Impact of biomass burning emissions on the composition of the South Atlantic troposphere: Reactive nitrogen and ozone

Abstract

In September/October 1992 an instrumented DC‐8 aircraft was employed to study the composition and chemistry of the atmosphere over the southern tropical Atlantic Ocean. Analysis of measurements, which included tracers of biomass combustion and industrial emissions, showed that this atmosphere was highly influenced by biomass burning emissions from the South American and African continents. Marine boundary layer was generally capped off by a subsidence inversion and its composition to a large degree was determined by slow entrainment from aloft. Insoluble species (such as PAN, NO, hydrocarbons, CO) were enhanced throughout the troposphere. Soluble species (such as HNO3, HCOOH, H2O2) were minimally elevated in the upper troposphere in part due to scavenging during cloud (wet) convection. Ozone mixing ratios throughout the South Atlantic basin were enhanced by ≈20 ppb. These enhancements were larger in the eastern South Atlantic (African emissions) compared to the western South Atlantic (South American emissions). In much of the troposphere, total reactive nitrogen (NOy) correlated well with tracers of biomass combustion (e.g., CH3Cl, CO). Although NOx (NO + NO2) correlated reasonably with these tracers in the lower (0–3 km) and middle troposphere (3–7 km), these relationships deteriorated in the upper troposphere (7–12 km). Stratospheric intrusions were found to be a minor source of upper tropospheric NOx or HNO3. Sizable nonsurface sources of NOx (e.g., lightning) as well as secondary formation from the NOy reservoir species (such as HNO3, PAN, and organic nitrates) must be invoked to explain the NOx abundance present in the upper troposphere. It is found that HNO3, PAN, and NOx were able to account for most of the NOy, in the middle troposphere (3–7 km); but a significant shortfall was present in the upper troposphere (7–12). This shortfall was also most pronounced in air masses with low HNO3. The reasons for the upper tropospheric reactive nitrogen shortfall is probably due to instrumental uncertainties and the presence of unidentified organic and inorganic nitrogen species.

Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data

“…Note that both trajectories and 3-D streamlines flowing from the east originally come from the low levels (P>800 hPa), with 28% and 21.5% respectively, due to synoptic upward motion below the AEJ. The absence of zonal gradient suggests that after ozone is initially formed at regional scale in the Harmattan layer by rapid photochemical processes over the band of fires (Jonquieres et al, 1998;Singh et al, 1996), the slower photochemical enhancements of ozone during the transport along the AEJ is either weak or balanced by mixing processes; otherwise the zonal gradient of ozone should be stronger. A Lagrangian aircraft campaign and model studies would be needed to investigate whether the dominant process creating the dry-season ozone maximum can be attributed to rapid and regional photochemistry or to slower and continental-scale photochemistry.…”

Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons

“…The dynamic processes allow redistribution of such emissions on a more global scale. During the TRACE-A campaign, plumes loaded with high O 3 over the Atlantic were attributed to biomass burning emissions from Africa (Singh, 1996). More recently high CO mixing ratios over the Indian Ocean have been attributed to African biomass burning (Edwards, 2006).…”

Seasonal and interannual variations in HCN amounts in the upper troposphere and lower stratosphere observed by MIPAS

“…the southern hemispheric biomass burning plume peaking in October and November, caused by combustion throughout the preceding dry season in austral spring in South America, central and southern Africa, and Australia. The spatial extension and composition of this plume has been investigated using various ground-based, airborne and spaceborne observations (Singh et al, 1996(Singh et al, , 2000Rinsland et al, 2001Rinsland et al, , 2005von Clarmann et al, 2007;Glatthor et al, 2009). In El Niño years, characterised by dry periods in Indonesia, fire emissions from this region are a considerable additional contribution to tropical biomass burning in austral spring.…”