Nitrous Acid Chemistry in the Urban Boundary Layer

Nitrous acid (HONO) plays an important role in tropospheric photochemistry as a major morning source of the hydroxyl radical (OH), which initializes and catalyzes ozone formation in urban areas. It has been reported that HONO photolysis in the morning can contribute to up to 34% of the daytime OH budget. However, its formation mechanisms at night are still not completely understood. Furthermore, recent field observations of unexpected high daytime HONO concentrations in both urban and rural areas point to unrecognized daytime HONO sources, which may be photolytically enhanced. Taking into consideration of these elevated HONO, studies have shown that HONO can be responsible for up to 55% of the daytime OH budget.;The vertical distributions of HONO, which allow further understanding of HONO formation and its impact on OH, were measured at three height intervals (20--70 m, 70--130 m and 130--300 m) over 5 km distance using Long-path Differential Optical Absorption Spectroscopy (LP-DOAS) in Houston, TX during two major field campaigns in 2006 and 2009. The observations were interpreted to determine the dominant HONO formation pathways at night and during the day using a 1-D chemistry and transport model (RCAT8.2).;Nighttime observations showed larger HONO mixing ratios accumulated near the ground, indicating that its source was near or at the ground. Model calculations based on the observations confirmed that NO2 conversion at the ground was the dominant formation pathway of HONO in the lowest 300 m. Vertical transport was the source of HONO aloft. Aerosol played a role in the removal of HONO rather than a source of HONO.;Daytime measurements showed HONO mixing ratios were much larger than the theoretical gas-phase photo-stationary steady state. Larger mixing ratios near the ground than aloft indicated there was an unknown HONO source located near or at the ground. The unknown daytime HONO formation rate (Punknown ), calculated using a steady-state approach, showed a dependence on NO2 and sunlight. Better correlations of the NO2 normalized Punknown with solar irradiance than with actinic flux suggested that photo-enhanced NO2 to HONO conversion occurred at the ground. The daytime HONO observations were reproduced by the 1-D model only if a photolytic source was included predominately at the ground and to a smaller degree on aerosol. Further analysis of model calculations showed a height dependence of daytime HONO formation and removal pathways in the daytime boundary layer.