Heterogeneous photochemical aging of soot by NO2 under simulated sunlight

The aging process of soot in the atmosphere can significantly influence its physical and chemical properties. Here, the heterogeneous photochemical aging of soot by NO2 under simulated sunlight was investigated using in situ attenuated total internal reflection infrared spectroscopy. It was observed that simulated sunlight can enhance the aging process of soot by NO2. The photochemistry of soot with NO2 led to the formation of various nitro compounds and oxygen-containing species on the soot surface. The photolysis of the surface nitro compounds resulted in various carbonyl groups and gas-phase NO and HONO. The photochemical aging depended on the NO2 concentrations and combustion conditions of soot production. The soot samples with high content of organic carbon (OC) showed high photochemical reactivity towards NO2. This means that the active species mainly belong to OC components, which mainly consist of various polycyclic aromatic hydrocarbons (PAHs) and some unidentified components.

► Sunlight can enhance the photochemical aging process of soot by NO2.
► Formed nitro compounds can be photolyzed and produce carbonyl species, NO and HONO.
► Combustion conditions can modify photo-reactivity of soot to NO2.
► Organic carbon provides main active sites to photochemical aging of soot by NO2.