Formation mass accommodation coefficient

HONO also undergoes deposition at surfaces in competition with its formation by the N02 heterogeneous reaction with water. For example, the mass accommodation coefficient for HONO on water has been reported to be in the range of 4 X 10 3 to 0.15 over temperatures from 278 to 297 K (e.g., Kirchner et al., 1990 Bongartz et al., 1994 Mertes and Wahner, 1995). Thus aqueous particles and surfaces having adsorbed water can also act as a sink for gaseous HONO. This is consistent with the observations of Harrison et al. (1996) on the direction of HONO fluxes from the surface at various concentrations of N02 at N02 concentrations below 10 ppb in rural areas, surfaces were observed to be a net sink of HONO (e.g., see Harrison et al., 1996 and Harrison and Peak, 1997). 

Evidence for the uptake of NO, by aqueous solutions has been sought in both laboratory and field studies. A lower limit for the mass accommodation coefficient for NO, on liquid water of > 2.5 X 10 3 was reported by Thomas et al. (1989) and Mihelcic et al. (1993). Li et al. (1993) followed the formation of particulate nitrate in a rural area and, by comparing their measurements to model predictions, suggested that the mass accommodation coefficient for NO, on aqueous (NH4)2S04-NH4HS04-H2S04 aerosols is approximately unity, i.e. NO, is taken up into the particle on every collision. 

We consider methods for describing how molecules interact with aerosol particles and how to obtain molecular properties and rate constants of relevance when studying the molecular level mechanisms for the formation of aerosol particles and how these provide the basis for heterogeneous chemistry. For understanding mass and heat transfer to and from aerosol particles we need to focus on the processes related to a gas molecule as it approaches the surface of an aerosol particle. A macroscopic property related to these processes is the sticking probabilities/ mass accommodation coefficients that are used when modelling evaporation. 

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