Impact of environmental variables on the reduction of nitric acid (HNO3) by volatile organic compounds emitted by motor vehicles


Nitric acid (HNO3) has long been thought of as a sink for nitrogen oxides in the atmosphere but recently has been identified as a potential precursor of nitrous acid (HONO). The mechanisms proposed for the conversion of HNO3 to HONO included heterogeneous reactions on surfaces such as primary organic aerosols, vegetation, and glass. Recent work in our laboratory has indicated that the reduction of HNO3 to HONO occurs homogeneously in the presence of volatile organic compounds (VOCs) emitted by motor vehicles. These experiments were conducted using a temperature-controlled quartz flow-tube reactor, and measurements of HNO3 and HONO were collected using a mist-chamber/ion-chromatography system. The average observed HONO formation rate was 0.54 ± 0.08 ppb hr-1 at 25 °C and ~20% relative humidity; these data are comparable to data from a 2006 field study in Houston, TX during which enhancements in HONO during morning rush hour were observed. This study focuses on the impact of environmental variables on the rate of formation of HONO. This rate decreases at higher relative humidity but increases with decreasing reactor temperature. The reaction rate does not change when initial HNO3 concentration is varied between 400 to 4600 ppt, suggesting that the concentration of reactive VOCs was the limiting factor. The reduction of HNO3 to HONO appears not to occur heterogeneously on the aerosol surfaces tested (motor oil, alumina, and carboxylated silica aerosol). The presence of ~120 ppb of ammonia has no observable impact on the reaction. The homogeneous conversion of HNO3 to HONO can have significant atmospheric implications due to the “renoxification” of non-reactive HNO3 into the reactive nitrogen species HONO.

Publication Date

Fall 2014

Journal Title

Abstracts of Papers of the Aamerican Chemical Society


American Chemical Society

Document Type

Conference Proceeding



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