CONCENTRATION GRADIENTS AND DRY DEPOSITION OF NITRIC-ACID VAPOR AT THE MAUNA-LOA-OBSERVATORY, HAWAII

From August 14 to September 5, 1991, we measured the mixing ratio of nitric acid vapor and several aerosol species (nitrate, sulfate, ammonium and sodium) at three heights (2 m, 10 m and 30 m) at the Mauna Loa Observatory. Well-defined logarithmic gradients of temperature and HNO3 vapor were measured throughout the experiment, indicating rapid HNO3 vapor deposition to the ground. Measured dry deposition velocities for HNO3 vapor ranged from 0.27 to 4 cm s-1. The collection efficiency for HNO3 vapor varied with sampler location and orientation on the sampling tower, due to concentration depletion by the tower or the sampler itself. We found that our long-term samplers at 8 m may underestimate free tropospheric HNO3 mixing ratios by about 20% due to dry deposition.