Deposition and adsorption of the air pollutant HNO3 vapor to soil surfaces

Deposition of nitric acid (HNO3) vapor to soils has been evaluated in three experimental settings: (1) continuously stirred tank reactors with the pollutant added to clean air, (2) open-top chambers at high ambient levels of pollution with and without filtration reducing particulate nitrate levels, (3) two field sites with high or low pollution loads in the coastal sage plant community of southern California. The results from experiment (1) indicated that the amount of extractable NO3- from isolated sand, silt and clay fractions increased with atmospheric concentration and duration of exposure. After 32 days, the highest absorption of HNO3 was determined for clay, Followed by silt and sand. While the sand and silt fractions showed a tendency to saturate, the clay samples did not after 32 days of exposure under highly polluted conditions. Absorption of HNO3 occurred mainly in the top 1 mm layer of the soil samples and the presence of water increased HNO3 absorption by about 2-fold. Experiment (2) indicated that the presence of coarse particulate NO; could effectively block absorption sites of soils for HNO3 vapor. Experiment (3) showed that soil samples collected from open sites had about 2.5 more extractable NO; as compared to samples collected from beneath shrub canopies. The difference in NO3- occurred only in the upper 1-2 cm as no significant differences in NO; concentrations were found in the 2-5 cm soil layers. Extractable NO3- from surface soils collected from a low-pollution site ranged between 1 and 8 mug NO3-N g(-1), compared to a maximum of 42 mug NO3-N g(-1) for soils collected from a highly polluted site. Highly significant relationship between HNO3 vapor doses and its accumulation in the upper layers of soils indicates that carefully prepared soil samples (especially clay fraction) may be useful as passive samplers for evaluation of ambient concentrations of HNO3 vapor. Published by Elsevier Science Ltd.