Atmospheric chemistry of volatile organic compounds (VOCs) has significant influence on human health and global climate change. As the most globallyabundant non-methane VOC in the atmosphere, isoprene is an important precursor of both ozone (O3) and secondary organic aerosol (SOA). A number of gas-phase chemical mechanisms have been developed to predict O3 formation from isoprene in the atmosphere. However, most of these chemical mechanisms have limitations, including being too condensed to represent most major gas-phase products or being too large for use in current air quality models (AQM). In addition, recent laboratory, mechanistic, and field studies have demonstrated new chemistry for isoprene photooxidation such as the regeneration of HOx (OH+HO2) and the formation of SOA. Hence, there is a necessity to develop a new chemical mechanism based upon the recent results. In this thesis, a new condensed gas-phase isoprene mechanism is developed and evaluated against over fifty experiments that were performed in the UNC dual outdoor smog chamber under natural sunlight. The mechanism implements the regeneration of HOx, the intermediate products that form SOA, and other results from recent studies. This new mechanism is able to reasonably simulate most experimental data and also performs well comparing to the other currently used chemical mechanisms. Unlike the well understood gas-phase isoprene chemistry, the isoprene SOA formation mechanism remains elusive. This thesis investigated SOA formation from isoprene as well as methacrolein (MACR), which is the major first-generation product of isoprene photooxidation, under varied initial nitric oxide (NO) levels, relative humidities (RHs), and seed aerosol acidities. Results indicate that both SOA mass concentration and chemical composition largely depend on the initial VOC/NO ratios and RH conditions.Most particle-phase oligomers, which have been previously observed to form from the oxidation of methacryloylperoxynitrate (MPAN), were enhanced under dry conditions. In addition, a nitrogen-containing organic tracer compound was found to form substantially in both isoprene/MACR chamber-generated and ambient aerosol samples. Moreover, increased RH and aerosol acidity were both observed to enhance organosulfate formation; however, elevating RH mediates organosulfate formation, suggesting that wet sulfate aerosols are necessary in forming organosulfates in atmospheric aerosols.