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 (WSBBOAfreshandWSBBOAaged) and three
oxygenated OA (WSOOAs).WSBBOAfreshis the most light-absorbing, with a mass absorption coefficient
(MAC365 nm) of 1.1 m2g–1, while theWSOOAs are the least (MAC365 nm= 0.01–0.1 m2g–1). These results,
together with the high abundance ofWSBBOAs (∼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 (1O2*), and oxidizing triplet excited states of organic
carbon (3C*), were also measured in the PM extracts during
illumination. Oxidant production potentials (PPOX) of the
five WSOA factors were explored. The photoexcitation of BrC chromophores
from BB emissions and in OOAs is a significant source of1O2* and3C*. By applying our PPOXvalues to archived AMS data at dozens of sites, we found that oxygenated
organic species play an important role in photooxidant formation in
atmospheric waters.