Seasonal variability and long-term evolution of tropospheric composition in the tropics and Southern Hemisphere

Abstract

Abstract. Present-day and future impacts of biomass burning and other sources in the tropics and Southern Hemisphere are studied by global chemical transport model (GCTM), satellites retrievals and surface measurements. The spring CO peaks found at Mahe Island (Western Indian Ocean) are attributed to the burnings in India but not those from Northern Africa. Easter Island (Eastern Pacific Ocean) is impacted indirectly by the hemispheric zonal transport of CO due to the burnings in Southern Africa/Latin America, via the westerlies. An increasing trend for CO by 0.33 ppb yr-1 in the past decade at Ascension Island is attributed to the combined effects of Latin American/Southern Africa burnings and increase of CH4 level. Changes in water vapour and UV over Southern Atlantic Ocean (SAO) in future January have dominated effects on the O3 distribution. More than 55% of O3 concentrations over SAO in both present-day and future September are not directly affected by the emissions (including lightning) over the adjacent two continents but attributable to transport of O3 from outside due to CO and CH4 oxidation and stratospheric intrusion. High NOx emissions in both continents in future increase the PAN concentrations over remote oceans at higher southern latitudes (> 35° S) as far as those near Australia, affecting the O3 budget over there. Future changes of biomass burning and anthropogenic NOx emissions in Southern Africa lead to a new area of O3 maximum near South Africa. The resulted O3 outflow to the Indian Ocean is pronounced due to the effects of the persistent anti-cyclone. A general reduction of future OH radical concentrations is predicted over the remote marine boundary layer in the tropics and Southern Hemisphere, due to the increases in CH4 and CO emissions combined with the low-NOx environment.