CRYSTALLIZATION OF MIXTURES OF AMMONIUM-NITRATE, AMMONIUM-SULFATE ANDSOOT

Crystallisation of laboratory aerosols of ammonium nitrate and of inte rnal mixtures of this salt with ammonium sulphate were investigated us ing humidity controlled nephelometry. The aerosol was produced via neb ulizing of solutions and then dried to 25% RH, which is a realistic mi nimum value for ambient air. Pure ammonium nitrate and aerosols consis ting of internal mixtures of ammonium nitrate and ammonium sulphate wi th a mixing ratio of more than 1.2 did not crystallize, which is in co ntrast to assumptions that crystallisation of such mixtures occurs in the atmosphere. Addition of realistic amounts of ''soot'' did not prom ote crystallisation. Aerosols with equal ratios of ammonium nitrate to ammonium sulphate crystallized and showed a (first) deliquescence poi nt close to that of ammonium nitrate. Ambient aerosol with a similar m ixing ratio behaved similarly. Because the laboratory tests demonstrat ed that the crystallisation of ammonium nitrate is not promoted by soo t but only by the ammonium sulphate, it is concluded that the ambient aerosol particles consisted of internal mixtures of ammonium nitrate a nd ammonium sulphate. (C) 1998 Elsevier Science Ltd. All rights reserv ed.