Anthropogenic and biogenic are two major air pollutants emitted from human activity and natural sources, respectively that generate primary atmospheric aerosols which are known for their direct or indirect adverse effects. These air pollutants can undergo photooxidation process by natural UV lights during the daytime and produce secondary atmospheric aerosols that adversely affect air quality, human health, visibility, and climate change. This study aided to understand the parameters that affect the secondary aerosol formation from both anthropogenic and biogenic precursors.
In-use diesel and natural gas heavy-duty vehicles (HDVs) are important mobile source emissions in urban areas. Heavy-duty diesel vehicles (HDDV) emit extremely high nitrogen oxide (NOx) and particulate matter (PM) emissions, while CNG heavy-duty vehicles produce more aged emissions. In this dissertation, we first conducted a comprehensive fuel and aftertreatment system effect analysis on the primary and secondary emissions from HDDVs when operated with hydrogenated vegetable oil (HVO) and ultra-low sulfur diesel (ULSD) fuel on the chassis dynamometer. The SOA formations were then analyzed by collecting and photooxidizing the diluted vehicle exhaust using a 30 m3 mobile atmospheric chamber. The results showed that in the vehicles with no selective catalytic reduction (SCR), the secondary aerosol (SA) formations were ~2 times higher for ULSD compared to HVO. Moreover, both primary and secondary compositions were mostly organics for no-SCR vehicles. Then, we tested the primary and secondary emissions from natural gas heavy-duty vehicles equipped with three-way catalysts (TWC) using the same approach. Particulate matter, particle number, and non-methane organic gas (NMOG) emissions were found to be higher for the natural gas vehicles equipped with TWCs compared to diesel vehicles to the diesel trucks equipped with diesel oxidation catalysts (DOC), diesel particulate filters (DPF), and selective catalytic reduction (SCR) systems. The contribution of lubricant oil in the primary emissions of the CNG vehicles led to enhanced SOA formations compared to HDDVs equipped with advanced aftertreatments. The secondary inorganic aerosols were also significantly higher in CNG vehicles.
We also studied the SOA yields and chemical compositions from α-pinene (a biogenic volatile organic compounds) photooxidation in a new 118 m3 fixed-volume environmental chamber under atmospherically relevant and controlled chemical conditions. The experiments were conducted at fixed branching ratios (β=0, 0.3, and 1.0), which were controlled via continuous NO injection throughout the experiment to resemble real atmospheric conditions. The corresponding classical experiments with variable β values with instantaneous NO injection prior to photooxidation were also performed to compare the SOA yields and gas-phase chemistry. The SOA yields for β=0.3 experiments (Y= 21.47%-30.45%) and β=0 conditions (Y= 20.22%-29.98%) were substantially higher compared to β=1 experiments with elevated NO level. It was found that the higher RO2 lifetime led to higher SOA yield due to the formation of next generation oxidized peroxy radicals (R'O2 and R"O2) from autoxidation reaction. Similarly, in elevated HO2/RO2 conditions, the higher SOA yields were obtained. The continuous NO injection method (constant β) revealed lower SOA yield than classical experiments (variable β) at lower initial hydrocarbon (HC) concentration and higher SOA yield at higher HC conditions.
