Carbonaceous aerosols (CAs) are major components of fine particulate matter (PM 2.5 ) that dramatically influence the energy budget of Earth. However, accurate assessment of the climatic impacts of CAs is still challenging due to the large uncertainties remaining in the measurement of their optical properties . In this respect, a modified versatile aerosol concentration enrichment system integrated into optical instruments (VACES-OPTS) was set up to increase particle concentration and amplify signal-noise ratio during optical measurement . Based on the novel technique, this study was able to lower the detection limit of CAs by an order of magnitude under high temporal resolution (2 h) and small sampling flow (6 L min −1 ). Besides, stable and reliable optical data were obtained for absorption apportionment and source identification of black carbon (BC) and brown carbon (BrC). In the field application of the new system, high absorption coefficient of CAs in Shanghai, China was witnessed. Further analysis of the contribution of black carbon BC and BrC to light absorption revealed that BrC could account for over 15% of the total absorption at 370 nm. According to the potential source contribution function model (PSCF) classification, CAs with strong light absorption in urban Shanghai originated not only from highly polluted inland China but also from active marine ship emissions.
Imidazole compounds are important constituents of atmospheric brown carbon. The imidazole components of aqueous secondary organic aerosol (aqSOA) that are generated from the evaporation of droplets containing pyruvaldehyde and inorganic ammonium are on-line characterized by an aerosol laser time-of-flight mass spectrometer (ALTOFMS) and off-line detected by optical spectrometry in this study. The results demonstrated that the laser desorption/ionization mass spectra of aqSOA particles that were detected by ALTOFMS contained the characteristic mass peaks of imidazoles at m/z = 28 (CH2N+), m/z = 41 (C2H3N+) and m/z = 67 (C3H4N2+). Meanwhile, the extraction solution of the aqSOA particles that were measured by off-line techniques showed that the characteristic absorption peaks at 217 nm and 282 nm appeared in the UV-Vis spectrum, and the stretching vibration peaks of C-N bond and C=N bond emerged in the infrared spectrum. Based on these spectral information, 4-methyl-imidazole and 4-methyl-imidazole-2-carboxaldehyde are identified as the main products of the reaction between pyruvaldehyde and ammonium ions. The water evaporation accelerates the formation of imidazoles inside the droplets, possibly owing to the highly concentrated environment. Anions, such as F−, CO32−, NO3−, SO42−and Cl−in the aqueous phase promote the reaction of pyruvaldehyde and ammonium ions to produce imidazole products, resulting in the averaged mass absorption coefficient () in the range of 200–600 nm of aqSOA increases, and the order of promotion is: F−> CO32−> SO42−≈ NO3−≈ Cl−. These results will help to analyze the constituents and optics of imidazoles and provide a useful basis for evaluating the formation process and radiative forcing of aqSOA particles.Keywords:pyruvaldehyde;inorganic ammonium;aqueous secondary organic aerosol;imidazole compounds;brown carbon
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