Sulfate Formation in Incense Burning Particles: A Single-Particle Mass Spectrometric Study

Incense burning is a common ritual in Asian communities, and it emits massive amounts of particles. These particles can undergo atmospheric aging upon exposure to sunlight and other pollutants. In this work, we observed sulfate formation in fresh incense particles upon exposure to SO₂, using a single-particle aerosol mass spectrometer (SPAMS). Analysis of the positive mass spectra classified the particles as K-type and OC-type. In both dark and light experiments, SO₂ uptake and oxidation were found preferentially in OC-type particles over K-type particles. Sulfate formation, as represented by the number fraction of sulfate-containing particles (FS), under dark was likely due to gaseous oxidants. FS increased with UV, mainly attributable to photosensitization reactions. While more sulfate formed at higher relative humidity (RH) under dark, sulfate formation under light was independent of RH. The increase in FS due to photochemistry was more effective under lower RH, where the photoactive compounds were more concentrated and likely generated more photo-oxidants. This effect outweighed that due to reduced SO₂ dissolution. Since incense burning particles contain organic photosensitizers that have also been found in biomass burning particles, our findings could shed light on the potential of biomass burning particles to trigger sulfate formation.