Effects of clouds on ozone column retrieval from GOME UV measurements

The Global Ozone Monitoring Experiment (GOME) on board the European Space A gency's ERS-2 satellite is a spectrometer measuring the Earth's reflectivit y between 240 and 790 nm. The main geophysical product from GOME is the ozo ne vertical column density, or ozone column. To obtain the ozone column the ozone slant column density, which is derived from the GOME reflectivity me asurements between 325 and 335 nm using the Differential Optical absorption Spectroscopy (DOAS) method, is divided by the air mass factor. The air mas s factor represents the optical path length in the atmosphere. Clouds gener ally enhance the air mass factor; however, clouds also screen part of the o zone below them. Therefore, in an accurate ozone retrieval method the prese nce of clouds should be taken into account. We investigated the influence o f errors in cloud parameters on the retrieved ozone column by performing a sensitivity study with a detailed radiative transfer model including polari zation. We show that the influence of clouds on the retrieved ozone column depends mainly on the cloud top height, cloud albedo, and cloud fraction. F br low and midlevel clouds an overestimation of the cloud fraction yields a n underestimation of the ozone column; for high clouds, the opposite is tru e. An overestimation of the cloud top height always yields an overestimatio n 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, respect ively, the errors in the retrieved ozone column are within 8, 6, and 5%, fo r solar zenith angles of 30 degrees, 60 degrees, and 75 degrees, respective ly.