Characterization of a new higher-resolution time-of-flight aerosol chemical speciation monitor: Application for measurements of atmospheric aerosols

Le résumé fourni par la source

Long-term measurements of the composition and mass concentration of particulate matter (PM) are important for source apportionment, epidemiological studies, and trends in atmospheric chemistry. The Aerosol Chemical Speciation Monitor (ACSM) has been widely used for in situ, real time measurements of PM. However, ACSMs provide unit mass resolution data, meaning isobaric ions (same unit mass, different exact m/z) cannot be separated, which can impact detection limits and separation and identification of different organic ions (e.g., C2H3O+ vs C3H7+). Here, we present a new Time-of-Flight ACSM with eXtended resolution (TOF-ACSM-X). With a mass resolving power of ∼2000 m/Δm, the TOF-ACSM-X enables higher-resolution, multi-peak fitting of individual ions compared to the two other existing ACSM models, namely the Time-of-Flight ACSM (TOF-ACSM) and quadrupole ACSM (Q-ACSM). This improved resolution leads to a factor of 25 improvement in ammonium detection limits, from ∼0.200 to ∼0.008 μg m−3 (TOF-ACSM versus TOF-ACSM-X, respectively), for 10-minute integration times, allows for elemental analysis (O/C and H/C) of organic aerosol, and enables improved mass spectral separation of the CH2O+ and NO+ signals at m/z = 30 for improved quantification of organic and inorganic particle nitrate. Comparisons of the TOF-ACSM-X with ambient measurements from two separate instruments show that the TOF-ACSM-X agrees quantitatively and that the TOF-ACSM-X provides unconstrained positive matrix factorization results for the organic aerosol that would not be possible with the unit mass resolution TOF-ACSM. Finally, we are now recommending a more direct and unifying calculation of nitrate mass concentration for both AMS and ACSM ionization efficiency calibrations.

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