Quantification Of Total Alkyl Nitrates And Peroxy Nitrates Using Thermal Dissociation Cavity Attenuated Phase Shift Spectroscopy (TD-CAPS)

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Eris, Gamze
Ng, Nga Lee
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Particulate matter or atmospheric aerosols are liquid or solid particles suspended in the air. The contribution of organic nitrates to total organic aerosols is found to be substantial from measurements conducted at various ambient sites worldwide. Alkyl nitrates (AN) and peroxy nitrates (PN), are temporary NOx reservoirs in the troposphere, therefore understanding the formation and fate of organic nitrates is critical to determine global and regional distributions of NOx, its cycling and impact on O3 and secondary organic aerosol production. Accurately measuring AN and PN has been challenging because they exist in low concentrations and are structurally distinct. To measure total AN and PN, 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 regarding interference from other atmospheric constituents is limited. The overall objective of the propose work is to develop TD-CAPS instrument that can measure total AN and PN in gas and particle phase by making use of the thermal decomposition of these compounds. 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. The difference of NO2 between channels is used to derive AN and PN concentration. We identified and quantified potential chemical interferences from side reactions due to presence of atmospheric constituents such as O3, NO and NO2 using isopropyl nitrate (IPN) and peroxy acetyl nitrate (PAN) as representative AN and PN compounds. We compare the accuracy of our tool with the accuracy of existing/established measuring instruments by producing secondary organic aerosol in Georgia Tech Environmental Chamber (GTEC) facility. Total gas phase and particle phase AN and PN are measured with the addition of Teflon filter and activated carbon denuder to the TD-CAPS inlet. Our results show that measurements of total particle-phase AN and PN made with the TD-CAPS correlate strongly with measurements by the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The measurement of total gas phase AN and PN with TD-CAPS agrees with High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) after accounting for measurement uncertainties in each instrument. The direct measurements of gas-phase and particle-phase organic nitrates allow us to calculate their bulk partitioning coefficients, an important parameter determining SOA formation and AN/PN fate.
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