Atmospheric aerosol particles have been widely recognized as a significant source of total fixed nitrogen (N) in the biosphere. A wide variety of nitrogenous species, including both inorganic nitrogen (IN) and organic nitrogen (ON) molecules, are found in ambient aerosols. IN species, mainly existing as NH₄⁺ and NO₃^-, are commonly measured using ion chromatography (IC). Benefitted from the easy and accurate measurement method, the ambient abundance and atmospheric chemistry of IN species are well understood. In comparison, the ON fraction is much less understood owing to a lack of simple analytical methods. Traditional methods for ON usually take ON to be the difference of total nitrogen (TN) and IN, i.e., ON = TN – IN, and IN = NH₄⁺(N) + NO₃^-(N). Three independent measurement q...[ Read more ]

Atmospheric aerosol particles have been widely recognized as a significant source of total fixed nitrogen (N) in the biosphere. A wide variety of nitrogenous species, including both inorganic nitrogen (IN) and organic nitrogen (ON) molecules, are found in ambient aerosols. IN species, mainly existing as NH₄⁺ and NO₃^-, are commonly measured using ion chromatography (IC). Benefitted from the easy and accurate measurement method, the ambient abundance and atmospheric chemistry of IN species are well understood. In comparison, the ON fraction is much less understood owing to a lack of simple analytical methods. Traditional methods for ON usually take ON to be the difference of total nitrogen (TN) and IN, i.e., ON = TN – IN, and IN = NH₄⁺(N) + NO₃^-(N). Three independent measurement quantities are involved and often they are of comparable levels. As such, ON by this difference approach is compromised with large uncertainty, an inherent problem with any analytical approaches that take difference of two large numbers.

In this work, a combined aerosol nitrogen and carbon analyzer was developed for the simultaneous determination of IN and ON species in aerosol samples collected on a filter substrate. This instrument integrates a combustion-based aerosol carbon analyzer and a chemiluminescent NO_x analyzer to enable the detection of both carbon (C) and N. The analyzer utilizes stepwise heating of aerosol materials to transform all nitrogenous species into NO_x for subsequent chemiluminescence detection. The simultaneous measurements of C and N allow approximate differentiation of ON and IN at each temperature step, as no C signal is companied with the thermal evolution of IN species. Two empirical approaches were proposed to estimate ON in real aerosol samples, namely, the proportion-based method and the molar N/C ratio-based method. Comparison of the two empirical methods and “the difference” method indicates N/C ratio-based method provides a more reliable and precise ON estimations.

The application of this analyzer was demonstrated through analyzing 62 PM_2.5 samples collected in 2016 at Tsuen Wan, an urban background site in Hong Kong. Using the N/C ratio-based method, we found ON concentrations in this set of PM_2.5 samples is in the range of 0.10-1.64 μgN/m³ and account for 12 - 58% of TN with an annual average percentage of 24%. The concentration of IN components ranges from 0.10 to 4.76 μgN/m³. ON concentrations reported in other places are comparable with our findings, further supporting the reliability of N/C ratio-based method.

In summary, a combined aerosol nitrogen and carbon analyzer together with the molar N/C ratio-based method is capable of providing a convenient and powerful tool for quantifying ON in aerosols.