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Comment on "The effects of molecular weight and thermal decomposition on the...

Jimenez-Palacios, J., M. R. Canagaratna, F. Drewnick, J. D. Allan, M. R. Alfarra, A. M. Middlebrook, J. G. Slowik, Q. Zhang, H. J. T. J. &. Coe, and D. R. Worsnop (2016), Comment on "The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer”, Aerosol Sci. Tech., 50, 9, doi:10.1080/02786826.2016.1205728.
Abstract: 

Recently a manuscript was published in Aerosol Science and Technology (50:2, 118–125, 2016; doi: 10.1080/02786826.2015.1136403) by D. M. Murphy, entitled, “The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer.” That manuscript presents a simple model (hereafter “M16 model”) for the vaporization and ionization of molecules in a vacuum system, similar to the processes occurring in the Aerodyne Aerosol Mass Spectrometer (AMS). The idealized model is based on the dispersion of molecules in the free molecular regime and predicts an additional square root dependence of the ionization efficiency on the molecular weight (MW) of the molecule of interest. This model correctly accounts for only one of the many complex processes that affect AMS sensitivity to aerosol chemical components.

The AMS instrument is widely used in the international atmospheric sciences community. As experienced AMS users, we find the Murphy (2016 Murphy, D. M. (2016). The Effects of Molecular Weight and Thermal Decomposition on the Sensitivity of a Thermal Desorption Aerosol Mass Spectrometer. Aerosol Sci. Technol., 50:118–125. doi: 10.1080/02786826.2015.1136403.) conclusions regarding AMS data quality to be inconsistent with our understanding of and findings on AMS performance. This inconsistency and the seriousness of the conclusions make it imperative to us to present a response providing a more balanced perspective to our atmospheric science colleagues inside and outside the AMS user community.

We present here a variety of evidence and reasoning demonstrating the inadequacy of the M16 model to represent important aspects of AMS performance. In the following, we use previously published results to address key issues raised in Murphy (2016 Murphy, D. M. (2016). The Effects of Molecular Weight and Thermal Decomposition on the Sensitivity of a Thermal Desorption Aerosol Mass Spectrometer. Aerosol Sci. Technol., 50:118–125. doi: 10.1080/02786826.2015.1136403.) associated with basic AMS features (vaporization temperature, single-particle timescales, and ionization efficiencies), AMS calibration (relative ionization efficiency), and evaluation of possible biases on organic aerosol field data. Rather than being an exhaustive review, the evidence put forth in this letter is representative of the AMS performance experienced by the large and varied AMS users' community. We conclude that the generalizations and assertions based on the M16 model are not consistent with many experimental AMS results and, hence, this model cannot be used to draw conclusions about the sensitivity of the AMS as a function of MW and the accuracy of ambient AMS measurements.

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Research Program: 
Tropospheric Composition Program (TCP)