(42b) Effect of Humidity and Particle Acidity SOA Derived from Reaction of Beta-Pinene with Nitrate Radical

Secondary organic aerosol (SOA) is a major component of atmospheric aerosol, which has important implications on health, climate, and visibility. SOA is formed when gas-phase products oxidize in the atmosphere to form lower volatility products that can partition onto pre-existing particles. Studies have shown that nighttime chemistry can account for about 20-50% of biogenic precursor oxidation in polluted urban areas. The primary nighttime oxidant is the nitrate radical (NO₃) formed by reaction of ozone and anthropogenic NO₂.  The interactions of biogenic VOCs with NO₃ radical represent a direct way for positively linking anthropogenic and biogenic emissions. Since reaction with nitrate radical is predominant at night, it primarily reacts when the relative humidity is high due to the cooler nighttime temperatures. Beta-pinene is an important biogenic precursor in nighttime aerosol chemistry because of its relatively high abundance and high SOA yields when oxidized by NO₃. In the ambient, the products of this reaction will condense onto aerosol of high anthropogenic sulfate content. The sulfate in this pre-existing aerosol increases the particle acidity. In this study, chamber experiments with beta-pinene are performed at the Georgia Institute of Technology Environmental Chamber facility. Reactions are carried out at high humidity to simulate elevated night-time humilities in the ambient environment. Seed aerosol acidity is controlled by the addition of either neutral ammonium sulfate ((NH₄)₂SO₄) or an acidic mixture of (NH₄)₂SO₄/H₂SO₄. N₂O₅ is injected into the chamber by pre-reacting NO₂ and O₃ in a flow tube to initiate beta-pinene oxidation. Aerosol chemical composition is characterized by a High Resolution-Time of Flight-Aerosol Mass Spectrometer (HR-TOF-AMS) with corresponding gas-phase oxidation products measured by a Chemical Ionization Mass Spectrometer (CIMS).  The hydrolysis of organic nitrate products from the beta-pinene/NO₃ reaction under humid conditions represents an important organic nitrate sink in the night-time environment. More detailed analysis will discuss the influence of relative humidity and particle acidity on gas-phase and aerosol composition.