Chemical and Light-Absorption Properties of Water-Soluble Organic Aerosols in Northern California and Photooxidant Production by Brown Carbon Components.

Atmospheric brown carbon (BrC) can impact the radiative balance of the earth and form photooxidants. However, the light absorption and photochemical properties of BrC from different sources remain poorly understood. To address this gap, dilute water extracts of particulate matter (PM) samples collected at Davis, CA over one year were analyzed using high resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectroscopy. Positive matrix factorization (PMF) on combined AMS and UV-vis data resolved five water-soluble organic aerosol (WSOA) factors with distinct mass spectra and UV-vis spectra: a fresh and an aged water-soluble biomass burning OA (BBOA and BBOA) and three oxygenated OA (OOAs). BBOA is the most light-absorbing, with a mass absorption coefficient (MAC) of 1.1 m g, while the OOAs are the least (MAC = 0.01-0.1 m g). These results, together with the high abundance of BBOAs (∼52% of the WSOA mass), indicate that biomass burning activities such as residential wood burning and wildfires are an important source of BrC in northern California. The concentrations of aqueous-phase photooxidants, i.e., hydroxyl radical (·OH), singlet molecular oxygen (O*), and oxidizing triplet excited states of organic carbon (C*), were also measured in the PM extracts during illumination. Oxidant production potentials (PP) of the five WSOA factors were explored. The photoexcitation of BrC chromophores from BB emissions and in OOAs is a significant source of O* and C*. By applying our PP values to archived AMS data at dozens of sites, we found that oxygenated organic species play an important role in photooxidant formation in atmospheric waters.