Photochemical ozone production in tropical squall line convection during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A

Abstract

We have examined the role of convection in trace gas transport and ozone production in a tropical dry season squall line sampled on August 3, 1985, during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A (NASA GTE/ABLE 2A)in Amazonia, Brazil. Two types of analyses were performed. (1) Transient effects within the cloud are examined with a combination of two-dimensional cloud and one-dimensional photochemical modeling. Tracer analyses using the cloud model wind fields yield a series of cross sections of NOₓ, CO, and O₃ distribution during the lifetime of the cloud; these fields are used in the photochemical model to compute the net rate of O₃ production. At noon, when the cloud was mature, the instantaneous ozone production potential in the cloud is between 50 and 60% less than in no-cloud conditions due to reduced photolysis and cloud scavenging of radicals. (2) Analysis of cloud inflows and outflows is used to differentiate between air that is undisturbed and air that has been modified by the storm. These profiles are used in the photochemical model to examine the aftereffects of convective redistribution in the 24-hour period following the storm. Total tropospheric column O₃ production changed little due to convection because so little NOₓ was available in the lower troposphere. However, the integrated O₃ production potential in the 5- to 13-km layer changed from net destruction to net production as a result of the convection. The conditions of the August 3, 1985, event may be typical of the early part of the dry season in Amazonia, when only minimal amounts of pollution from biomass burning have been transported into the region.