Dear authors. I like your paper very much!
Please consider these suggestions when uploading your final version:
1). The sentence in the abstract “The total mass of Si in PM2.5 was found to be uncorrelated with the secondary aerosol formation process, which suggested that Si is a new conservative tracer for aerosol chemistry.” does not accurately describe your findings because it implies there is secondary chemistry involving Si. I think it should say instead: “The total mass of Si in PM2.5 was found to be uncorrelated with the secondary aerosol formation process, which suggested that Si is a new conservative tracer for the amount of primary material in PM2.5, and can be used to estimate the relative amount of secondary and primary compounds in PM2.5.”
2). I think the paper should state that one would expect this method to be useful only in regions when crustal particles represent a measurable amount of PM2.5. Beijing is a good example of an area where this method is going to be useful, but the method is not likely to be useful in areas dominated by biogenic aerosols, such as forests.