Mechanism of the atmospheric chemical transformation of acetylacetone and its implications in night-time second organic aerosol formation

Recently, a high concentration of acetylacetone (AcAc) has been measured in China, and its day-time chemistry with OH reaction has been evaluated. The phenomenon has profound implications in air pollution, human health and climate change. To systematically understand the atmospheric chemistry of AcAc and its role in the atmosphere, the night-time chemistry of AcAc with O3 and NO3 radical were investigated in this work in detail using density functional theory. The results show that for O3- and NO3-initiated atmospheric oxidation reactions of AcAc, the barrier energies of O3/NO3-addition are found to be much lower than those of H-abstraction, suggesting that O3/NO3-addition to AcAc is a major contributing pathway in the atmospheric chemical transformation reactions. The total degradation rate constants were calculated to be 2.36 × 10-17 and 1.92 × 10-17 cm3 molecule-1 s-1 for the O3- and NO3-initiated oxidation of AcAc at 298 K, respectively. The half-life of AcAc+O3 in some polluted areas (such as, Pearl River Delta and Yangtze River Delta) is close to 3 h under typical tropospheric conditions. Due to its short half-life, the ozonolysis of AcAc plays a more significant role in the night-time hours, leading to fast transformations to form primary ozonides (POZs). A prompt, thermal decomposition of POZs occurred to yield methylglyoxal, acetic acid and Criegee intermediates, which mainly contributed to the formation of secondary organic aerosol (SOA). Subsequently, using the high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), a non-negligible concentration of AcAc was measured in the field observation during the night-time in Nanjing, China. The obtained results reveal that the atmospheric oxidation of AcAc can successively contribute to the formation of SOA under polluted environments regardless of the time (day-time or night-time). This is due to its high reactivity to tropospheric oxidant species (such as, O3 and NO3 radicals at night-time).