Using Micro-Raman Spectroscopy to Investigate Chemical Composition, Mixing States, and Heterogeneous Reactions of Individual Atmospheric Particles.

Abstract

Measuring the chemical composition of individual atmospheric aerosol particles can provide direct evidence of their heterogeneous reactions and mixing states in the atmosphere. In this study, micro-Raman spectroscopy was used to measure the chemical composition of 1200 individual atmospheric particles in 11 samples collected in Beijing air. (NH4)2SO4, NH4NO3, various minerals, carbonaceous species (soot and organics), and NaNO3 were identified in the measured particles according to their characteristic Raman peaks. These species represented the main components of aerosol particles. In individual particles, NH4NO3 and (NH4)2SO4 either existed separately or were internally mixed. Possible reaction pathways of CaCO3 particles in the atmosphere were proposed based on the results of this study and laboratory simulations on heterogeneous reactions in the literature. CaCO3 reacted with N- and S-containing (nitrogen- and sulfur-containing) acidic gases to produce Ca(NO3)2 and CaSO4. Ca(NO3)2 further reacted with S-containing acidic gases and oxidants to produce CaSO4. Of the soot-containing particles, 23% were internal mixtures of soot and inorganic material. Of the organics-containing particles, 57% were internal mixtures of organic and inorganic materials. Micro-Raman spectroscopy directly identified functional groups and molecules in individual atmospheric particles under normal ambient conditions, rendering it a powerful tool for measuring the chemical composition of individual atmospheric particles with a diameter of ≥1.0 μm.