Nitrous acid (HONO) photolysis constitutes a primary source of OH in the early morning, which leads to
changes in model gas-phase and particulate matter concentrations. However, state-of-the-art models of
chemical mechanisms share a common representation of gas-phase chemistry leading to HONO that fails
in reproducing the observed profiles. Hence, there is a growing interest in improving the definition of
additional HONO sources within air quality models, i.e. direct emissions or heterogeneous reactions. In
order to test their feasibility under atmospheric conditions, the WRF-ARW/HERMES/CMAQ modeling
system is applied with high horizontal resolution (4 4 km2) to Spain for November 24e27, 2008. HONO
modeled sources include: (1) direct emissions from on-road transport; NO2 hydrolysis on aerosol and
ground surfaces, the latter with (2) kinetics depending exclusively on available surfaces for reaction and
(3) refined kinetics considering also relative humidity dependence; and (4) photoenhanced NO2
reduction on ground surfaces. The DOMINO measurement campaign performed in El Arenosillo
(Southern Spain) provides valuable HONO observations. Modeled HONO results are consistently below
observations, even when the most effective scenario is assessed, corresponding to contributions of direct
emissions and NO2 hydrolysis with the simplest kinetics parameterization. With the additional sources of
HONO, PM2.5 predictions can be up to 14% larger in urban areas. Quantified impacts on secondary
pollutants have to be taken as a low threshold, due to the proven underestimation of HONO levels. It is
fundamental to improve HONO sources definition within air quality models, both for the scientific
community and decision makers.Peer ReviewedPostprint (published version
