Development of an Automatic Linear Calibration Method for High Resolution Single Particle Mass Spectrometry: Improved Chemical Species Identification for Atmospheric Aerosols

Abstract. The mass resolution of laser desorption ionization (LDI) single particle aerosol mass spectrometry (SPAMS) is usually low (~500), which has been greatly improved by recent development of delayed ion extraction technique. However, due to large fluctuations among LDI processes during each laser shot, accurate calibration of mass-to-charge ratio for high resolution SPAMS spectra is challenging. Here we developed an automatic linear calibration method to improve the accuracy of mass-to-charge (m/z) measurement for single atmospheric aerosol particles. Laboratory generated sea spray aerosol and atmospheric ambient aerosol were tested. After the calibration, the fluctuation ranges of the reference ions (e.g. Pb+ and SO4+) m/z reaches ±0.018 for sea spray aerosol and ±0.024 for ambient aerosol in average mass spectra. With such m/z accuracy, the HR-SPAMS spectra of sea spray aerosol can easily identify elemental compositions of organic peaks, such as Cx, CxHy and CxHyOz. While the chemical compositions of ambient aerosols are more complicated, CxHy, CxHyOz and CNO peaks can also be identified based on their accurate mass. With the improved resolution, the time series of peaks with small m/z differences can be separated and measured. In addition, it is also found that applying high resolution data with enhanced mass calibration can significantly affect particle classification (identification) using the ART-2a algorism, which classify particles based on similarities among single particle mass spectra.



Major comments
This study reports the development of an automatic linear calibration method for analyzing mass spectral data acquired with single particle mass spectrometers with mass resolution of ∼ 2000. The paper also shows the successful application of this method to analyzing lab generated sea spray particles and some ambient aerosols. This work is important given the broad application of single particle mass spectrometry in atmospheric studies and aerosol research and improvement of the chemical resolution of this technique is important. The scope of the work fits well within AMT and the manuscript is generally well written. I recommend acceptance for publication after following comments are addressed.

Response:
We are grateful for the comments given by the referee and hope our new automatic linear calibration method can be applied into more versions of aerosol mass spectrometry for atmospheric studies. Also we have addressed all the comments in the following paragraphs.

Specific comments： 1. Line 119,change to "dried by"
We have made correction to that. Changes in manuscript: Line 135-136: "The ambient particles were dried by a diffusional dryer before being sampled by the HR-SPAMS." 2. Fig. S2, the caption for this figure needs to be rewritten to better present the information content.
We have rewritten the caption of the Fig.S2 to better present the information content. Changes in supplementary material: " Fig.S2 partial enlarged detail in the sing particle mass spectra for the

The description on
Step 3 given in the paragraph on pages 6 and 7 is a bit hard to follow. How exactly is the calibration conducted? Are the measured m/z bins determined from the "traditional method" mentioned in Step 0? What exactly is the "traditional method" involved? How many bins are selected for each m/z? (1)The detailed statement of the Step 3 is that: First, we will get the measured m/z bins determined from the "traditional method", just as described in the Step 0. And these bins were not sufficiently accurate. Then, we picked up some reference ions to make a linear regression between the two set of variables (measured vs. theoretic reference ion m/z bin values). The obtained calibration parameters (a slope and an intersect) from linear regression were used to calibrate every bin value for this mass spectrum. However, the m/z bin values are fixed numbers (they are not continuous). So we had to assign the calibrated m/z value to its closest m/z bin value. Finally, the correct aerosol mass spectra can be acquired. We had made some revisions to make it easier to read in the Step3; (2)The measured m/z bin values were determined from the "traditional method" mentioned in the Step 0; (3)The "traditional method" was more like a coarsely-calibrated method, which usually selected a few particles with distinct ion patterns. Fig3 and Table S1&S2 had reported there were around five of larger bin numbers offset for the ionized species. What's more important is that every particle mass spectra were different from each other and needed its own calibration parameters. So the "traditional method" is not enough for the SPAMS data processing.
(4)One specific bin was selected for each m/z as shown in the Fig.S2 Changes in manuscript: The measured m/z bin values of the reference ions mentioned in the Step0 were calibrated based on their theoretic (or true) m/z bin values." : "Then we used these parameters to make the calibration for every bin value in this mass spectra. Finally, the m/z of the whole spectrum had been corrected." 5. Give units for "measurement m/z" and "theoretical m/z" on the axis labels in all the Figures presented in this paper. The unit is Dalton. We have updated all m/z axis labels in in the revised paper. Figure 3, the symbols are hard to differentiate, consider to revise. The spectra a and b look identical, are they really represent sea spray aerosol and ambient aerosol respectively? Thanks for pointing this out. It was a mistake and has been corrected now. We have also separated the figure into 2 panels to make them easier to differentiate. Changes in manuscript: Line 548-553:  Figure 4, what is "error limits"? How was is calculated?

6.
The "error limit" is the delta bin number which is concluded from the Table S1&S2 and Fig.3, representing the accepted error range (around 3 bin numbers) after the calibration for a m/z bin value. Changes in manuscript: Line 252-254: " Figure 4 reports the average positive and negative mass spectra for the laboratory generated sea spray aerosols and the error limits mean the concluded accepted error range." 7. Line 250, change to "Ca2+" Actually the ion fragment generated from the SPAMS can only carry one charge, so the Ca + is reasonable.

Line 266, define "LR-SPAMS"?
We have give a definition for that. Changes in manuscript: Line 295-297: "In contrast, it is impossible for a low resolution (LR)-SPAMS to provide such detailed time variation measurement of these peaks."