Enhancing characterization of organic nitrogen components in aerosols and droplets using high-resolution aerosol mass spectrometry

Abstract. This study aims to enhance the understanding and application of the Aerodyne high-resolution aerosol mass spectrometer (HR-AMS) for comprehensive characterization of organic nitrogen (ON) compounds in aerosol particles and atmospheric droplets. To achieve this, we analyzed seventy-five N-containing organic compounds, representing a diverse range of ambient ON types, including amines, amides, amino acids, N-heterocycles, protein, and humic acids. Our results show that ON compounds can produce significant levels of NH_x⁺ and NO_x⁺ ion fragments, which are typically recognized as ions representative of inorganic nitrogen species. We also discovered the presence of CH₂N⁺ at m/z = 28.0187, an ion fragment that is rarely quantified in ambient datasets due to substantial interference from air-related N₂⁺. As a result, we determined that an updated calibration factor of 0.79 is necessary to accurately quantify ON content using aerosol mass spectrometry.

We also assessed the relative ionization efficiencies (RIEs) for different ON species and found that the average RIE of ON compounds (1.52 ± 0.58) aligns with the commonly used default value of 1.40 for organic aerosol (OA). Moreover, through a careful examination of the HR-AMS mass spectral features of various ON types, we propose fingerprint ion series that can aid the ON speciation analysis. The presence of C_nH_2n+2N⁺ ions is closely linked with amines, with CH₄N⁺ indicating primary amines, C₂H₆N⁺ suggesting secondary amines, and C₃H₈N⁺ representing tertiary amines. C_nH_2nNO⁺ ions (especially for n values of 1–4) are very likely derived from amides. The co-existence of three ions, C₂H₄NO₂⁺, C₂H₃NO⁺, and CH₄NO⁺, serves as an indicator for the presence of amino acids. Additionally, the presence of C_xH_yN₂⁺ ions indicates the occurrence of 2N-heterocyclic compounds. Notably, an elevated abundance of NH₄⁺ is a distinct signature for amines and amino acids, as inorganic ammonium salts produce only negligible amounts of NH₄⁺ in HR-AMS.

Finally, we quantified the ON contents in submicron particles (PM₁) and fog waters in Fresno, California and PM₁ in New York City (NYC). Our results revealed the substantial presence of amino compounds in both Fresno and NYC aerosols, whereas concurrently collected fog waters contained a broader range of ON species, including N-containing aromatic heterocycle (e.g., imidazoles) and amides. These findings highlight the significant potential of employing the widespread HR-AMS measurements of ambient aerosols and droplets to enhance our understanding of the sources, transformation processes, and environmental impacts associated with ON compounds in the atmosphere.