posted on 2014-01-07, 00:00authored byMelissa
A. Donaldson, Andrew E. Berke, Jonathan D. Raff
Nitrous
acid (HONO) is an important OH radical source that is formed
on both ground and aerosol surfaces in the well-mixed boundary layer.
Large uncertainties remain in quantifying HONO sinks and determining
the mechanism of HONO uptake onto surfaces. We report here the first
laboratory determination of HONO uptake coefficients onto actual soil
under atmospheric conditions using a coated-wall flow tube coupled
to a highly sensitive chemical ionization mass spectrometer (CIMS).
Uptake coefficients for HONO decrease with increasing RH from (2.5 ±
0.4) × 10–4 at 0% RH to (1.1 ± 0.4) ×
10–5 at 80% RH. A kinetics model of competitive
adsorption of HONO and water onto the particle surfaces fits the dependence
of the HONO uptake coefficients on the initial HONO concentration
and relative humidity. However, a multiphase resistor model based
on the physical and chemical processes affecting HONO uptake is more
flexible as it accounts for the pH dependence of HONO uptake and bulk
diffusion in the soil matrix. Fourier transform infrared (FTIR) spectrometry
and cavity-enhanced absorption spectroscopy (CEAS) studies indicate
that NO and N2O (16% and 13% yield, respectively) rather
than NO2 are the predominant gas phase products, while
NO2– and NO3– were detected on the surface post-exposure. Results are compared
to uptake coefficients inferred from models and field measurements,
and the atmospheric implications are discussed.