Formation of Brown Aqueous Secondary Organic Aerosol during Multiphase Cloud Simulations using the CESAM Chamber Facility

We investigated the formation of aqueous brown carbon (aqBrC) from methylglyoxal and methylamine in multiphase reactions using the CESAM chamber facility at the University Paris-Est Creteil. Following reaction in the chamber, droplets and particles were sampled with a Particle-Into-Liquid-Sampler (PILS), a capillary waveguide cell for UV/visible spectroscopy, and a total organic carbon analyzer (TOC). Particle size distributions were measured with a scanning mobility particle sizer and used to determine the mass absorption coefficient (a normalized absorbance measurement). Absorption spectra were recorded while aerosol or gas phase aqBrC precursors were introduced into the humid chamber. Sampling was continuous during and after cloud events. The events lasted 5-10 minutes and produced measurable brown carbon signal at 365 nm. When lights were used, absorbance at 365 nm decreased steadily indicating photobleaching of aqBrC products or preferential formation of different, non-absorbing products. Although absorptivity increases prior to cloud formation, cloud events produce sharp increased in aqBrC absorptivity. While measurable absorbance at 365 nm indicates aqBrC formation, very little absorbance was recorded beyond 450 nm indicating that the products were not as oligomerized as products observed in prior work in multi-day, bulk phase simulations.