Role of Relative Humidity in New Particle Formation from Ozonlysis of Atmospheric Volatile Organic Compounds

Christopher Snyder, University of Vermont


The impact of relative humidity (RH) on organic new particle formation (NPF) from ozonolysis of biogenic volatile organic compounds (BVOCs) remains an area of active debate. Previous reports provide contradictory results indicating both depression and enhancement of NPF under conditions of moderate RH, while others ignore the potential impact. Only several reports have suggested that the effect may depend on absolute mixing ratio of the precursor volatile organic compound (VOC, ppbv). However, before any experiments could be completed, development of new methods was necessary to overcome the limitation of sampling ultrafine nanoparticles (<50 nm aerodynamic>diameter) with aerosol mass spectrometry. This dissertation includes a report on a new Particle Growth Apparatus (PaGA) that artificially grows particles from as small as 17 nm to over 110nm. Considerable effort was made to identify the most suitable growth matrix (squalane) and optimize particle growth for reproducibility and sensitivity.

The PaGA was then utilized in the subsequent reports on the impact of RH on NPF from dark ozonolysis of cis-3-hexenyl acetate (CHA) and α-pinene. We show that RH inhibits NPF by CHA, essentially shutting it down at low RH. New oxidation products dominant under humid conditions were identified that, based on estimated vapor pressures (VPs), should enhance NPF; however, it is possible that the vapor phase concentration of these low volatility products is not sufficient to initiate NPF. Furthermore, reaction of C3-excited state Criegee intermediates (CIs) with water may lead to the formation of small carboxylic acids that do not contribute to NPF. This hypothesis is supported by experiments with quaternary O3 + CHA + α-pinene + RH systems, which showed decreases in total α-pinene-derived NPF at ~ 0% RH and subsequent recovery at elevated RH. We then report on the impact of RH on NPF from dark ozonolysis of α-pinene at mixing ratios ranging from 4 to 60 ppbv. We show that RH enhances NPF (factor of 8) at the lowest mixing ratio, with a very strong dependence on α-pinene mixing ratio from 4 to 22 ppbv. For the case of α-pinene, NPF is enhanced at low mixing ratios due to a combination of chemistry, accelerated kinetics and, potentially albeit counterintuitively, reduced partitioning of semi-volatile oxidation products to the particulate phase. The different NPF behavior between the CHA and α-pinene in the presence of water is likely due to the fate of the respective CIs formed from the ozonolysis and their ability to form low volatility products.

The results from this work showed how water affects SOA generation based on the VOC precursor and its concentration. These important findings highlight the knowledge gaps in our understanding of the complex chemical and physical process leading to atmospheric aerosols. Ultimately, the work presented in this dissertation and future work along the directions proposed will serve to advance our understanding of the atmospheric chemistry of secondary organic aerosol and improve current approaches to modeling atmospheric aerosol, especially as regards its impacts on climate change.