Seasonal variations in the production of singlet oxygen and organic triplet excited states in aqueous PM_2.5 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.

Singlet oxygen is produced from brown carbon-containing cooking organic aerosols (BrCOA) under indoor lighting

“…These concentrations are comparable to illuminated ambient particulate matter extracts collected in Switzerland 23,24 and in Hong Kong. 26 These concentrations are an order of magnitude lower than particulate matter collected in Davis, California. 20,48 Yet indoor are an order of magnitude higher than PM 2.5 in Colorado.…”

“…18 Faust and Allen 19 reported the first measurements of steady state concentrations () in cloud water, 2.7–110 × 10 −13 M. Subsequently, fog water, cloud water and aqueous solutions of ambient particulate matter have been shown to produce under laboratory irradiated conditions. 20–27…”

Light Exposure of Wood Smoke Aerosol: Connecting Optical Properties, Oxidation, Radical Formation, and Chemical Composition

“…These larger molecules could potentially lead to more absorption. Another route to the formation of organic peroxy radicals could involve excited carbon-centered radicals in the triplet state ( 3 C*; R3), which have been measured as the second most concentrated oxidant in atmospheric water samples after 1 O 2 and before OH radicals. ,,, Unlike 1 O 2 and OH radicals, 3 C* can be formed through a variety of molecules with different reactivities. They can react through direct oxidation by electron transfer, forming organic radicals.…”

“…Another type of reactive oxygen species that has been observed in particulate matter is singlet oxygen ( 1 O 2 ) which is formed by electronic excitation of ground state triplet molecular oxygen (reaction R2). ,,,− In natural waters, it is generated by photosensitized chromophoric organic molecules, which transfer their energy once in their triplet excited state. , In the particle state, it has also been hypothesized to be formed from sensitized BrC molecules which can transfer their energy to triplet oxygen, forming singlet oxygen . Higher in chemical reactivity and energy than ground-based molecular oxygen, it acts as an electrophile due to its low energy antibonding LUMO (lowest unoccupied molecular orbital) orbitals.…”