Crystallisation of mixtures of ammonium nitrate, ammonium sulphate and soot

Crystallisation of laboratory aerosols of ammonium nitrate and of internal mixtures of this salt with ammonium sulphate were investigated using humidity controlled nephelometry. The aerosol was produced via nebulizing of solutions and then dried to 25% RH, which is a realistic minimum value for ambient air. Pure ammonium nitrate and aerosols consisting of internal mixtures of ammonium nitrate and ammonium sulphate with a mixing ratio of more than 1.2 did not crystallize, which is in contrast to assumptions that crystallisation of such mixtures occurs in the atmosphere. Addition of realistic amounts of “soot” did not promote crystallisation. Aerosols with equal ratios of ammonium nitrate to ammonium sulphate crystallized and showed a (first) deliquescence point close to that of ammonium nitrate. Ambient aerosol with a similar mixing ratio behaved similarly. Because the laboratory tests demonstrated that the crystallisation of ammonium nitrate is not promoted by soot but only by the ammonium sulphate, it is concluded that the ambient aerosol particles consisted of internal mixtures of ammonium nitrate and ammonium sulphate.