Experiment (3) showed that soil samples collected from open sites had about 2.5 more extractable NO 3 − as compared to samples collected from beneath shrub canopies. Experiment (2) indicated that the presence of coarse particulate NO 3 − could effectively block absorption sites of soils for HNO 3 vapor. Absorption of HNO 3 occurred mainly in the top 1 mm layer of the soil samples and the presence of water increased HNO 3 absorption by about 2-fold. 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. After 32 days, the highest absorption of HNO 3 was determined for clay, followed by silt and sand. The results from experiment (1) indicated that the amount of extractable NO 3 − from isolated sand, silt and clay fractions increased with atmospheric concentration and duration of exposure. Deposition of nitric acid (HNO 3) 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.
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