Long-term observation of NHx gas/particle partitioning and the potential impacts of organic ammonium salts

Abstract:

Abstract: The phase partitioning of total ammonium (NH_x) is largely determined by aerosol pH and the available balancing anions in atmosphere. We studied a 10-year record (2007-2016) of PM_2.5 chemical composition in Toronto atmosphere, which reveals that particulate nitrate and sulfate have substantially decreased along with the reduction of gaseous precursors. On the other hand, NH_x concentrations did not show the decreasing trend. With NH_x becoming more abundant with respect to inorganic acidic components, it is possible to have a larger relative contribution from organic acids in particulate ammonium formation. By using a custom designed sampling apparatus to overcome the potential sampling loss of organic ammonium salts, we found evidence of a significant amount of organic acids balancing particulate ammonium but both of them evaporated during collection as sampling artefacts. Chemical reactions of low-volatility organic acids producing formic and acetic acid were very likely involved. We derived a double phase partitioning method that can evaluate the impact of particulate organic ammonium formation on aerosol pH. This approach can also be used to evaluate the extent to which non-volatile cations are participating in the phase partitioning of nitrate.

Key words: PM_2.5, NHx, organic acids, aerosol pH, phase partitioning