Naeem Lodhi, Phd candidate, presented his works at the 30th Annual American Association for Aerosol Research Conference.
Abstract: The hygroscopic properties of secondary organic aerosol formed by the OH initiated oxidation of isoprene and several terpenes were investigated in the York University smog chamber facility. Pure organic particles were formed in nucleation experiments using either the individual hydrocarbons or the sequential oxidation of multiple hydrocarbons. In addition, to examine the interaction of organic and inorganic phases, monodisperse ammonium sulfate seed particles were allowed to undergo condensational growth due to partitioning of oxidation products from the gas phase. Humidograms (plots of hygroscopic growth factor as function of relative humidity) were measured using a humidified tandem differential mobility analyzer (HTDMA).
The humidograms of pure organic particles formed in nucleation experiments don’t show any deliquescence or efflorescence. As relative humidity is raised to 40%, particles formed from isoprene show apparent diameter decreases of up to 5%. Particles formed by oxidation of a-pinene, limonene, and delta$^3-carene exhibit very little or no water uptake. For the oxidation products of b-pinene, the humidograms show slight but smooth uptake of water; isoprene and delat$^3-carene oxidation products show similar behavior above 40% RH and 60% respectively. The results were fitted with an empirical equation and give hygroscopicity parameters (+/- one standard error) of 0.0154 +/- 0.0014 for b-pinene and 0.036 +/- 0.002 for isoprene. These correspond to diameter growth factors of 1.031 and 1.069 at 85% RH.
Water uptake by multi-component secondary organic particles obey the volume additivity rule. Humidograms of seeded particles show both smooth hygroscopic growth and deliquescence. These experimental results were fitted with a numerical model that accounts for water uptake by both phases and for the gradual dissolution of ammonium sulfate. The results show that volume additivity is a reasonable approximation for this system and that HTDMA results can be inverted to obtain the organic hygroscopicity parameter and the relative amounts of organic and inorganic material.