Infrared Spectroscopic Evidence for a Heterogeneous Reaction between Ozone and Sodium Oleate at the Gas–Aerosol Interface: Effect of Relative Humidity

The heterogeneous ozonolysis of sodium oleate aerosols in an aerosol flow tube (AFT) is reported under different relative humidity (RH%) conditions. Sub-micron sodium oleate particles were exposed to a known ozone concentration and the consumption of sodium oleate was monitored by infrared spectroscopy. When the experimental results are treated as a surface-mediated reaction (i.e. following a Langmuir-Hinshelwood type mechanism), the following parameters are obtained: at low RH%, K(O3 )=(3 � 1)�10-16 cm3 molecule-1 and k_max^I=(0.046 � 0.006) s-1; at high RH%, K(O3 )=(6 � 2)�10-16 cm3 molecule-1 and k_max^I=(0.21 � 0.05) s-1. From these pseudo first-order coefficients, the reactive uptake coefficients for dry and aqueous sodium oleate aerosols are calculated as (1.5 ? 0.5) � 10-7 and (1.7 ? 0.7) � 10-6, respectively. Hydrated oleate aerosols display both an increase in the ozone trapping ability and in the effective rate reaction at the droplet surface compared to dry aerosol surfaces. These observations may provide an explanation for some of the variability observed between lab studies of dry ozonolysis and real-world, atmospheric lifetimes of oleic acid-related species.