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The phase transitions and hygroscopic growth of two humic acid aerosols(Aldrich sodium salt and Leonardite Standard (IHSS)) andtheir mixtures with ammonium sulphate have been investigated using acombination of two techniques, Fourier transform infra-red (FTIR)  spectroscopyand tandem differential mobility analysis (TDMA).  A growth factor of 1.16 at 85% relative humidity (<i>RH</i>) was found for the Aldrich humic acid which can be regarded as an upper limit forgrowth factors of humic-like substances (HULIS)found in atmospheric aerosol and issignificantly smaller than that of typical atmosphericinorganics. We find that the humic acid aerosols exhibit water uptake overall relative humidities with no apparent phase changes, suggesting thatthese aerosols readily form supersaturated droplets. In the mixedparticles, the humic acid component decreases the deliquescencerelative humidity (DRH) and increases the efflorescence relative humidity(ERH) of the ammonium sulphate component, and there is some degree ofwater uptake prior to ammonium sulphate deliquescence. In addition, at lowRH, the FTIR spectra showthat the ammonium is present in a different chemicalenvironment in the mixed aerosols than in crystalline ammonium sulphate,perhaps existing as a complex with the humic materials. The growth factorsof the mixed aerosols are intermediate between those of thesingle-component aerosols and can be predicted assuming that the inorganicand organic fractions take up water independently

Phase transitions and hygroscopic growth of aerosol particles containing humic acid and mixtures of humic acid and ammonium sulphate