Droplet phase (heterogeneous) and gas phase (homogeneous) contributions to secondary ambient aerosol formation as functions of relative humidity.

It is shown that liquid phase reactions tend to be the most important mechanism of sulfate aerosol formations for relative humidities greater than 75%, while gas phase reactions are predominant at lower humidities. In either case, although measurements have been made both during daylight and nighttime hours, aerosol formation has been observed only in air parcels exposed to sunlight. -from Authors