Secondary Organic Aerosol Formation from Biogenic and Anthropogenic VOCs

Atmospheric aerosols are solid or liquid particles suspended in the atmosphere and are either directly emitted into the atmosphere as primary organic aerosols (POA) or formed from gas phase oxidation reactions as secondary organic aerosol (SOA). Sesquiterpenes (C₁₅H₂₄) are a class of biogenic volatile organic compounds (BVOCs) that are significant SOA precursors due to their high reactivity with oxidants and their high SOA yields. Previous studies have focused almost exclusively on β-caryophyllene, and there is relatively little known about the other sesquiterpenes. This has led to chemical transport models (CTMs) frequently only using β-caryophyllene as the only sesquiterpene contributor to SOA in these models. In chapter 1 we focus on another major sesquiterpene, α-humulene, which has three endo-cyclic double bonds. A series of experiments quantified the SOA production during the ozonolysis of α-humulene in the Carnegie Mellon atmospheric simulation chamber. The experiments resulted in high SOA yields ranging from 30 to 70% for SOA concentration in the range 10 to 100 μg m-3 . Most of the SOA had effective volatility equal or less than 1 μg m^-3 at 298 K and the average SOA effective vaporization enthalpy was 115 ± 23 kJ mol^-1. The SOA aerosol mass spectrometer (AMS) spectrum was slowly evolving during experiments, which suggested modest differences, from the AMS point of view, between the SOA compounds produced initially and the SOA compounds produced towards the end of the experiment. The α-humulene SOA mass spectrum resembled that of β-caryophyllene SOA but it was less similar to α-pinene SOA. 

Inconsistencies have often been observed between the measurements obtained from the AMS and the Scanning Mobility Particle Sizer (SMPS) during laboratory experiments focused on SOA formation. In chapter 2 we show that these discrepancies are due to the use of a sizeindependent collection efficiency (CE) especially in experiments in which new particle formation takes place. A new methodology for the estimation of a composition and size dependent CE is proposed. This approach improves significantly the agreement between the measurements from the two instruments. The average deviation of the two types of measurements remained below 10% for both total aerosol and SOA concentration. We show that in these experiments, ammonium sulfate particles both pure and thinly coated with SOA have a lower CE than the pure SOA particles or particles thickly coated with SOA. 

Many laboratory studies have studied the oxidation of VOCs by the hydroxyl radical (OH) ozone (O₃), and the nitrate radical (NO₃). Not only have they studied the first-generation SOA products, but they have gone to study later generations produced through various methods of chemical aging, with oxidation by OH being one of the more common methods. The goal is to use these chemical aging experiments to better understand the complexity of ambient SOA formation and improve the CTMs so that they do not underestimate the ambient SOA. Chapter 3 discusses our field campaign to a forested area in Greece to investigate the formation of SOA due to OH oxidation and using a dual chamber system. In 10 out of the 13 performed experiments under both high and low NO_x conditions the SOA formation was below the detection limit (0.1 µg m^-3). In two more it was also very low, but it could be detected for approximately 30 min due to incomplete mixing of the chamber and higher concentrations near the injection/sampling area. Clear SOA formation of 1.2 µg m^-3 was observed in only one experiment which was characterized by the higher monoterpene concentration levels during the study. The ambient OA is quite oxidized (initial O:C around 0.7), the VOC concentrations are quite low due to their small lifetime, and the later generations of reactions contribute little to both the SOA mass concentration and its oxidation state. The SOA formed is similar to that of the MO-OOA factor of the SOAS campaign in the southeast US.

Many volatile organic compounds (VOCs) are also used as ingredients in essential oils. Sesquiterpenes, for example, create the fragrance for these oils. However, VOCs like the sesquiterpenes are hard to dissolve in aqueous solutions and are very volatile. Therefore, scientists use sugary substrates, mainly cyclodextrins, to absorb and encapsulate the VOCs before dissolving them in solutions to make the oils. In chapter 4, we test the capacity of α-cyclodextrin to absorb and trap β-caryophyllene by injecting β-caryophyllene into a smog chamber filled with αcyclodextrin/ammonium sulfate particles and using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) to track various organic fragments indicative of βcaryophyllene absorbing into the particles. We take it a step further by also oxidizing the βcaryophyllene via dark-ozonolysis. We found several nominal masses in the high-resolution analysis that have strong peaks for pure hydrocarbon fragments (C_xH_y). Because of how αcyclodextrin is structured, these pure hydrocarbon fragments could only come from βcaryophyllene, which means any change in these fragments would indicate a gain or loss of βcaryophyllene (after the initial injection but before the ozonolysis) in the particles. Taken together, the high-resolution pure hydrocarbon time traces suggested strongly that β-caryophyllene does absorb into α-cyclodextrin at 30% RH.