Sources and chemistry of nitrogen oxides over the tropical Pacific

We examine the sources and chemistry affecting nitrogen oxides (NO x = NO + NO 2 ) over the tropical Pacific (30°S–20°N) using observations from the Pacific Exploratory Mission to the Tropics B (PEM‐Tropics B) aircraft mission conducted in March–April 1999. A global model of tropospheric chemistry driven by assimilated meteorological data is used to interpret the observations. Median concentrations observed over the South Pacific during PEM‐Tropics B were 7 pptv NO, 16 pptv peroxyacetyl nitrate (PAN), and 34 pptv nitric acid (HNO 3 ); the model generally reproduces these observations but overestimates those over the North Pacific. Lightning was the largest source of these species in the equatorial and South Pacific tropospheric column and in the tropical North Pacific upper troposphere. The oceanic source of acetone implied by high observations of acetone concentrations (mean 431 pptv) allows an improved simulation of PAN/NO x chemistry. However, the high acetaldehyde concentrations (mean 78 pptv) measured throughout the troposphere are inconsistent with our understanding of acetaldehyde and PAN chemistry. Simulated concentrations of HNO 3 and HNO 3 /NO x are highly sensitive to the model representation of deep convection and associated HNO 3 scavenging. Chemical losses of NO x during PEM‐Tropics B exceed chemical sources by a factor of 2 in the South Pacific upper troposphere. The chemical imbalance, also apparent in the low observed HNO 3 /NO x ratio, is explained by NO x injection from lightning and by frequent convective overturning which depletes HNO 3 . The observed imbalance was less during the PEM‐Tropics A campaign in September 1996, when aged biomass burning effluents over the South Pacific pushed the NO x budget toward chemical steady state.