Trends of tropical tropospheric ozone and its precursors

Abstract

As part of the Tropospheric Ozone Assessment Report Phase 2 (TOAR-II), the Ozone and Precursors in Tropics (OPT) working group counts three deliverables dedicated to quantifying 1) the distribution, 2) the trends of tropospheric ozone (O3) and its precursors (carbon monoxide, CO; formaldehyde, HCHO; nitrogen dioxide, NO2) in the tropics over the past 20-25 years, and 3) their impacts on a global scale. This presentation will focus on the trends estimate. We answer the following scientific questions: How have tropical tropospheric ozone and its precursors changed with time? What sources (e.g. anthropogenic emissions, biomass burning, lightning) drive these trends and to what extent?To accomplish this, we use both observations and model output.  The observations include in situ measurements of O3 and its precursors from surface sites, sounding balloons (SHADOZ) and instrumented aircraft (IAGOS), as well as ground-based (FTIR) and spatial (IASI, OMI, GOME-2) remote-sensed observations. Global model output come from ECHAM6-HAMMOZ, LMDZ-OR-INCA, MIROC-CHASER, CAM4-Chem, GISS-E2 and the CESM2-WACCM6 ensemble.The trends estimates are based on monthly anomalies and are calculated after considering climate variabilities such as El Niño- Southern oscillation (ENSO) and quasi-biennial oscillation (QBO).From IAGOS and SHADOZ ozone profiles, we estimate positive ozone trends between 1994 and 2019 throughout the troposphere above the Americas, Africa, India, Southeast Asia and Malaysia/Indonesia. Trends may reach up to 6 ± 1.6 ppb/decade in the free troposphere and up to 12.5 ± 2 ppb/decade in the boundary layer. There is also considerable regional variability. For example, trends are +0 to 4 ppb/decade in the free troposphere above the remote Pacific and Atlantic SHADOZ stations (1998-2019). According to OMI satellite retrievals, tropospheric ozone burden increases between 2004 and 2021 across the tropical latitude band (20˚N-20˚S) and the trends range between 0.09 Tg/yr (OMI CCD retrieval in the southern hemisphere) and 0.3 Tg/year (OMI/MLS retrieval in the northern hemisphere).The presentation will also include trends estimates of observed ozone’s precursors such as CO, NO2 and formaldehyde as well as trends estimate of ozone and its precursors from model output.