Evolution of biomass burning smoke particles in the dark

The evolution in the dark of physiochemical properties and chemical composition of smoke particles emitted from wheat straw burning, as well as the effect of relative humidity (RH) on these properties, was investigated in an aerosol chamber. The smoke particles are composed primarily of carbonaceous materials and a considerable amount of inorganic salts (∼25 wt.%). During aging, the fraction of inorganic salts in smoke PM1.0 increases, mainly due to the formation of more sulfate and nitrate at the expense of chloride; this heterogeneous conversion is facilitated at high RH. The hygroscopicity parameter κH of fresh smoke particles is 0.27 and this is estimated to decrease by 0.01 after 4 h dark aging. Both aging and high RH lead to increases of particle size and density. The effective densities of smoke PM2.5 and PM1.0 deduced from concurrent mass and volume concentration measurements gradually increase from about 1.18 to 1.44 g/m3 within 4 h aging at 45%-55% RH, in line with the results obtained both from size-resolved particle density analysis using an aerosol particle mass analyzer (APM) and from estimation using composition-weighted bulk densities. The density of smoke particle is size-, RH-, and aging extent-dependent; the size effect becomes less pronounced with aging.