1. Effect of surfactant Effect of morphology
Measuring Uptake Coefficients for Polycyclic Aromatic Hydrocarbons onto Aerosol Particles Using Photoionization Detection
Effect of surfactant
Effect of morphology
Surfactant covered aerosol particles retain a thin soapy film on the surface at low RH. We measured relative uptake coefficients for the NaCl/SDS system over a wide range of RH in both a hydration and dehydration mode to highlight the important of morphology and hysteresis effects. This figure shows some of the data for the NaCl/SDS system. In the dehydration experiment, we find that the lowest value of g corresponds the aqueous morphology (>45% RH), and that g increases by a factor of ~2.5 upon efflorescence. This feature changes with SDS coverage and is most pronounced when the shrinking that accompanies the efflorescence of the particle concentrates the SDS from sub-monolayer to super-monolayer coverages. Interestingly, this thin film morphology is the most efficient of all particle morphologies in taking up pyrene. In the hydration experiment, the uptake coefficient changes only slightly with increasing RH as expected. There is no phase change in this experiment and the particle morphology is similar throughout. These experiments highlight the important of particle morphology in working with models of atmospheric particles.
This figure shows uptake coefficient for pyrene onto SDS-coated NaCl aerosol particles for several surface coverages of SDS. These experiments were all carried out under dry conditions. For both 144 and 200 nm particles, g is near 1.5x10-4 for pure NaCl particles. The uptake coefficient jumps dramatically with nominally only 1 monolayer of SDS absorbed to the surface and remains relatively constant at higher coverage. Under dry conditions, our experiments suggest that SDS is not necessarily evenly distributed on the surface; however the effect of the surfactant is insensitive to the amount of SDS. It is likely that the SDS domains increase in thickness as the nominal coverages increase rather than spreading out to cover more of the surface.