Formation of Nitrous Acid on Urban Surfaces

Abstract

Nitrous acid (HONO) has been observed in the nocturnal urban atmosphere for decades. During daylight hours, the rapid photolysis of HONO is a significant source of OH-radicals, which drive tropospheric chemistry and ozone-formation. Recently, unexpected high values of HONO have been detected during the day. Despite its importance, sources of HONO are still poorly understood. Direct emission of HONO or homogeneous chemical formation alone cannot explain the high HONO-to-NO2 ratios often measured in the boundary layer. Today it is thus generally accepted that HONO is formed by heterogeneous hydrolysis of NO2. However, large uncertainties about the nature of the surfaces and the chemical conversion mechanism remain. Here, we present direct measurements from three field campaigns detecting daytime HONO mixing ratios of ~200 ppt using DOAS. A chemical transport model (RCAT 8.1.2) was modified to quantify the individual contribution of the vertical transport effects and chemical processes for different times of the day. While aerosols were found to be of minor importance under all circumstances, vertical transport and heterogeneous HONO production on the ground surface (at ~5%) and on canopies (at ~45%) were found to be of major influence on the daytime production of atmospheric HONO. The heterogeneous interactions of HONO with real urban surfaces were further investigated in a smog chamber using a White-type DOAS multi-reflection system. The NO2 uptake coefficients on these surfaces were calculated to be gNO2 ~10-8 on Teflon, ~10-7 on PE foil, ~10-5 on asphalt and concrete, ~3 x 10-6 on roof-tiles and flagstone-tiles, and ~2 x 10-5 on grass. The higher values were found to be well correlated to an enhanced BET surface. The HONO concentrations were found to scale with the relative humidity, and thus the HONO uptake coefficient is not independently determinable. Therefore, a model (HeCSI) was developed using Langmuir adsorption-desorption isotherms to describe the concentration-time series of all trace gases. Based on this new model approach, HONO uptake coefficients, the amount of HONO adsorbed on the surfaces of the smog chamber, and the out-gassing frequency could be determined. It was found that the physical-chemical equilibrium underlying the model describes the chemical NOX - system at all times.