Nitric Acid

CAS RN: 7697-37-2

Environmental Fate

TERRESTRIAL FATE: During transport through the soil, nitric acid will dissolve some of the soil material, in particular, the carbonate based materials. The acid will be neutralized to some degree with adsorption of the proton also occurring on clay materials. However, significant amounts of acid are expected to remain for transport down toward the ground water table. Upon reaching the ground water table, the acid will continue to move, now in the direction of the ground water flow. A contaminated plume will be produced with dilution and dispersion serving to reduce the acid concn.
ATMOSPHERIC FATE: In Colorado, nitric acid vapor is scavenged by incorporation into snow.
ATMOSPHERIC FATE: A mesoscale model of pollutant transport, transformation and deposition was used to perform a detailed analysis of acidic deposition to the states of New York and Ohio (USA) during a 3 day springtime deposition episode. This model can be used to assess the roles of wet and dry deposition to individual land types in the removal of pollutants from the atmosphere. Over two-thirds (67%, Ohio; 78%, New York) of the acidic deposition during this rainy period fell as wet deposition, primarily in the form of sulfuric acid. Dry deposition of sulfur dioxide accounted for 70-75% of the total dry acidic deposition in both areas, and most of the remaining dry deposition occurred as nitric acid. Over both deposition areas, particulate sulfate deposition accounted for < 1% of the total acid deposition. Due to the highly surface specific nature of the dry deposition process, individual land types displayed unique patterns of pollutant uptake. Water surfaces absorbed primarily sulfur dioxide, while rougher forested areas absorbed a larger proportion of nitric acid vapor. Urban areas, with their associated material surface, were found to absorb significantly less acid in the dry form, and during dry periods most of this deposition may occur as nitric acid vapor, although considerable uncertainty exists regarding the treatment of rainfall wetted surfaces. These model results suggest that dry pollutant fluxes to individual surface types will show significant variability from any averaged flux estimates over larger areas encompassing numerous land types.
ATMOSPHERIC FATE: Dry deposition of nitric acid (HNO3) to forests is controlled by aerodynamic properties of the canopy. Most surfaces are strong sinks for HNO3, and measurements show that deposition rates to vegetation are determined entirely by atmospheric transport, i.e. there are no surface resistances limiting uptake rates. For a typical forest 10 m high in a wind speed of about 5 m/sec, values of deposition velocity for HNO3 are likely to be in the range 50-100 mm/sec. For an avg air concn of HNO3 of 0.5 nL/L this would result in the deposition of about 4-8 kg N/ha/yr. A multi-layer canopy gas and radiation exchange model (Maestro) was modified to calculate air pollutant deposition. Leaf boundary layer resistances, and stomatal resistances in the model were adjusted for gas molecular diffusivity, and leaf surface resistances and internal resistances were added. A comparison between HNO3 deposition on Keilder Forest (300 m above sea level), United Kingdom and Whitetop Mountain (1682 m above sea level), Virginia for 6 mo (spring-summer) gave gas concn of 0.3 nL/L and 0.7 nL/L, respectively.
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