Anion-cation balance in particle phase

I'm now analysising the water-soluble ions in atmopsheric aerosol. With ultrasonic concussion and filtering impurity, the anion and cation from the particle sample are examined in Ion Chromatography System(IC) separately. 


We test $\ce{Cl-, SO4^2-, NO3-}$ and $\ce{Ca^2+, Mg^2+, Na+, NH4+, K+}$ as 8 major ion species to quantify the concentration level and seasonal variation in the sampling area. 


But when I combine the anion and cation data together for same particle sample, the ratio of [ Total charge of anion]/[Total charge of cation] are much larger than 1. 


In my opinion, the cation and anion inside particle should keep the balance of charge. The 8 ion species I choose should take up the majority of the total water-soluble ions (added with $\ce{F-}$, organic acid radical, etc). 


I have also searched some paper, some results of their ion-sample are also not keep the balance, but some observed. 


Are the ratio of (+-)charges differed by sampling sites? The anion especially $\ce{SO4^2-}$ are larger comparing with the cation species. Are there any missing cation that I have not deteced?




Yes, you have left out one very important cation from your cation/anion balance: $\ce{H+}$


The primary pathway for new particle formation in the atmosphere is the photo-oxidation, via hydroxyl radicals, of volatile reduced sulfur species to form very non-volatile sulfuric acid. For the most part then, new particles start out as puny droplets of sulfuric acid and grow via accretion, etc. from there. 


I recommend measuring the pH of the filtered aerosols, and I strongly suspect that will account for your missing cations. 


If I have missed the point of the question, please feel free to ask for any clarifications in the comments.


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