The Effect of Relative Humidity on Secondary Organic Aerosol Formation

Nano-scale particles in the atmosphere, known as aerosols, have important impacts on human health and in cloud formation. When aerosols are able to take on water under cloud-forming conditions, they are known as cloud condensation nuclei (CCN). Alpha pinene gas is emitted primarily by vegetation, and can react with a number of atmospheric components, including ozone gas, thereby generating secondary organic aerosols (SOA). The purpose of this investigation was to study the effects of relative humidity during the SOA-forming process on the CCN activity and particle size distribution of the products of the reaction of alpha pinene and ozone. This effect was studied by reacting atmospherically-relevant concentrations of ozone and alpha pinene in a one cubic meter Teflon smog chamber at varying levels of relative humidity and then sampling the resulting aerosol over time to measure the changing particle concentrations, size distributions, and ultimate CCN activity of the newly formed SOA.