Removal of NO_<i>x</i> and NO_<i>y</i> in biomass burning plumes in the boundary layer over northern Australia

The Biomass Burning and Lightning Experiment Phase B (BIBLE‐B) aircraft measurement campaign was conducted over the western Pacific and Australia in August and September 1999. In situ aircraft measurements of carbon monoxide (CO), nitric oxide (NO), total reactive nitrogen (NO y ), ozone (O 3 ), nonmethane hydrocarbons (NMHCs), and other species were made during BIBLE‐B. Meteorological analysis shows that the trace gases emitted from biomass burning in northern Australia were mostly confined within the planetary boundary layer (below ∼3 km) by strong subsidence in the free troposphere. Removal processes of NO x (equal to measured NO + calculated NO 2 ) and NO y in biomass burning plumes in the boundary layer are examined on the basis of correlation analysis. The photochemical lifetime of NO x in biomass burning plumes during the daytime is estimated to be 0.1 to 0.3 days using the correlations of NO x with short‐lived NMHCs and hydroxyl radical (OH) concentration calculated from a constrained photochemical model. Correlation of NO y with CO shows that ∼60% of the NO y molecules originating from biomass burning were removed in the boundary layer within 2–3 days. This result is consistent with dry deposition of nitric acid (HNO 3 ) in the plumes. It is likely that only a small fraction of NO y emitted from biomass burning was exported from the boundary layer to the free troposphere during the BIBLE‐B period.