Photochemical aging represents an important transformation process of aerosol particles in the atmosphere, which greatly influences the physicochemical properties and the environmental impact of aerosols. In this work, we find that Beijing urban PM aerosol particles release substantial HONO, a significant precursor of •OH radicals, into the gas phase during the photochemical aging process. The generation of HONO exhibits a high correlation with the amount of nitrate in PM. The formation rate of HONO becomes gradually decreased with the irradiation time, but can be restored by introducing the acidic proton, indicative of the essential role of the acidic proton in the HONO production. Other environmental factors such as relative humidity, light intensity, and reaction temperature also possess important influences on HONO production. The normalized photolysis rate constant for HONO ( J) is in the range of 1.22 × 10 s ∼ 4.84 × 10 s, which is 1-3 orders of magnitude higher than the reported photolysis rate constant of gaseous HNO. The present study implies that the photochemical aging of Beijing PM is an important atmospheric HONO production source.

Soot, which consists of organic carbon (OC) and elemental carbon (EC), is a significant component of the total aerosol mass in the atmosphere. Photochemical oxidation is an important aging pathway for soot. It is commonly believed that OC is photoactive but EC, albeit its strong light absorption, is photochemically inert. Here, by taking advantage of the different light absorption properties of OC and EC, we provide direct experimental evidence that EC also plays an important role in the photochemical aging of soot by initiating the oxidation of OC, even under red light irradiation. We show that nascent soot, in addition to undergoing photochemical oxidation under blue light with a wavelength of 440 nm, undergoes similar oxidation under red light irradiation of λ = 648 nm (L). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L These observations indicate that EC plays a pivotal role in photoaging of soot by adsorbing light to initiate the oxidation of OC. Comparison of in situ IR spectra and photoelectrochemical behaviors suggests that EC-initiated photooxidation of OC proceeds through an electron transfer pathway, which is distinct from the photoaging induced by light absorption of OC. Since the absorption spectra of EC have a much larger overlap with the solar spectra than those of OC, our results provide insight into the chemical mechanism leading to rapid soot aging by organic species observed from atmospheric field measurements.

The photochemical aging of soot in the aqueous phase could have an important influence on water environments such as fog water and wet aerosols in the atmosphere, as well as lakes and oceans. In this study, we systematically investigated the photochemistry of soot in the aqueous phase. Soot releases dissolved black carbon into the aqueous phase during photoreactions, which is attributed to the phototransformation of the nonpolar unsaturated C–H species in soot to polar carbonyl-containing species. More importantly, we found that soot suspensions, particularly those of the dissolved part of soot, were effective photosensitizers for the generation of singlet oxygen (1O2). The 1O2 apparent quantum yield of the dissolved part reached 33 ± 2% under 377 nm irradiation, which is an order of magnitude higher than those of most types of well-studied dissolved organic matter in water. As a result, soot could impact the environmental fate of coexisting organic contaminants, such as the photodegradation of bisphenol A. This study will not only give insight into the photochemistry of soot in the liquid phase but also reveal the significant implications of soot photoaging in the aqueous phase by the release and degradation of organic matter.