Monday, 10 February 2003
Heterogeneous formation of nitrous acid in laboratory systems
Nitrous acid (HONO) is known to accumulate overnight and is the major source of OH at dawn in polluted areas. The formation of HONO from heterogeneous NO2 hydrolysis has been studied for many decades, but the mechanism remains unclear. It is well known that surfaces are required for HONO formation and that the overall reaction is represented by 2 NO2 + H2O --> HONO + HNO3, where some HONO is released to the gas phase while HNO3 remains on the surface. Several previous studies show that the reaction is first order with respect to NO2 at ambient relative humidity (RH). We present studies of NO2 hydrolysis at 298 K and 1 atm using long path FTIR spectroscopy to measure gaseous species at 20%, 50%, and 80% RH with initial NO2 levels of 10 - 100 ppm. The reaction order with respect to NO2 was determined by observing both NO2 decay and HONO formation. In addition to HONO production, NO and small amounts of N2O were observed and are believed to be due to secondary reactions of HONO on the walls of the reaction chamber. These gaseous products were quantified and together account for approximately 50% of the NO2 loss, consistent with the above reaction stoichiometry. We present a mechanism for NO2 hydrolysis and modeling studies that explain the results of these experiments and are consistent with most previous studies. The atmospheric implications of heterogeneous NO2 hydrolysis will be discussed.