Reply on Anonymous Referee #2

This manuscript describes FTIR analysis coupled with multivariate calibration of specific functional groups (aliphatic C-H, carboxylic COOH, oxalate O=C-O-, non-acid and carbonyl C=O and alcohol O-H) in filter-based fine PM samples collected at the surface over 8 years at 4 sites in the southeastern United States as part of the former SEARCH air quality network. The authors find that there is a decline in organic mass that is driven primarily by carboxylic acid and oxalate functional groups. They attribute this to reductions in anthropogenic emissions of SO2 and/or VOCs. There is a lot of analysis and the supplemental information is extensive.

This critique provides an important perspective for us on this matter. We have moved material from the SI to the main text to more visibly discuss the possibility of sample degradation as a cause of OM decline over the sample years.
In order to further clarify and support our hypotheses, we have taken the following measures: Discussion points from our first paper, where these storage studies were initially reported, have been moved to the Discussion section of this paper (Section 3.2;. Specifically, we have expanded upon the variability of re-analyzed filters and differences between shorter-and longer-term storage. We have additionally highlighted changes in COOH and oxOCO in particular, since these two FGs were observed to decline over time in the present work. The observation that the 2009-2010FG and OM concentrations were actually increasing (due to the 2008 also supports the lack of OM loss during storage; this has been further highlighted in Section 3.2 (page [13][14]. The cold storage of filters (< 4 •C) to minimize loss of volatile species is now mentioned in the Methods section (Section 2.1, page 5) and within the Discussion section (Section 3.2,.
Further, the authors attempt to make links to temporal trends among NO x or O 3 with specific FTIR-derived functional groups almost exclusively with literature concerning ambient data. It's my opinion that links to laboratory literature with reference to specific spectra wavenumbers help make their arguments stronger.
There have only been limited studies of laboratory (chamber) experiments published thus far using infrared spectrometry, and studies quantifying specific molecules (for which FG composition is known) explain only a small fraction of the overall OM mass. Due to aging times, specific fuels or precursor molecules, and photooxidation conditions, the resonances of functional groups in condensed molecules may vary according to a variety of intramolecular and intermolecular interactions. Therefore, broad ranges rather than specific wavenumbers are used to interpret the FG composition in this study. However, biogenic secondary organic aerosols observed in various ambient environments are consistent with reported FG composition and spectral profiles due to oxidation of biogenic precursors in chamber experiments (for example, Corrigan et al., 2013;Liu et al., 2018). Corrigan, A. L., Russell, L. M., Takahama, S., Aijälä, M., Ehn, M., Junninen, H., Rinne, J., Petäjä,T., Kulmala, M., Vogel, A. L., Hoffmann, T., Ebben, C. J., Geiger, F. M., Chhabra, P., Seinfeld, J. H., Worsnop, D. R., Song, W., Auld, J., and Williams, J.: Biogenic and biomass burning organic aerosol in a boreal forest at Hyytiälä, Finland, during HUMPPA-COPEC 2010, Atmospheric Chemistry and Physics, 13, 12 233-12 256, doi:10.5194/acp-13-12233-2013, 2013 Liu, J., Russell, L. M., Ruggeri, G., Takahama, S., Claflin, M. S., Ziemann, P. J., Pye, H. O. T., Murphy, B. N., Xu, L., Ng, N. L., McKinney, K. A., Budisulistiorini, S. H., Bertram, T. H., Nenes, A., and Surratt, J. D.: Regional Similarities and NO x -Related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern United States, Journal of Geophysical Research: Atmospheres, 123, 10,620-10,636, doi:10.1029/2018JD028491, 2018 In response to the reviewer's concern, we have simplified and moved much of the material in this literature-based discussion to the Supplement. The discussion in the manuscript highlights the findings of other FT-IR spectrometry studies, including the work cited above, and particularly work done using samples from the SE U.S. However, as mentioned above, the FT-IR spectrometry literature is still somewhat limited, so we hope to only suggest probable links to atmospheric sources or processes in other discussions. Table 2 very confusing. It appears the aCH trend is driven by 'just' BHM summer, COOH 'just' in the summer, but many other values are presented. I understand how this would be value for careful readers. However there are more compelling plots in the SI that I think would make better use of the main text space.

I found
We appreciate the reviewer's precise suggestions for improving the clarity of our paper. We have moved Table 2  "Fossil fuel combustion contributes substantially to OC in the SE U.S., especially at urban sites: ~50 % of primary and secondary OC at urban BHM was estimated to be from fossil sources, but at rural CTR, >80 % of primary and all of secondary OC was from modern sources such as trees (in the early 2000s;Blanchard et al., 2008). In addition, other emissions from fossil fuel combustion, including NO x , SO 2 , and primary OM, also affect OM concentrations by modulating aerosol surface area, aerosol water content, aerosol acidity, and oxidant concentrations (Nguyen et al., 2015;Carlton et al., 2010;Al-Naiema et al., 2018)."