Ozone retrieval errors associated with clouds in total ozone mapping spectrometer (TOMS) data

This study characterizes TOMS Ozone Retrieval Errors (ORES) associated with incorrect Cloud-Top Pressures (CTPs) and with assuming opaque Lambertian clouds, investigates these errors' effects on tropospheric ozone derivation, and analyzes ozone anomalies over TOMS data. Large errors occurring in TOMS assumed CTPs and inaccurate CTP-caused ORES are most significantly from inappropriately added ozone below clouds. Because OREs are usually within the TOMS retrieval precision when Cloud Optical Depth (COD) ≥ 20, assuming angular-independent cloud reflection is good. Because of In-Cloud Ozone Absorption ENhancement (ICOAEN), assuming opaque clouds can introduce large positive OREs even for optically thick clouds. For a 2–12 km water cloud of COD 40 with 20.8 DU ozone homogeneously distributed inside the cloud, the ORE is 17.8 DU at nadir view. The ICOAEN effect depends strongly on viewing geometry and inter-cloud ozone amount and distribution; it is typically 5–13 DU over the tropical Atlantic and Africa and 1–7 DU over the tropical Pacific for deep convective clouds. The TOMS Partial Cloud Model (PCM) is good because negative PCM effect partly cancels other positive errors. At COD ≤ 5, the TOMS algorithm retrieves approximately the correct total ozone because of compensating errors. With increasing COD up to 20–40, negative PCM effect decreases more dramatically than positive effects, so overall positive ORE increases and is dominated by the ICOAEN effect. The ICOAEN effect can largely underestimate tropospheric ozone derived from cloudy/clear difference techniques. The convective cloud differential and cloud-clear pair methods use minimum ozone above clouds to cancel positive errors. A Positive or Negative Ozone Anomaly (POA/NOA) is defined to occur if the ozone/reflectivity correlation coefficient in a region is ≥0.5 or ≤−0.5. Average fractions of OA occurrence are 31.8% and 35.8% in Nimbus-7 and Earth-Probe TOMS data, respectively. Most tropical NOAs result from large cloud-height errors; corrections lead to 50–70% POAs in the tropics because of mainly the ICOAEN effect. POAs with fractions of 30–60% occur in marine stratocumulus regions west of South Africa and South America. ORES over clear and cloudy areas cause about half the ozone/reflectivity slope; greater ozone production from frequent low-altitude clouds and rich ozone precursors may cause the remainder. The knowledge of TOMS OREs has important implications for ozone/trace gas retrieval from other satellites.