Effects of clouds on ozone column retrieval from GOME UV measurements

The Global Ozone Monitoring Experiment (GOME) on board the European Space Agency's ERS-2 satellite is a spectrometer measuring the Earth's reflectivity between 240 and 790 nm. The main geophysical product from GOME is the ozone vertical column density, or ozone column. To obtain the ozone column the ozone slant column density, which is derived from the GOME reflectivity measurements between 325 and 335 nm using the Differential Optical absorption Spectroscopy (DOAS) method, is divided by the air mass factor. The air mass factor represents the optical path length in the atmosphere. Clouds generally enhance the air mass factor; however, clouds also screen part of the ozone below them. Therefore, in an accurate ozone retrieval method the presence of clouds should be taken into account. We investigated the influence of errors in cloud parameters on the retrieved ozone column by performing a sensitivity study with a detailed radiative transfer model including polarization. We show that the influence of clouds on the retrieved ozone column depends mainly on the cloud top height, cloud albedo, and cloud fraction. Fbr low and midlevel clouds an overestimation of the cloud fraction yields an underestimation of the ozone column; for high clouds, the opposite is true. An overestimation of the cloud top height always yields an overestimation of the ozone column. Using a cloud albedo of 0.8, and assuming errors in the GOME cloud fraction and cloud top height of +/-0.2 and +/-3 km, respectively, the errors in the retrieved ozone column are within 8, 6, and 5%, for solar zenith angles of 30 degrees, 60 degrees, and 75 degrees, respectively.