Investigating The Effect Of Relative Humidity On Organic New Particle Formation From The Dark Ozonolysis Of Biogenic Volatile Organic Compounds

Austin Callum Flueckiger, University of Vermont


Solid or liquid particulate matter suspended in the air, also known as atmospheric aerosols, are a ubiquitous component of Earth’s atmosphere. It is important to understand the chemical and physical processes that lead to the formation of these aerosols as they have an impact on climate health and human health. An important subset of atmospheric aerosols are secondary organic aerosols (SOA) that form from the gas-phase oxidation of volatile organic compounds (VOCs). VOCs can be emitted via biogenic and anthropogenic pathways, however, global estimates place biogenic sources as the major contributor. Although widely studied, some of the fundamental mechanisms that lead to the formation of SOA are still poorly understood. The impact of relative humidity (RH) on organic new particle formation (NPF) of SOA particles via ozonolysis of biogenic VOCs remains an area of active debate. Most laboratory chamber studies have conducted experiments under dry (0 % RH) conditions and at artificially high VOC mixing ratios (ξVOCs) not reflective of the lower troposphere. For the few studies that do account for RH (the third most abundant atmospheric gas), most report a negative correlation between RH and NPF. However, these artificially high ξVOCs are not indicative of real-world chemical processes taking place in the lower troposphere. Herein, the effect of relative humidity on organic NPF from the ozonolysis of biogenic VOCs is studied specifically at atmospherically relevant ξVOCs. α-Pinene, βpinene, cis-3-hexenyl acetate, myrtenol, sabinene, and α-terpineol were all subjected to ozonolysis experiments. It is shown that RH has a multi-fold enhancement on NPF from α-pinene, β-pinene, sabinene, and α-terpineol-derived SOA at each respective lowest ξVOC, with a very strong dependence on ξVOC. Although cis-3-hexenyl acetate and myrtenol showed an attenuation in NPF at all ξVOCs, evidence is provided to indicate that there likely exists a chemical mechanism leading to the observed behavior. In addition, measurements made on the rate of NPF for α-pinene-derived SOA suggest that the presence of elevated RH at particle genesis has a major influence on the enhancement in NPF, for the VOCs that demonstrated an enhancement event.