Abstract

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 O3 concentrations but not NOx concentrations. It therefore tends to depress OH
concentrations over the tropical Pacific. These effects contrast to the large enhancements of
O3 and NOx concentrations and the moderate increase of OH concentrations due to biomass
burning outflow during the PEM-Tropics A experiment (September-October 1996). Observed
CH3I 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 CH3I 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 HO2, CH2O, H2O2, and CH3OOH 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 HNO3 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
O3 accounts for only 50% of chemical loss; significant transport of O3 into the region appears
to take place within the tropics. Convective transport of CH3OOH enhances the production of
HOx and O3 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging
of H2O2. Convective transport and scavenging of reactive nitrogen species imply a
necessary source of 0.4-1 Tg yr-1 of NOx 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
SO2 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.