Impact of ammonium nitrate chemistry on the AOT in Cabauw, the Netherlands

A 1D column model representing detailed microphysics and chemistry associated with sulfate, ammonia and nitric acid is applied to study local processes which influence aerosol optical thickness (AOT). We focus on Cabauw, the Netherlands, on May 8th 2008 during the Intensive Measurement campaign At Cabauw Tower (IMPACT). The 8th of May was characterized by a strong diurnal cycle of RH and aerosol concentration in the boundary layer. Furthermore, the lower troposphere was characterized by high concentrations of aerosol ammonium and nitrate as well as ammonia and nitric acid. Production of ammonium nitrate in the aerosol contributes to the particle hygroscopicity. This leads to additional condensational growth, which, in turn, enables further uptake of ammonium nitrate. It is found to predominantly affect particles smaller than 0.1 μm, so that the simulated wet aerosol size distribution is considerably narrower compared to a situation without ammonium nitrate. Our results show that the daily evolution of aerosol ammonium nitrate influences the variability of AOT more than the daily variability of RH and aerosol concentration. On this particular day, aerosol ammonium nitrate effectively enhances AOT by a factor of 2–4, depending on the time of the day. A relatively large discrepancy between modeled and observed AOT in the afternoon of May 8th is probably associated with secondary organic aerosol production in the afternoon which is not considered in our study.