Surface modification of bioaerosol by physical, chemical, and biological ageing processes

Primary biological aerosol particles (PBAP) are a significant fraction of total atmospheric aerosol burden and can exhibit unique properties in terms of ice nucleation. In current atmospheric models, it is usually assumed that the physicochemical properties of PBAP are constant during their atmospheric residence time. However, several experimental studies have shown that PBAP undergo microphysical, chemical, and biological ageing processes in the atmosphere. These processes include bacterial agglomeration, modification of protein surfaces by chemical reactions (e.g., nitration) and cellular responses to changing ambient conditions. In addition, possible biological ageing processes such as cell growth and multiplication may change cell size and number. Here, we explore by means of process models the modification of the ice nucleating, hygroscopic and optical (scattering/absorption) properties of PBAP by such ageing processes with an emphasis on biological ageing. We show that cell growth/multiplication of ice-nucleating bacteria could enhance IN activity. Besides, cell modficaiton by ageing processes might change bacteria scattering properties due to size and surface composition modfication. Modification of protein surfaces  decreases IN activity for certain types of PBAP over atmospeherically relevant time scales. We perform model sensitivity tests over wide ranges of chemcial and biological parameters to identify conditions, under which these and other ageing processes have a significant effect on physicochemical properties of aged PBAP. Based on this analysis, we develop parameterizations for PBAP ageing processes to be included in aerosol and cloud models of different scales.