Organic aerosol (OA) is a ubiquitous component of atmospheric particulate that influences both human health and global climate. A large fraction of OA is secondary in nature (SOA), being produced by oxidation of volatile organic compounds (VOCs) emitted by biogenic and anthropogenic sources. Despite the integral role of SOA in atmospheric processes, there remains a limited scientific understanding of the chemical and physical changes induced in SOA as it ages in the atmosphere. This thesis describes work done to increase the knowledge of processes and properties of atmospherically relevant SOA. In the work presented in this thesis, I have worked on improving an existing innovative, soft ionization aerosol mass spectrometer and utilized it to establish chemical mechanisms for oxidation of atmospherically relevant organic precursors (i.e., Green Leaf Volatiles). I discovered that SOA formation from cis-3-hexen-1-ol is dominated by oligomer and higher molecular weight products, whereas the acetate functionality in cis-3-hexenylacetate inhibited oligomer formation, resulting in SOA that is dominated by low molecular weight products. One of the most important factors contributing to uncertainties in our estimations of SOA mass in the atmosphere, remains our basic assumption that atmospheric SOA is liquid-like, which we have found to be untrue. Hence, I developed a methodology to estimate the phase state of SOA and identified new parameters that can have significant influence on the phase state of atmospheric aerosol. This simplified method eliminates the need for a Scanning Mobility Particle Sizer (SMPS) and directly measures Bounce Factor (BF) of polydisperse SOA using only one multi-stage cascade Electrostatic Low Pressure Impactor (ELPI). The novel method allows for the real time determination of SOA phase state, permitting studies of the relationship between SOA phase, oxidative formation and chemical aging in the atmosphere. I demonstrated that SOA mass loading (CSOA) influences the phase state significantly. Results show that under nominally identical conditions, the maximum BF decreases by approximately 30% at higher CSOA and suggests that extrapolation of experiments not conducted at atmospherically relevant SOA levels to simulate the chemical properties may not yield results that are relevant to our natural environment. My work has provided a better understanding of the mechanisms of aerosol formation at atmospheric concentrations, which is necessary to understand its physical properties. This improved understanding is fundamental to accurately model aerosol formation in the atmosphere, and subsequently evaluate their large-scale effect on human health and environment.