Estimation of Tropospheric OH and Cl concentrations inferred from Non-Methane Hydrocarbons ratios over Southern California

Non Methane Hydrocarbons (NMHC) play an important role as chemical precursors to ozone and secondary organic aerosol (SOA) in the troposphere via their photochemical oxidation. In turn, ozone and SOA affect global climate, air quality, and human health. Though oxidation of NMHCs generally occurs via reaction with the hydroxyl radical (OH), oxidation can also occur via reaction with atomic chlorine (Cl), often at rates that are orders of magnitude greater than with OH. During the 2015 NASA Student Airborne Research Program (SARP), whole air samples (WAS) were collected onboard the NASA DC-8 over Southern California. In this study, four regions were focused on: The Los Angeles area, Santa Barbara (which also include samples collected on the ground), the Central Valley and the Salton Sea. Using ratios of select NMHCs that react with OH and Cl at different rates, Cl concentrations over the study regions were estimated to be 1.5 × 10⁴, 1.2 × 10⁴, 1.8 × 10⁴, 8.2 × 10³, and 7.5 × 10³, molecules cm^-3 respectively. These results were used to calculate theoretical OH radical concentration, which were in the range of 2.5 × 10⁵ to 8.2 × 10⁵ molecules cm^-3 over the whole study region. The magnitude of OH and Cl concentrations has reasonable impact on ozone formation. Although the Central valley is far away from the ocean we estimated a relatively high Cl concentration, indicating the importance of Cl in ozone production even in inland regions of California.