Study on Several Imidazoles by Aerosol Time of Flight Mass Spectrometry

Secondary organic aerosol (SOA) has an important impact on the atmospheric environment, climate and human health．It has become clear that aqueous chemical processes occurring in cloud droplets and wet atmospheric particles are an important source of SOA. Reactions of glyoxal and ammonium sulfate (AS) in bulk solution and aerosol water can irreversibly formation of light absorbing organic nitrogen compounds. The generation of nitrogen-containing organics, for example, imidazoles are of particular interest because this reaction is not photochemically induced and therefore can occur in the dark, has the potential to form light-absorbing “brown carbon” products that can directly impact radiative forcing. However, due to the low yield of imidazoles in the aqueous phase reaction, it is very difficult to identify and analyze the products. Most of the existing results are obtained based on solution systems using off-line methods. The importance of these reactions in the atmosphere, especially the surface of atmospheric aerosols, needs further research. In this study, the mass spectra of seven imidazole samples (imidazole, imidazole-2-carbaldehyde, 2-methylimidazole, 4-methylimidazole, 1-ethylimidazole, 2-ethylimidazole, 1-n-butylimidazole) were analyzed by aerosol laser time-of-flight mass spectrometer (AL-TOF MS) and thermal desorption/tunable vacuum ultraviolet time-of-flight photoionization aerosol mass spectrometer (TD-VUV-TOF-PIA MS) based on synchrotron radiation source. A series of solutions of imidazoles were prepared using purified water or ethanol. Each solution was atomized by compressed air using a TSI constant output atomizer, the droplets output from the atomizer were passed through a diffusion dryer, then introduced into the mass spectrometer. AL-TOF MS used a 266 nm pulsed laser as the ionization source, which provided both positive and negative mass spectra, more than 200 particles mass spectra were used to obtain an average MS for each solution. The main advantages of TD-VUV-TOF-PIA MS areits tuneability of synchrotron photoionization photon energy and “soft ionization”, which minimise molecular fragmentation. The energy range used in this research was 8-12 eV. The two complementary techniques had a different ionization mechanism and would favour the formation of different ions. The aim of these works was to identify tracer ions for glyoxal and ammonium sulfate aqueous phase reaction. Fingerprint spectra of each sample in two different ionization modes were obtained. AL-TOF MS mainly obtained the fragment peaks information of these samples, and the other mainly obtained the mass spectrum containing the intact parent peak. The mass spectrum of the above mentioned standard sample was used to identify the secondary organic aerosol sprayed by the glyoxal-ammonium sulfate solution. Through the combination of AL-TOF MS characteristic peaks of the imidazole samples, it can be identified that the imidazoles are formed by the reaction of glyoxal and ammonium sulfate in the aqueous phase aerosol. The mass spectrometric characteristics of the imidazole samples obtained by TD-VUV-TOF-PIAMS under different photon energies confirmed that the imidazole species produced by the above aqueous phase reaction contained imidazole and imidazole-2-formaldehyde.