Intercomparison of AMS and ACSM Measurements for Particulate Organic Nitrates (pON)
Abstract
Particulate organic nitrates (pON) are important atmospheric species that are formed through the reactions of volatile organic compounds (VOCs) with atmospheric oxidants (OH/NO3 radicals) and NOx. They may account for a considerable fraction of fine particulate matter (PM2.5) but their concentrations, sources and formation processes remain nearly unexplored aspects of atmospheric chemistry. Recently, a methodology based on time-of-flight aerosol mass spectrometer (ToF-AMS) measurements of NO+ and NO2+ fragments of nitrate aerosol has been proposed to distinguish between inorganic nitrates and pON. However, this methodology has not been applied to quadrupole or time-of-flight Aerosol Chemical Speciation Monitors (Q-ACSM/ToF-ACSM) or to anthropogenic pON. In the present study, the response of 8 different ACSM, and 1 Long-ToF-AMS, instruments to pON was explored through a unique experimental setup under controlled conditions at the Aerosol Chemical Monitor Calibration Centre (ACMCC). pON were generated in a Potential Aerosol Mass (PAM) oxidation flow reactor from the reaction of NO3 radical with single VOC precursors, two biogenic (limonene and b-pinene) and two anthropogenic (acenaphthylene and guaiacol). The results of this intercomparison will be presented, with a focus on variations in the mass spectra of pON (NO+/NO2+, organics fragments and organic-to-nitrate ratios) as a function of: (1) instrumental configuration (AMS vs ACSM, Q-ACSM vs ToF-ACSM, standard vs capture vaporizers) (2) pON precursor (3) particle size and (4) particle mass concentration. Possible improvements of the default fragmentation table used to calculate the contributions to the signal for organic and nitrates will be proposed in order to account for interfering signals from other species. The impact of the observed variabilities on the NO+/NO2+ methodology will also be investigated.