Climatology and trends of tropospheric ozone over the eastern Pacific Ocean: The influences of biomass burning and tropospheric dynamics

To study the latitudinal variation of tropospheric ozone over the eastern Pacific Ocean, we derive ozone in the lower troposphere by subtracting TOMS column ozone over the Andes from the ozone column over the eastern Pacific Ocean. The analysis of tropospheric ozone climatology in conjunction with meteorological and biomass-burning data suggests three latitude bands with different characteristics for tropospheric ozone. (1) Air in the latitude range 3°N-7°N experiences persistent upward motion in the middle troposphere over the eastern equatorial Pacific Ocean. This condition results in relatively low ozone amounts in this region. (2) Tropospheric ozone between latitudes 2°N 22°S shows strong seasonal variation that is well correlated with the biomass burning season over southern tropical South America. Persistent subsidence in the middle troposphere and prevailing easterlies over this region provide favorable conditions for enhancing ozone amounts in the lower troposphere. (3) Tropospheric ozone over 23°S-36°S does not appear to be influenced by biomass burning. Regression analysis of deseasonalized tropospheric ozone shows positive trends from 12°S to 23°S and from 2°N to 5°N with essentially zero trends elsewhere. Trends north of the equator are generally smaller and less consistently positive than trends from 12°S-23°S. With tropospheric ozone seasonally in phase in the presence of enabling horizontal and vertical winds, the average increasing trend of +0.9±0.3(2σ)[%/yr] or +0.14±0.04(2σ)[DU/yr] between 12°S and 23°S over the 14 years analyzed here is consistent with an increasing biomass-burning source.