Reply to “Comment on the effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer” by Jimenez et al.

Measuring the composition of atmospheric aerosols is a challenging problem because of the presence of a wide variety of both inorganic and organic compounds as well as varying volatility, phase, and water content. The Aerodyne Aerosol Mass Spectrometer (AMS) has made significant contributions to our knowledge of aerosols. In Murphy (2016), I pointed out that the framework used for understanding the sensitivity of the AMS to various species did not include the effects of molecular weight and thermal decomposition during vaporization. A comment by Jimenez et al. (2016) supports the accuracy of the AMS and questions the relevance of these effects to the sensitivity of the AMS. Murphy (2016) and Jimenez et al. (2016) agree on some substantial points. We agree that the time spent by various molecules in the ionization region affects the sensitivity to those molecules. We agree on the need for additional calibrations. There is also agreement that the great majority of AMS data for ambient conditions do not need to be revised by factors of 2 or 3. In discussing why there is no need for such a revision, Jimenez et al. seem to have misunderstood Murphy (2016), in which I stated that the data are unlikely to be off by such factors. It remains important to recognize the importance of molecular weight and thermal decomposition. In free molecular flow, heavier molecules spend more time in the ion source, increasing the chance of ionization and the sensitivity to those molecules. The molecular weight effect is modulated by thermal decomposition because, for many organics, the species that evaporate off of a 600 C vaporizer have smaller molecular weights than the original molecule. Figure 1 shows the expected range of sensitivity relative to nitrate. Depending on the amount of thermal decomposition, the sensitivity of the AMS to a given molecule can be anywhere in the shaded region. One can see why the standard AMS organic relative sensitivity of 1.4 (horizontal line) works reasonably well: it is an approximation for species that partially thermally decompose on the vaporizer. That many organics partially decompose is known not only from the overall sensitivity but also from vacuum ultraviolet ionization measurements of CO2 and other fragment molecules (Canagaratna et al. 2015). Besides providing a context for the standard sensitivity, Figure 1 makes two predictions that should be recognized. First, the range of possible sensitivities is quite large and therefore it is possible, even likely, that some classes of organic aerosol, especially in the laboratory or primary emissions, will have significantly different sensitivities than the standard approximation. Second, thermal decomposition has a large influence on sensitivity. This makes it important to systematically calibrate whether or not the thermal decomposition of organics depends on particle properties such as size, oxygen to carbon ratio, what inorganics are present, or other factors. In their comment, Jimenez et al. (2016) suggest the use of a more empirical approach to AMS calibrations. Indeed, thermal decomposition on the vaporizer is difficult to predict from first principles, making calibrations especially important. Hopefully, the comment by Jimenez et al. will be the start of routine publication of calibrations by many AMS users for the specific types of compounds they are analyzing, rather than the use of a single number for organic sensitivity.