Seasonal variation of NOx loss processes coupled to the HNO3 formation in a daytime urban atmosphere:: A model study

We studied the seasonal cycle of the coupling between atmospheric denoxification processes and in-situ daytime formation of gas phase HNO3 using a photochemical air pollution model. The model is constrained with urban atmospheric boundary layer observations of O-3, NO2 and NO made in Ciudad Real, central Spain. The highest daytime HNO3 mixing ratio of 0.3 ppbv was predicted to occur in summer, following a modelled OH concentration peak of similar to 1.4x10(6) molecules cm(-3) and subsequent reaction with NO2. During winter, calculated values of HNO3 are lower due to less incoming radiation and higher wet removal of atmospheric HNO3. The predicted mixing ratios are in good agreement with observations of atmospheric HNO3 at similar urban environments in central Spain. Additionally, a marked seasonal cycle is predicted with minimum HNO3 concentrations occurring in winter, indicative that traffic emissions and photochemistry dominate the in-situ formation of gas phase HNO3 at this location. This process has implications in the removal of NOx from the urban atmosphere.