The impact of different nitrous acid sources in the air quality levels of the Iberian Peninsula

Abstract. Hydroxyl radical (OH) is a primary oxidant in the atmosphere and affects both gas-phase pollutants and particulate matter levels. Nitrous acid (HONO) acts as an important source of OH in the urban atmosphere. Therefore it is important to account accurately for HONO sources within air quality models in order to predict air pollution dynamics. HONO observations in urban areas are characterized by high concentrations at night and low concentrations around midday. Existing gas-phase chemical mechanisms do not reproduce the observed HONO levels, suggesting a lack of sources, such as direct emissions or heterogeneous reactions. Specific HONO emission rates, heterogeneous chemical mechanisms leading to its formation and related kinetics are still unclear. Therefore, most air quality models consider exclusively gas-phase chemistry related to HONO. This work applies the WRF-ARW/HERMES/CMAQ modeling system to quantify the effect of the addition of HONO sources in the predictability of HONO profiles, and its subsequent effect on secondary pollutants formation (mainly O₃ and PM_2.5). The modeling episode is based on a 2004 severe summertime pollution event in the Iberian Peninsula, using high resolution of 4 × 4 km². Two different parameterizations for emissions and the hydrolysis of NO₂ on wet surfaces are added as HONO sources in the atmosphere. Emissions have the largest impact on HONO levels, especially in urban areas, where they can contribute from 66% to 94% to the HONO peak concentration. Additionally, in urban environments, NO₂ hydrolysis on building and vegetation surfaces contributes up to 30% to the HONO peak. Both, the available surface area and the relative humidity must be included as parameters affecting the NO₂ hydrolysis kinetics. As a result, NO₂ hydrolysis is negligible on aerosol surfaces, due to the small surface area available for reaction, and it is more effective in producing HONO below high relative humidity conditions. The addition of HONO sources affects the concentration of secondary pollutants. In particular, major changes are produced in the early morning, due to the higher OH release via HONO photolysis. Significant changes in PM_2.5 concentrations are predicted, that can be 16% (2.6 μg m⁻³) higher in the new scenarios. When accounting for HONO sources, nitrate levels increase especially in urban areas and sulfates in areas downwind from conventional power plants in the Iberian Peninsula. Also, O₃ peak concentrations are slightly affected (from 0.7 to 4 ppb, 1% to 4.5%). The improvement of the HONO sources representation within air quality models produces changes in O₃ peak predictions and significantly affects the reaction pathways leading to aerosols formation. Therefore, HONO sources other than gas-phase chemistry should be accurately included within modeling frameworks.