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

[1] 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+).