Modelling the partitioning of ammonium nitrate in the convective boundary layer

Aerosols have a pronounced influence on the climate system, both directly by scattering and absorbing incoming solar radiation (Hess et al., 1998; Haywood and Boucher, 2000; IPCC, 2007) and indirectly by altering cloud properties (Rosenfeld et al., 2008; 5 Kaufman et al., 2002). The combined climate effect of aerosols is poorly understood compared to the climate effect of greenhouse gases. In the Netherlands, nearly half (42 %–48 %) of the fine aerosol (PM2.5) mass consists of secondary inorganic aerosols (ammonium nitrate and ammonium sulphate) (Weijers et al., 2011), which are the dominant anthropogenic aerosol species in the size range with maximum light scattering 10 (0.4–1.0 μm) (ten Brink et al., 1997). Also, these secondary inorganic aerosols are effective cloud condensation nuclei, because of their size and water solubility. Due to intensive agriculture, the ammonia concentrations in the Netherlands have always been sufficiently high to neutralise sulphuric and nitric acid. During the last twenty-five years, sulphur dioxide emissions have decreased much more than those of nitrogen oxides 15 in Europe and especially in the Netherlands (Vestreng et al., 2007, 2009). Therefore, ammonium nitrate has become increasingly important in comparison to ammonium sulphate. There are two major physical differences between ammonium nitrate and ammonium sulphate. First, the water uptake by ammonium nitrate aerosol depends stronger on the ambient humidity than the water uptake by ammonium sulphate 20 aerosol (Tang, 1996). Hygroscopic growth of aerosols evidently affects the interaction with solar radiation. Second, ammonium nitrate resides in both the gas phase and in the aerosol phase while ammonium sulphate resides exclusively in the aerosol phase. The equilibrium between the gas phase and the aerosol phase depends strongly on temperature and relative humidity. As a result, the interaction of ammonium nitrate 25 with solar radiation is much stronger at lower temperatures and higher relative humidities. With ammonium nitrate becoming increasingly important, systematic investigation of these properties seems appropriate.