Factors controlling tropospheric O₃, OH, NO_<i>x</i> and SO₂ over the tropical Pacific during PEM‐Tropics B

Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)‐Tropics B experiment (March‐April 1999) are analyzed. Concentrations of CO and long‐lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately O 3 concentrations but not NO x concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of O 3 and NO x concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM‐Tropics A experiment (September‐October 1996). Observed CH 3 I concentrations, as in PEM‐Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one‐dimensional model with CH 3 I observations yields a 10‐day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM‐Tropics A. Model simulated HO 2 , CH 2 O, H 2 O 2 , and CH 3 OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (∼25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO 3 concentrations during PEM‐Tropics B are too low (a factor of 2–4 below 6 km) compared to observations. Budget analyses indicate that chemical production of O 3 accounts for only 50% of chemical loss; significant transport of O 3 into the region appears to take place within the tropics. Convective transport Of CH 3 OOH enhances the production of HO x and O 3 in the upper troposphere, but this effect is offset by HO x loss due to the scavenging of H 2 O 2 . Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4–1 Tg yr −1 of NO x in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO 2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS‐OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection.