Passengers carrying the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a commercial aircraft cabin may infect other passengers and the cabin crew. In this study, a cabin model of the seven-row Airbus A320 aircraft is constructed and meshed for simulating the SARS-CoV-2 spread in the cabin with a virus carrier using the Computational Fluid Dynamics (CFD) modeling tool. The passengers’ infection risk is also quantified with the susceptible exposure index (SEI) method. The results show that the virus spreads to the ceiling of the cabin within 50 s of the virus carrier’s normal breathing. Coughing makes the virus spread to the front three rows with a higher mass fraction. While the high mass fraction areas always stay on the same side of the aisle as the virus carrier, the adjacent passengers and the passengers in the back two rows are affected more than the others when the virus carrier breathes normally. Spread patterns under the carrier’s two breath conditions, normal breath and cough, were numerically simulated.

The sudden outbreak of the Corona Virus Disease 2019 (COVID-19) broke the stable trend of air quality improvement in Chinese cities due to the emergency response measures, such as the lockdown policy. In this study, 333 Chinese cities are divided into six levels by economic factors, such as first-tier, new first-tier, second-tier, third-tier, fourth-tier, and fifth-tier, and the time of each year was defined as pre-lockdown, lockdown and reopen periods. The results show that, during the pre-lockdown period, the concentrations of NO₂ and PM_2.5 decreased by 10.13% and 6.15%, respectively, compared with the same period averages in 2017–2019. The fifth-tier cities have the minimum reductions of NO₂ and PM_2.5. The lockdown policy has significantly improved the air quality in Chinese cities during the COVID-19 pandemic. The concentrations of NO₂ and PM_2.5 decreased by 42.38% and 29.34%, respectively, but the air quality of first-tier cities was least affected by lockdown. During the reopen period, the concentrations of NO₂ and PM_2.5 decreased by 14.28% and 25.71%, respectively, and the NO₂ concentrations in each city are similar for the season. The NO₂ and PM_2.5 concentrations in 2020 without lockdown were estimated through a multivariate linear regression model. The actual values of both NO₂ and PM_2.5 concentrations during the lockdown period are lower than the projected values based on the 2017–2019 trend, again indicating that the lockdown policy influences their concentrations.