Browsing by Subject "Ozone formation sensitivity"

Ozone, an important and ubiquitous trace gas, protects lives from harm of solar ultraviolet (UV) radiation in the stratosphere but behaves as a toxic compound in the troposphere to living organisms. Also, tropospheric ozone is a vital oxidant or source of daytime oxidant (i.e., OH radical) for e.g., different volatile organic compounds (VOCs). Affecting global radiation balance directly or indirectly by acting as cloud condensation nuclei and having negative impact to human health, aerosols are widely studied for over a century. Highly oxygenated organic molecules (HOM) were proved to be a large source of secondary organic aerosol (SOA) and their oxidation formation pathways from VOCs can also trigger the production of ozone once involving NOx (=NO+NO2) and UV light. The highly nonlinear relationship among ozone, NOx, and VOCs (O3-NOx-VOC sensitivity or O3 formation sensitivity) has been researched since last century. The complex system was recently reflected during COVID-19 lockdowns: reduction of NOx increased the ozone production. This is because the system was in VOC-limited regime, where reducing VOCs is the most efficient way to reduce O3. However, the determination of O3 formation regimes (either VOC-limited or NOx-limited) is challenging in different environmental conditions. The intrinsic connection between HOM and O3 formation provide a new insight: the proportions of VOCs and NOx not only affect the O3 formation regimes but also impact the distribution of HOM species. Therefore, in this study, we try to unveil the indicating role of HOM species on the O3 formation sensitivity by chamber experimental works with a nitrate chemical ionization mass spectrometer (CI-APi-TOF) and gas monitors. Injected NOx and VOCs step by step, the experiments were designed to make the atmosphere-mimicking system change between those two regimes. The ratio between HOM-dimers and HOM organic nitrate monomers was selected as the indicator for O3 formation sensitivity due to their closely connected chemical reactions, involving peroxy radicals. Furthermore, a simple box model was developed for simulating chamber results and obtaining O3 isopleths to visually show the O3 formation regimes. Through experimental and model results, it can be inferred that ratios below 0.2 consistently correspond to the VOC-limited regime, whereas ratios above 0.5 consistently correspond to the NOx-limited regime. This study demonstrates that the ratio based on HOM species could additionally indicate the O3 formation sensitivity of ambient air when we use CI-APi-TOF to investigate the chemical compounds and aerosol formation, helping to elaborate the O3 pollution in the real troposphere.