Present-day distribution of tropospheric ozone and precursors in the tropics 

<p>As part of the Ozone and Precursors in the Tropics (OPT) working group of the Tropospheric Ozone Assessment Report Phase 2 (TOAR-II), we present the first results on the distribution of tropospheric ozone (O3) and its precursors (carbon monoxide, CO; formaldehyde, HCHO; nitrogen dioxide, NO2) in the tropics over the past 20-25 years. The goal is to give an overview of the seasonal, geographical and vertical variabilities of tropical tropospheric O3 and its precursors. To do so, we use in situ measurements of O3 and its precursors from surface sites, sounding balloons (SHADOZ) and instrumented aircraft (IAGOS and ATOM), as well as ground-based (FTIR) and spatial (IASI, OMI, GOME-2) remote-sensed observations. The observations are averaged monthly over the longest available time-period as well as over the first five years of the time period. The results for these two time periods give the context to interpret distributions and variabilities of O3 and its precursors over the most recent five years as we call it &#8220;Present-day&#8221;. &#160;Special emphasis is given to the differences of O3 and its precursors&#8217; distributions between remote and polluted regions and to the relationships between the gaseous species. Model output will be included to fill gaps in space and time when necessary to help the interpretation of the analysis based on observations.</p><p>From IAGOS measurements, the highest O3 and CO maxima occur in the lower troposphere of Northern Hemisphere Africa, which remains the most influenced by biomass burning. CO maxima are attributed using SOFT-IO model to anthropogenic emissions by 60%. Second maxima are observed in the lower troposphere of Asia, mostly due to anthropogenic emissions. The highest amount of transported CO in the tropics originates from Africa.</p>