The effect of biological particles and their ageing processes on aerosol radiative properties: Model sensitivity studies

Abstract. Biological aerosol particles (BAPs) such as bacteria, viruses, fungi and pollen, represent a small fraction of the total aerosol burden. However due to their unique properties, they have been suggested to be important in for radiative forcing by the aerosol direct and indirect effects. By means of process model studies, we compare the sensitivity of these radiative effects to various physicochemical BAP properties (e.g. number concentration, diameter, hygroscopicity, surface tension, contact angle between ice and particles). Exceeding previous sensitivity studies, we explore not only the variability of these properties among different BAP types, but also the extent to which chemical (e.g. nitration), physical (e.g. fragmentation) and biological (e.g. bacteria cell generation) ageing processes of BAPs can modify these properties. Our model results lead to a ranking of the various properties for the radiative effects: (i) Given that BAPs contribute ~ 0.1 % to total cloud condensation nuclei (CCN) number concentration, their effect on total CCN is likely small. (ii) BAPs number fraction of large particles (diameter > ~ 0.5 μm) is much higher, resulting in a relatively more important effect on direct radiative forcing. (iii) In mixed-phase clouds at T > −10 °C, BAPs can contribute ~ 100 % to ice nuclei (IN), which makes their role as IN the most important. Our study highlights the need of implementing ageing processes of different BAPs into models as BAP size, CCN and IN activity and optical properties may be sufficiently altered to affect BAP's residence time and survival in the atmosphere. In particular, we suggest the potential role of biological processes, that are currently not included in aerosol models due to the sparsity of comprehensive data, could affect physicochemical BAP properties