10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Characterization of Thermal Dissociation Cavity Attenuated Phase Shift Spectroscopy (TD-CAPS) for Total Gas-Phase and Particle-Phase Alkyl Nitrates and Peroxy Nitrates Measurements
GAMZE ERIS, Masayuki Takeuchi, Ezra Wood, David Tanner, Greg Huey, Nga Lee Ng, Georgia Institute of Technology
Abstract Number: 1263 Working Group: Instrumentation
Abstract Recent field studies at various ambient sites demonstrate that the contribution of organic nitrates to total submicron organic aerosols is substantial. Organic nitrates, including alkyl nitrates (AN) and peroxy nitrates (PN), are important NOx reservoirs or sinks in the troposphere. Formation of AN and PN terminates the chain reactions of ROx and NOx radicals; therefore the quantification of these nitrates is critical to understanding the global and regional distributions of NOx as well as its cycling and impact on ozone and secondary organic aerosol (SOA) production. Organic nitrates consist of structurally distinct compounds that are usually in low concentration, which makes their quantification, challenging. To measure organic nitrates, a thermal dissociation (TD) inlet is typically coupled with various NOy detection techniques such as Chemical Ionization Mass Spectrometry (TD-CIMS), Laser Induced Fluorescence (TD-LIF), Cavity Ring-down Spectrometer (TD-CRDS) and Cavity Attenuated Phase Shift Spectroscopy (TD-CAPS). TD-CAPS is advantageous since the CAPS monitor is commercially available, easy to setup and use, and requires little maintenance. Ambient measurements by TD-CAPS have been recently conducted in a remote region. However, characterization of the instrument with regard to interference from other atmospheric constituents and limits of detection are limited. In this work, we developed a TD-CAPS instrument to measure total AN and PN in gas and particle phase. The instrument consists of two quartz tube reactors at 563 K and 473 K (enabling decomposition of AN and PN, respectively, to NO2) and a reference channel that measures the ambient NO2. The NO2 concentration in each channel is measured by a CAPS monitor. Isopropyl nitrate (IPN) and peroxy acetyl nitrate (PAN) are used as representative AN and PN compounds to identify and quantify potential chemical interferences from side reactions due to presence of atmospheric constituents such as O3, NO and NO2. Correction factors are derived for binary reactions of these compounds over a wide range of concentrations that is representative of rural and urban ambient environments and laboratory chamber reaction conditions. Moreover, SOA is produced from oxidation of biogenic volatile organic compounds in the Georgia Tech Environmental Chamber (GTEC) facility to evaluate the performance and further improve the TD-CAPS instrument. Various oxidants, i.e., OH, NO3 and O3, are reacted with isoprene and monoterpenes such as α-pinene, β-pinene and limonene to produce different organic masses with different organic nitrate products in the chamber. The gas-phase and particle-phase AN and PN are measured by the addition of a Teflon filter and activated charcoal denuder to the TD-CAPS inlet, respectively. Particle-phase organic nitrates measured by TD-CAPS are compared with those measured by High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). Quantitative agreement between these two methods is observed. Gas-phase and particle-phase organic nitrates measured by TD-CAPS are also compared with observations by Filter Inlet for Gases and AEROsols (FIGAERO) High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) . These direct measurements of gas-phase and particle-phase organic nitrates allow us to calculate their bulk partitioning coefficients, an important parameter determining SOA formation.