Measurements of the Nitric Acid to NOx Ratio in the Troposphere

Nitric acid concentrations, measured by both teflon/nylon filter packs (FP) and the tungstic oxide denuder (DEN), are compared with the average NOx concentrations from laser-induced fluorescent and chemiluminescent methods. The HNO3/NOx ratio based on filter packs ranged from 0.8 to 10.4, with a mean of 3.4. The DEN nitric acid concentrations produced ratios ranging from <0.3 to 9.8, with a mean of 2.6. Average marine ratios were larger than those from continental regions, in part due to continental anthropogenic sources of NOx. Although we collected very few boundary layer samples, their average ratios were smaller than those in the free troposphere, apparently because of the effect of dry surface removal of nitric acid. The nitric acid to NOx ratio was greatest when the NOx/NOy ratio was smallest, such that the nitrogen photochemistry was nearing completion.