Retrieval of vertical profiles of NO3 from zenith sky measurements using an optimal estimation method -: art. no. 4587

The generic optimal estimation method (OEM) developed by Rodgers [1976, 1990, 2000] for solving atmospheric data inversion problems has been successfully developed to retrieve vertical profile information of NO3 from zenith sky spectroscopic measurements of column abundance made through sunrise. The technique has been shown to be robust and yields profile information to an altitude of 20 km. The rapid photolysis of NO3 at dawn ensures that a time series of column density measurements provides significant vertical information. The method has been extensively tested and a thorough error analysis was performed. The largest source of error arises from the undersampling of the column density through sunrise, the so-called smoothing error. The measurement errors and uncertainties in the forward model are also significant. Although a simple forward model involving only photolysis of NO3 during sunrise is able to successfully map the retrieved profile onto the column density measurements on some days, on other days systematic errors arise in the retrieval. These can be accounted for by either increased cloudiness or thermal decomposition of N2O5 in polluted boundary layer air, both effects extending the lifetime of NO3 through sunrise. The technique allows NO3 profile information to be retrieved in a variety of conditions over an extended period of time in a reliable and consistent manner.