Integrated theoretical and experimental studies of organic atmospheric aerosols

Abstract

A combination of different projects are presented to (a) give the theoretical effects of organic aerosol on the nucleation and growth of cloud droplets and (b) present measurements of organic aerosol material at a land-based site used to study the remote marine atmosphere.The basis of the theoretical work is that organic aerosol material tends to have solubility in water that is lower than hygroscopic salts, but higher than other aerosol constituents such as mineral dust or elemental carbon (soot) particles. When present in an internal mixture with salt particles that deliquesce to form cloud droplets, the sparingly-soluble organic material would undergo gradual dissolution such that the critical supersaturation is lowered compared to droplets nucleated on salt particles alone. Results of an adiabatic cloud model are presented to show that the size distribution of activated droplets is shifted towards a smaller number of larger droplets as a result of this gradual dissolution. This effect is dependent on the amount and solubility of the organic material present in the particles, and would lower the reflectivity of clouds compared to the reference case of a cloud nucleated on purely hygroscopic aerosol. Atmospheric particle solubility therefore is a variable that influences the global radiative balance.A thermal-optical carbon analyzer was built to measure ambient concentrations of organic and refractory carbon at Cheeka Peak, an atmospheric monitoring station on the coast of the Olympic Peninsula of Washington State. Measurements took place continuously over the course of approximately one year, which is the largest data set of carbonaceous material for a site of this type. The instrument is described in detail, as well as the sampling methods used. The data shows that when quartz filters are used to collect atmospheric material in remote marine locations with very low concentrations of organic material, there is an organic vapor artifact that is approximately equal in mass to the amount of organic particles collected, adding to the uncertainty in the measurement. The average concentration of organic aerosol at this site is ca. 50 ng m$\sp{-3},$ compared with ca. 20 ng m$\sp{-3}$ for refractory (black, soot) carbon.