The influence of biogenic emissions from Africa on tropical tropospheric ozone during 2006: a global modeling study

We have performed simulations using a 3D global chemistry-transport model to investigate the influence that biogenic emissions from the African continent exert on the composition of the tropopause in the tropical region. For this purpose we have applied two recently developed biogenic emission inventories provided for use in large-scale global models (Granier et al, 2005; Lathiére et al, 2006) whose seasonality and temporal distribution for isoprene, biogenic NO and biogenic volatile organic compounds is markedly different. The use of the climatological values for biogenic emissions provided by Lathiére et al, 2006 results in an increase in the amount of nitrogen sequestrated into longer lived reservoir compounds which contributes to the reduction in tropospheric ozone burden in the tropics. The associated re-partitioning of nitrogen between PAN, HNO3 and organic nitrates also results in a ~5% increase in the loss of nitrogen by wet deposition. At a global scale there is a reduction in the oxidizing capacity of the model atmosphere which increases the atmospheric lifetimes of CH4 and CO by ~1.5% and ~4%, respectively. By the use of sensitivity studies we show that the release of NO from soils in Africa accounts for between ~5-45% of tropospheric ozone in the African troposphere, ~10% in the upper troposphere and between ~5-20% of the total tropical ozone column over the tropical Atlantic Ocean. The subsequent reduction in OH over the source regions allows enhanced transport of CO out of the region. For biogenic volatile organic C1 to C3 species released from Africa the effects on tropical tropospheric ozone are rather limited, although this source contributes to the global burden of VOC by between ~2-4% and has a large influence on the organic composition of the troposphere over the tropical Atlantic Ocean. Comparisons against a range of different measurements indicate that applying the climatology of Lathiére et al (2006) improves the performance of TM4 for 2006 in the tropics.