Seasonal variations in the production of singlet oxygen and organic triplet excited states in aqueous PM2.5 in Hong Kong SAR, South China

Abstract

Abstract. Photooxidants drive many atmospheric chemical processes. The photoexcitation of light-absorbing organic compounds (i.e., brown carbon, BrC) in atmospheric waters can lead to the generation of reactive organic triplet excited states (3C∗), which can undergo further reactions to produce other photooxidants such as singlet oxygen (1O2∗). To determine the importance of these aqueous photooxidants in secondary organic aerosol (SOA) formation and transformation, we must know their steady-state concentrations and quantum yields. However, there have been limited measurements of aqueous 3C∗ and 1O2∗ in atmospheric samples outside of North America and Europe. In this work, we report the first measurements of the steady-state concentrations and quantum yields of 3C∗ and 1O2∗ produced in aerosols in South China. We quantified the production of 3C∗ and 1O2∗ in illuminated aqueous extracts of PM2.5 collected in different seasons at two urban sites and one coastal semi-rural site during a year-round study conducted in Hong Kong SAR, South China. The mass absorption coefficients at 300 nm for BrC in the aqueous PM2.5 extracts ranged from 0.49 to 2.01 m2 g-C−1 for the three sites. Both 1O2∗ and 3C∗ were produced year-round. The steady-state concentrations of 1O2∗ ([1O2∗]ss) in the illuminated aqueous extracts ranged from 1.56×10-14 to 1.35×10-12 M, with a study average of (4.02±3.52)×10-13 M. At nearly 2 orders of magnitude lower than [1O2∗]ss, the steady-state concentrations of 3C∗ ([3C∗]ss) ranged from 2.93×10-16 to 8.08×10-14 M, with a study average of (1.09±1.39)×10-14 M. The quantum yields of 1O2∗ and 3C∗ also spanned wide ranges across samples, with a range of 1.19 % to 13.74 % and an average of (5.19±2.63) % for 1O2∗ and a range of 0.05 % to 3.24 % and an average of (0.56±0.66) % for 3C∗. The [1O2∗]ss and [3C∗]ss correlated with the concentration and absorbance of BrC, thus implying that the amount of BrC drives the steady-state concentrations of these photooxidants. The locations (urban vs. semi-rural) did not have a significant effect on [3C∗]ss and [1O2∗]ss, which indicated that BrC from local sources did not have a significant influence on the year-round 3C∗ and 1O2∗ production. 3C∗ and 1O2∗ production were found to be the highest in winter and the lowest in summer for all three sites. The observed seasonal trends of 1O2∗ and 3C∗ production could be attributed to the seasonal variations in the long-range air mass transport. Our analysis highlighted the key role that regional sources play in influencing the composition and concentrations of water-soluble BrC in winter PM2.5 in Hong Kong SAR, which contributed to their highest 3C∗ and 1O2∗ production. The current results will be useful for modeling seasonal aqueous organic aerosol photochemistry in the South China region.