Seasonal Variation of NOx Loss Processes Coupled to the HNO3 Formation in a Daytime Urban Atmosphere: A Model Study

Saiz-Lopez, Alfonso; Notario, Alberto; Albaladejo, José; McFiggans, Gordon

Seasonal Variation of NOx Loss Processes Coupled to the HNO3 Formation in a Daytime Urban Atmosphere: A Model Study

2007

Saiz-Lopez, Alfonso | Notario, Alberto | Albaladejo, José | McFiggans, Gordon

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 O₃, 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 ∼1.4 × 10⁶ 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.

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