American Association for Aerosol Research - Abstract Submission

AAAR 39th Annual Conference
October 18 - October 22, 2021

Virtual Conference

Abstract View


Potential Underestimation of Particulate Organic Nitrate Concentration by an Aerosol Mass Spectrometer

MASAYUKI TAKEUCHI, Yuchen Wang, Benjamin A. Nault, Manjula Canagaratna, Nga Lee Ng, Georgia Institute of Technology

     Abstract Number: 221
     Working Group: Instrumentation and Methods

Abstract
Ambient measurements across the globe to date indicate the ubiquitous presence and substantial contribution of particulate organic nitrate (pON) to sub-micron organic aerosol. Accurate quantification of gas- and particle-phase ON is crucial to understand its role in secondary organic aerosol (SOA) formation and NOx cycling. Yet, a limited number of techniques is available for ON quantification and instrument inter-comparison efforts are scarce. An Aerodyne aerosol mass spectrometer (AMS) is widely used for pON measurements. In this study, we examined the response of the AMS towards aerosol species with hydroxyl nitrate moieties using authentic standards. Four hydroxyl nitrates derived from isoprene, α-pinene, β-pinene, and limonene were investigated. The methanol solution of each hydroxyl nitrate was atomized, size-selected at 250-400 nm via a differential mobility analyzer, and introduced into the AMS and a condensation particle counter (CPC). The measured NO3:Org molar ratio is substantially lower (0.3) than the expected ratio of 1 based on the molecular structure of the standards. The mass-based comparison with CPC suggests that the NO3 measured by the AMS (collection efficiency corrected) is lower than CPC-measured NO3, with a slope of 0.44-0.6 depending on the precursor compounds (isoprene vs. different monoterpenes). In other words, AMS underestimates pON-derived NO3 concentrations by approximately a factor of two. We hypothesize that relative ionization efficiency of lab-generated organic hydroxyl nitrate species differs from that of inorganic nitrate in the AMS. As pON quantification from AMS relies proportionally on the NO3 measurements, this study provides new insights for a more accurate pON quantification.