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

Abstract

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 O3, with net production at high altitudes compensating for weak net loss at low altitudes. This balance implies that O3 concentrations in the 0–12 km column can be explained solely by in situ photochemistry; influx from the stratosphere is negligible. Simulation of H2O2, CH3OOH, and CH2O concentrations measured aboard the aircraft lends confidence in the computations of O3 production and loss rates, although there appears to be a major gap in current understanding of CH2O chemistry in the marine boundary layer. The primary sources of NOx over the South Atlantic Basin appear to be continental (biomass burning, lightning, soils). There is evidence that NOx 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 HNO3 to NOx in the upper troposphere by a mechanism not included in current models. A general representation of the O3 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 NOx from combustion, soils, and lightning to the upper troposphere, leading to O3 production; eventually, the air subsides and net O3 loss takes place in the lower troposphere, closing the O3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NOx emissions in the tropics.

Observed OH and HO2 in the upper troposphere suggest a major source from convective injection of peroxides

“…These measurements were obtained as part Further support for our hypothesis of a major convective contribution of peroxides to HOx levels in the upper troposphere is offered by previous observations of H202 and CH3OOH concentrations over the south tropical Atlantic during TRACE-A [Jacob et al, 1996]. Diel steady state model calculations underestimated these two peroxides in the upper troposphere (8-12 km) by mean factors of 1.8 and 3.6 respectively, while agreement was much better at lower altitudes.…”

Peroxyacetyl nitrate (PAN) concentrations in the Antarctic troposphere measured during the photochemical experiment at Neumayer (PEAN'99)

“…The role of PAN acting as a precursor of NO V has been investigated for the remote troposphere over the tropical South Atlantic and South Paci"c Jacob et al, 1996;Schultz et al, 1999). Although PAN mixing ratios were less than 5 pptv in the marine boundary layers, PAN decomposition accounted for NO V production rates of a few pptv h\ which was mainly due to the much higher temperatures in tropical latitudes.…”

“…Despite lower PAN concentration at lower altitudes in remote marine areas, its decomposition has been found to be su$cient to maintain observed NO V ( "NO#NO ) mixing ratios (e.g. Heikes et al, 1996;Jacob et al, 1996;Schultz et al, 1999), thus demonstrating that PAN can act as a transport agent for NO V from its continental sources to remote tropospheric regions. As a result, PAN in#uences the NO V balance, which in turn critically controls photochemical production and destruction of ozone in many regions of the troposphere (e.g.…”

Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

“…During SONEX, formic acid was present in concentrations of =45 ppt in the UT and =120 ppt in the lower troposphere (Figure 9) , the process is quite slow, and this is at best a minor source of HOx radicals. In most cases, organic acids are removed from the atmosphere by wet and dry deposition processes, and their chemistry is at best highly uncertain [Talbot et al, 1990[Talbot et al, , 1995Jacob et al, 1996]. An accurate enough source inventory for these organic acids is not currently available to model their global distribution.…”