for the total column decrease slightly (around 5% on average). Moreover, we compared our new and operational retrieval results with other studies of tropospheric ozone (Eremenko et al., 2008; Thompson et al., 2008) and found that the new version is in better agreement with the other data sets than the operational one. Spatial correspondence is evidently improved but the retrieval still overestimates O3 in the troposphere.
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Sofieva, V.F., J. Tamminen, E. Kyroelä, T. Mielonen, P. Veefkind, B. Hassler, G. Bodeker (2013), Joint climatology of ozone profiles and tropopause height, submitted to J. Geophys. Res.
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