Photo-degradation of atmospheric chromophores: type conversion and changes in photochemical reactivity

Abstract

Abstract. Atmospheric chromophoric organic matters (COM) can participate in photochemical reactions because of the photosensitiveness, thus COM have a potential contribution to aerosols aging. The photochemical mechanism of atmospheric COM and the effect of photo-degradation on its photochemical reactivity are not fully understood. To address this knowledge gap, the characteristics of COM photo-degradation and the potential effects of COM photolysis on the photochemical reactivity are illustrated. COM are identified by excitation-emission matrices combined with parallel factor analysis. We confirm that both water-soluble and water-insoluble COM are photo-bleached, and an average 70 % of fluorescence intensities are lost after 7 days of light exposure. Furtherly, it is found that there is a transformation process of low oxidation to high oxidation HULIS. We propose that the high oxidation HULIS could be used to trace the aging degree of aerosols. In terms of photochemical reactivity, compared with before photolysis, the triplet state COM (3COM*) decrease slightly in ambient particle matter (ambient PM) samples and increase in primary organic aerosol (POA). However, the COM induce fewer singlet oxygen after photolysis. The photolysis and conversion of COM are the major cause of the change of photochemical activity. The result also enunciate that the photochemical reaction mechanisms and aerosol aging processes are relatively different in various aerosols. In conclusion, we demonstrated that the photo-degradation of COM not only change the chemical compositions, but also change the roles of the COM in the aerosol aging process.

Significant Promotion of Light Absorption Ability and Formation of Triplet Organics and Reactive Oxygen Species in Atmospheric HULIS by Fe(III) Ions

“…The complexation of different metal ions had significant effects and differences on the production of 3 HULIS*. As shown in Figure S9, atmospheric HULIS (about 17 mg·C·L –1 ) stressed by four metal ions (100 and 250 μmol·L –1 ), Zn 2+ , Al 3+ , Cu 2+ , and Fe 3+ , showed a gradual increase in cumulative triplet-state production with the increase of light radiation duration, which was similar to previous studies by Mu et al , Cu 2+ , Zn 2+ , and Al 3+ -coupled systems have decreased 64, 26, and 28% of the cumulative triplet-state generation in the atmospheric HULIS (0 min of light radiation), respectively. Also, with the increase of metal ion concentration, the inhibitory effect increased slightly, which was negligible.…”

“…Brown carbon (BrC) aerosols, which can absorb near-ultraviolet and visible light, − are important contributors to the radiative forcing and the greenhouse effect in the troposphere . As an important component of BrC, humic-like substances (HULIS) can also produce active intermediates, such as triplet organics and reactive oxygen species (ROS) via photochemistry, ,, and further initiate the aging process of organic aerosols in the particle phase. − For example, it has been found that triplet organics can oxidize isoprene and induce a portion of ROS, such as hydroxyl radical ( · OH), single oxygen ( 1 O 2 ), and superoxide (O 2 ·– ). , The atmospheric concentrations of HULIS, which mainly originated from terrestrial biomass burning (11–36%) and coal combustion (11–41%), are in the range of 0.39–13.4 μg·m –3 in China. − …”

Formation characteristics of aerosol triplet state and coupling effect between the separated components with different polarity

“…The experimental results show that 3 C* plays a leading role in the production of 1 O2 and • OH. When there are triplet substances in the environment, the triplet state transfers energy to O2 molecules to form 1 O2, and 1 O2 may undergo complex reactions to generate • OH (Chen et al, 2020;Mu et al, 2020). In this study, the genertation mechanism of • OH varies with the components.…”