EC-07. Challenges of estimating tropical tropospheric ozone trends

Abstract
Tropical tropospheric ozone (TTO) is important for the global radiation budget because the longwave radiative effect of tropospheric ozone is higher in the tropics than mid-latitudes. In recent decades the TTO burden has increased, partly due to the ongoing shift of ozone precursor emissions from mid-latitude regions toward the equator. In this study, we assess the trends of TTO using ozone profiles measured by high quality in situ instruments from the IAGOS (In-Service Aircraft for a Global Observing System) commercial aircraft, the SHADOZ (Southern Hemisphere ADitional OZozone sondes) ozonesonde network, and two satellite records reporting tropical tropospheric column ozone (TTCO): OMI and OMI/MLS. In situ observations show that TTO increased between 1994 and 2019, with the largest mid- and upper tropospheric increases above India, Southeast Asia and Malaysia/Indonesia (from 3.4 ± 0.8 to 6.8 ± 1.8 ppbv decade-1), reaching 11 ± 2.4 and 9.4 ± 0.9 ppbv decade-1 in the lower troposphere (India and Malaysia/Indonesia). The longest satellite records only span 2004-2019, but show increasing ozone across the tropics, with maximum trends of 2.3 ± 1.1 DU decade-1 over Southeast Asia (OMI) and 2.4 ± 0.8 DU decade-1 over India (OMI/MLS). In general, the sparsely sampled aircraft and ozonesonde records do not detect the 2004-2019 ozone increase, which could be due to sample sizes that are too small for trend detection on this relatively short 15-year time period. The fact that the satellite records also fail to consistently detect the positive trend when their sampling frequency is limited to that of the in situ observations is an argument for increased in situ sampling in the tropics. This study demonstrates the need to maintain and develop continuous observations (in situ and remote sensing) above the tropical Pacific Ocean, the Indian Ocean, Southern Africa and South Asia in order to estimate accurate and precise ozone trends for these regions.