UC Riverside

Secondary organic aerosol (SOA) impacts global climate change, visibility, and human health. Further, tropospheric ozone deleteriously affects human health and plant ecosystems. This thesis focuses on atmospheric SOA and ozone formation processes from the photo-oxidation of individual and mixtures of intermediate-volatility organic compounds (IVOCs) as well as unburnt gasoline and diesel in the presence of NOx and a surrogate reactive organic gas (ROG) mixture. These processes are evaluated using the state-of-the-art dual 90 m3 indoor environmental chamber facility at UCR CE-CERT.

IVOCs are normally considered exempt to volatile organic compounds (VOC) regulations due to their low evaporation rates; however, half of the 14 select IVOCs investigated in this work lost more than 95% of their mass due to evaporation in less than one month. Benzyl alcohol (0.41), n-heptadecane (0.38), and diethylene glycol monobutyl ether (0.16) all had measured SOA yields greater than 0.1 in the presence of NOx and a surrogate ROG mixture. These IVOCs also measurably influence ozone formation compared to the surrogate ROG mixture only by impacting radical levels and NOx availability. The overall SOA and ozone formation of the IVOC-containing generic consumer products could not be explained solely by the individual IVOC experiments.

Number 2 diesel fuel is also a complex mixture of IVOCs, which formed 14 times higher SOA than previously reported for diesel fuel. Further, doubling NOx concentrations within relevant urban concentration levels (NOx < 50 ppb) enhanced SOA formation by an additional 33%. However, when NOx levels were raised to the very high NOx concentrations (> 1.5 ppm) needed mimic the earlier studies of diesel fuel SOA, SOA formation was fourteen fold times less than the SOA formation consistent with earlier studies.

Direct evaporation from unburned gasoline is an established source of ozone and secondary organic aerosol (SOA) forming precursors. As new vehicle emission control technologies continue to decrease primary organic aerosol and gas-phase emissions, whole fuel evaporation becomes a more significant source of ambient organic aerosol and ozone. While SOA formation from some gasoline components has been individually studied, there are only a few studies on how these complex mixtures behave in the atmosphere. Given changes in fuel formulations, it is important to revisit whole gasoline as an important SOA precursor, especially in light of increased knowledge on the impact of reactivity on aerosol formation and improved atmospheric chambers and instrumentation. SOA formation from photo-oxidation of gasoline samples in the presence of NOx leads to an aerosol yield of approximately 0.055 and is consistent regardless of fuel manufacturer or octane rating. Aerosol formation, consistent with the work of Odum et al., (1996) was observed to be driven by aromatic content in the gasoline. Aromatic hydrocarbons are important to SOA formation while certain compounds in the gasoline play an additional role by suppressing OH and therefore SOA and ozone formation. Increasing NOx and hydroxyl radical concentrations enhance SOA and ozone formation during the photooxidation of gasoline.