Ca Mg and K are components of suspended "dust".

Na is a biprocuct in oil refining and then occurs in the submicron range often in combination with the tracer Vanadium.

Mg Ca in aerosol over the ocean with Origin in the Shara as example:

Article Chemical composition of mineral dust aerosol during the Saha...

Is that value really negative when the uncertainty is taken into account? If the zero is contained within the uncertainty range, for example -0.2±0.3, then it is a matter of data measurement accuracy and not a failure. However, if there is still some inconvenience, one can exclude the salt from the analysis since contained below the detection limit.

Dear Harry ten Brink , thanks for your reply. Actually, that is not my question but as you expected, I would like to identify the source by using the data. It was very helpful. Many thanks.

Dear Kengo Shibuya , thanks for your reply as well. I also thinking to exclude the the data row with negative value of non-sea salt. Your reply makes me more confident to do that. Many thanks.

How do you know the aerosol is pure marine?

As my example over the Atlantic, but even more so data from Pacific coast in the US shows there is long-range transport of Asian continental aerosol over thousands of kilometers

Similar issue here. Did you find reasonable explanation for these negative values?

Na+ mass concentrations really are overwhelming at our measurement site, so I guess the tracers equations I used might not really be suitable

Non sea salt components of potassium, nss-K+ (Hitchcock et al., 1980); sulfate, nss-SO4 2– (Millero and Sohn, 1992); calcium, nss-Ca2+ (Morales et al., 1998); magnesium, nssMg2+ (Virkkula et al., 2006); and chloride, nss-Cl– are calculated considering sodium as a reference species (Keene et al., 1986):

nss-K+ = K+ measured + Na+ × 0.037 (all positive)

nss-SO4 2– = SO4 2–measured – Na+ × 0.2516 (all positive)

nss-Ca2+ = Ca2+measured – Na+ × 0.0373 (mostly negative)

nss-Mg2+ = Mg2+measured – Na+ × 0.12 (mostly negative)

nss-Cl– = Cl– measured – Na+ × 1.174 (mostly negative)

What do you think?

best

Maybe Cl− or Mg2+ are better tracers alternatively?

Cl is a bad tracer because seasalt reacts with Nitric acid and sulphuric acid and the weaker HCl is driven out: google for it if you like to know more

Mg is a tracer for suspended dust

It is also present in a ratio of 1:8 with Na in seasalt but as such not a tracer for seasalt: it is rather the other way around. Based on coarse Na the contribution of sesalt Mg can be calculated

Thank you for your answer. I will stick with Na as a tracer for non-sea-salts then.

Still can't wrap my head around these negative values despite including the quantification limits as previously suggested.

edit: I guess it is because my aerosols aren't really marine at all (Na/Cl_mean=0.4 instead of the 1.8 you would expect for pristine sea water). Are there nss tracers for mixed sea water/continental origins maybe?

Really appreciate your help

Emmanuel Chevassus I'm sorry for my very late reply. Just check my account.

As what have been discussed here, in my understanding, the ratio of species X to sodium that we use in equation is the ratio found in bulk sea water, which mean that ss fraction should be high. The ratio is not completely suitable with our sample because ss fraction should be not as much as in sea water. That's why our sea salt fraction may be over estimated, resulting minus nss fraction in some samples. However, that is the best thing we can do to apportion ss and nss fraction. What do you think?