10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Quantifying the Hydrolysis Rate of Daytime and Nighttime Organic Nitrates in Aerosol Water
MASAYUKI TAKEUCHI, Thomas Berkemeier, Gamze Eris, Nga Lee Ng, Georgia Institute of Technology
Abstract Number: 1507 Working Group: Aerosol Chemistry
Abstract Ambient field studies have observed a varying level of atmospheric gaseous and particulate organic nitrates across different continents and many found that their contribution to fine organic aerosol mass is substantial. Depending on the fate, these organic nitrates could act as a reservoir and/or permanent sink of reactive oxidized nitrogen species (i.e. NOx), which eventually determines the production potential of tropospheric ozone. Modeling results using recent field measurement data in the Southeastern U.S. suggest that the lifetime of submicron particulate organic nitrates is on the order of several hours, though the loss mechanism is not known. Due to their direct impact on the NOx cycling, ozone and secondary organic aerosol (SOA) formation, it is crucial to understand the dominant loss mechanisms of organic nitrates in the atmosphere and parameters that affect the loss rate. Among various loss mechanisms, particle-phase hydrolysis is a known loss mechanism of particulate organic nitrates in the presence of aerosol water and is generally considered a major loss process. However, studies quantifying the hydrolysis rate of particulate organic nitrates are currently scarce, in part due to the difficulty of isolating the effect of hydrolysis from other confounding processes, such as potential RH-dependent loss of organic nitrate vapors to Teflon chamber walls. Here, our study aims at quantifying the hydrolysis rate of particulate organic nitrates from α-pinene and β-pinene under two oxidation conditions: photooxidation in the presence of NOx (i.e. daytime) and NO3 oxidation (i.e. nighttime), taking into account the difference in organic nitrate vapor wall loss rate at different RH. Experiments are conducted in the Georgia Tech Environmental Chamber (GTEC) facility. A large suit of oxidized, speciated gaseous and particulate organic nitrates are measured by Filter Inlet for Gases and AEROsols coupled to High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (FIGAERO-HR-ToF-CIMS). The total gaseous and particulate organic nitrates are monitored by Thermal-Dissociation Cavity Attenuated Phase Shift Spectrometer (TD-CAPS), and the total particulate organic nitrate concentration is estimated by High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). For the photooxidation win the presence of NO, there was a clear trend in the evolution of speciated particulate organic nitrates that C10H17NOx (x = 5-8) peaked first suggesting the first generation products, followed by the peaks of C10H15NOx and C10H13NOx indicating further OH aging to produce compounds with lower H:C and higher O:C ratios. This process is expected to take place by the abstraction of H, reaction with NO to create an alkoxy radical, which then forms a carbonyl via decomposition. However, regardless of the molecular formula, all speciated C10 particulate organic nitrates, when normalized humid to dry experiments, follow a similar decreasing trend that is identical to the bulk behavior observed in HR-ToF-AMS. This could indicate that the first oxidation step is more important than later steps in determining the susceptibility of particulate organic nitrates. Additionally, we experimentally determine the wall loss rate of organic nitrate vapors in our chamber facility under dry and humid conditions. Results from applying the experimental values in the aerosol kinetic model will be presented and the impact of organic nitrate vapor wall loss on the quantification of hydrolysis rate will be discussed.