Interference of organic signals in highly time resolved nitrate measurements by low mass resolution aerosol mass spectrometry

Highly time resolved measurements of nitrate in ambient aerosols were conducted by an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS or simply AMS) and a particle-into-liquid sampler (PILS) coupled to ion chromatography from field intensives at two sites: an urban site in New York City (Queens College; QC) for wintertime (22 January to 5 February 2004) and a rural site in southwestern New York state (Pinnacle State Park; PSP) for summertime (18 July to 6 August 2004). In this study, we report that in rural atmospheres the inorganic nitrate signal from Q-AMS may contain significant interferences from organic signals. Analysis of the QC data indicates a good agreement between the PILS-nitrate and AMS-nitrate measurements (R² = 0.94; linear regression slope = 1.05). In addition, the m/z 30 and m/z 46 (two dominant ion fragments in nitrate mass spectrum) signals tightly correlate at QC (R² = 0.98) and have an average ratio similar to that determined in the laboratory for NH₄NO₃ (m/z 30/m/z 46 = 2.4). In contrast, at the PSP site the correlation between PILS- and AMS-nitrate was poor (R² = 0.34), the AMS reported nitrate values were substantially higher, and the m/z 30 to m/z 46 ratios were generally much larger than 2.4. These observations, together with evaluations by aerosol phase ion balance, indicate that the AMS m/z 30 signals at PSP have been strongly influenced by organic compounds that also produce signals at m/z 30, e.g., organic nitrates (NO⁺), oxygenated organics (CH₂O⁺), hydrocarbon-like organics (C₂H₆⁺), and nitrogen-containing organic compounds (CH₄N⁺).