Comparison of FTIR and Particle Mass Spectrometry for the Measurement of Particulate Organic Nitrates

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH4NO3, NaNO3, and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO3 radical reactions at 22 degrees C and 1 atm in air with alpha- and beta-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the alpha-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [CxHyNzO8](+) were identified using HR-ToF-AMS. The NO+/NO2+ ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the alpha- and beta-pinene, 3-carene, and limonene reactions, similar to 5 for the isoprene reaction, 2.4 for NH4NO3 and 80 for NaNO3. The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO2/C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO+/NO2+ ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH4NO3 or NaNO3. However, unique identification of specific organic nitrates is not possible with either method.