Research Projects

Project at a Glance

Title: Formation of gaseous nitrous acid (HONO): a key determinant of tropospheric ozone and fine particles.

Principal Investigator / Author(s): Finlayson-Pits, Barbara J.

Contractor: UC Irvine

Contract Number: 97-311


Research Program Area: Atmospheric Processes

Topic Areas: Chemistry & Reactivity


Abstract:

Oxides of nitrogen (NOx = NO + NO2) are the only known anthropogenic source of ozone in polluted urban areas in California and are also a major contributor to the formation of particles. While the gas phase chemistry of oxides of nitrogen is reasonably well understood and represented in current airshed models, it is known that NOx also undergoes i.heterogeneousld reactions on surfaces, e.g. of particles, buildings, roads etc. These surface reactions can have a significant impact on the overall chemistry, but because they are not understood, they are not explicitly included in airshed models used for control strategy development. In this project, some of the heterogeneous reactions of oxides of nitrogen were studied using a variety of experimental systems where both the gas and surfaces could be probed spectroscopically. One of these heterogeneous reactions is the hydrolysis of nitrogen dioxide (NO2) on surfaces which generates nitrous acid (HONO). Nitrous acid is the major source of the hydroxyl radical in the morning in coastal urban areas in California, setting off the chain chemistry that leads to the formation of ozone, particles and associated pollutants. We have shown that the mechanism of the surface reaction of nitrogen dioxide (NO2) with water is different than proposed in earlier studies and as represented in current models. Nitric acid (HNO3) is also produced in the NO2 hydrolysis reaction, but rather than being released to the gas phase, it remains adsorbed on the surface. We established that, in contrast to current understanding where nitric acid is deposited out or forms particulate nitrate, this nitric acid on the surface can undergo further reactions with nitric oxide to regenerate nitrogen dioxide. This makes at least some of the nitric acid available for further regeneration of ozone. Initial airshed modelling incorporating this chemistry shows that it may have significant implications for the chemistry used in current models and hence potentially on the predicted impacts of various degrees of control of VOC and NOx.


 

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