Search

The city clusters in eastern coastal China have suffered from serious air pollution in the past decades, which is partially related to the complex local topography and meteorological conditions. The planetary boundary layer (PBL) scheme is a critical parameter for accurate meteorology simulations and air quality predictions. In this study, we analyze the impact of four typical PBL schemes, namely, Yonsei University (YSU), Mellor-Yamada-Nakanishi-Niino Level 2.5 (MYNN), Asymmetric Convective Model version 2 (ACM2), and Mellor-Yamada-Janjic (MYJ) PBL, within the Weather Research and Forecasting (WRF) model to assess their impacts on the simulations of air pollutant concentrations based on the Comprehensive Air Quality Model with Extensions (CAMx) for the Yangtze River Delta (YRD) region, one of the most developed city clusters in eastern China. The results indicate that the MYNN scheme performs best in terms of fine particulate matter (PM 2.5) and nitrogen dioxide (NO 2) simulations, with mean bias of 4.8 μg m−3 and 9.3 μg m−3 in summer and 11.7 μg m−3 and 5.4 μg m−3 in winter, respectively. The YSU scheme performs best for ozone (O 3) prediction, with better simulation results in summer than in winter. Notably, some discrepancies among different PBL schemes in the prediction of air pollution are directly associated with the complex topography. For the prediction of PM 2.5 , the MYJ and YSU schemes tend to overestimate the concentrations in the plains of Jiangsu and northern Anhui while underestimating PM 2.5 in the hilly areas compared with the MYNN scheme. Meanwhile, the performance of the ACM2 scheme is opposite to that of the MYJ and YSU schemes. For O 3 predictions, MYJ overestimates O 3 in the eastern coastal area and underestimates O 3 in the inland areas in summer compared with the YSU scheme, while the ACM2 scheme is the opposite of the MYJ scheme, and the MYNN scheme consistently overestimates O 3. • Impacts from four PBL schemes within WRF on the simulation of air pollutants are evaluated over the YRD region. • Seasonal and diurnal variations of surface pollutant concentrations are predicted and discussed. • The MYNN scheme shows good performance for PM 2.5 and NO 2 , while the YSU scheme is most suitable for O 3 prediction in the YRD. • Implementing a single PBL scheme for a large area with complex topography is insufficient. [ABSTRACT FROM AUTHOR]

The outbreak of COVID-19 has spreaded rapidly across the world. To control the rapid dispersion of the virus, China has imposed national lockdown policies to practise social distancing. This has led to reduced human activities and hence primary air pollutant emissions, which caused improvement of air quality as a side-product. To investigate the air quality changes during the COVID-19 lockdown over the YRD Region, we apply the WRF-CAMx modelling system together with monitoring data to investigate the impact of human activity pattern changes on air quality. Results show that human activities were lowered significantly during the period: industrial operations, VKT, constructions in operation, etc. were significantly reduced, leading to lowered SO 2 , NO x , PM 2.5 and VOCs emissions by approximately 16–26%, 29–47%, 27–46% and 37–57% during the Level I and Level II response periods respectively. These emission reduction has played a significant role in the improvement of air quality. Concentrations of PM 2.5 , NO 2 and SO 2 decreased by 31.8%, 45.1% and 20.4% during the Level I period; and 33.2%, 27.2% and 7.6% during the Level II period compared with 2019. However, ozone did not show any reduction and increased greatly. Our results also show that even during the lockdown, with primary emissions reduction of 15%–61%, the daily average PM 2.5 concentrations range between 15 and 79 μg m−3, which shows that background and residual pollutions are still high. Source apportionment results indicate that the residual pollution of PM 2.5 comes from industry (32.2–61.1%), mobile (3.9–8.1%), dust (2.6–7.7%), residential sources (2.1–28.5%) in YRD and 14.0–28.6% contribution from long-range transport coming from northern China. This indicates that in spite of the extreme reductions in primary emissions, it cannot fully tackle the current air pollution. Re-organisation of the energy and industrial strategy together with trans-regional joint-control for a full long-term air pollution plan need to be further taken into account. Unlabelled Image • Air quality changes during the COVID-19 in YRD region are analyzed. • The WRF-CAMx modelling system is applied to investigate impact of lowered human activities on air quality changes. • Sources of the residual pollution are figured out for policy implications for future air pollution control. [ABSTRACT FROM AUTHOR]