Origin of ozone and NO_x in the tropical troposphere: A photochemical analysis of aircraft observations over the South Atlantic basin

The photochemistry of the troposphere over the South Atlantic basin is examined by modeling of aircraft observations up to 12‐km altitude taken during the TRACE A expedition in September–October 1992. A close balance is found in the 0 to 12‐km column between photochemical production and loss of O 3 , with net production at high altitudes compensating for weak net loss at low altitudes. This balance implies that O 3 concentrations in the 0–12 km column can be explained solely by in situ photochemistry; influx from the stratosphere is negligible. Simulation of H 2 O 2 , CH 3 OOH, and CH 2 O concentrations measured aboard the aircraft lends confidence in the computations of O 3 production and loss rates, although there appears to be a major gap in current understanding of CH 2 O chemistry in the marine boundary layer. The primary sources of NO x over the South Atlantic Basin appear to be continental (biomass burning, lightning, soils). There is evidence that NO x throughout the 0 to 12‐km column is recycled from its oxidation products rather than directly transported from its primary sources. There is also evidence for rapid conversion of HNO 3 to NO x in the upper troposphere by a mechanism not included in current models. A general representation of the O 3 budget in the tropical troposphere is proposed that couples the large‐scale Walker circulation and in situ photochemistry. Deep convection in the rising branches of the Walker circulation injects NO x from combustion, soils, and lightning to the upper troposphere, leading to O 3 production; eventually, the air subsides and net O 3 loss takes place in the lower troposphere, closing the O 3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NO x emissions in the tropics.