Modeling Peroxy Radical (RO2) Fate and Secondary Organic Aerosol Formation during α-Pinene Perturbation Experiments
JEAN RIVERA-RIOS, Masayuki Takeuchi, Havala Pye, Nga Lee Ng,
Georgia Institute of Technology Abstract Number: 350
Working Group: Urban Aerosols
Abstractα-pinene is one of the most studied monoterpenes in atmospheric chemistry. Despite years of work, the contribution of α-pinene to ambient aerosol and the sensitivity of secondary organic aerosol (SOA) formation to ambient conditions remains to be fully understood. In this work, we utilize ambient perturbation experiments to understand the formation of SOA from α-pinene oxidation under real urban conditions. Ambient perturbation experiments are chamber experiments that use ambient air as the matrix and thus can have a wide variety of oxidation conditions. An outdoor chamber is filled with ambient air and perturbed with a fixed amount of α-pinene. We then monitor the oxidation products and SOA formation using aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS). These experiments are modeled using the Master Chemical Mechanism (MCM) as well as recently published and updated SOA formation mechanisms, that include among other changes, isomerization reactions. We find that the aerosol yield depends on the peroxy radical (RO
2) fate, decreasing as RO
2+NO becomes the dominant pathway. In addition, isomerizations are necessary to model the observed aerosol formation. The isomerization rates required to explain the observed SOA yields during perturbation experiments are compared to standard, high NO
X, laboratory chamber experiments. We find that the isomerization rates required to model the measured SOA during perturbation experiments are much slower than those needed to explain SOA yields from standard laboratory chamber experiments with high NO
X conditions. The observed SOA yields in the perturbation experiments are sensitive to relatively low nitrogen oxide concentrations, which implies that aerosol formation under high NO
X laboratory chamber conditions is likely due to mechanisms different from isomerization reactions. The data and mechanism updates used in this work provide new insights for improved representation of α-pinene SOA formation in global and regional models.