A modeling study of PM_2.5 transboundary transport during a winter severe haze episode in southern Yangtze River Delta, China

The WRF-CHEM (Weather Research Forecast-Chemistry) model was used to investigate a severe haze episode in Zhejiang province in southern Yangtze River Delta, China, during 28 November to 10 December 2013 to obtain the characteristics of PM_2.5 transboundary transport and its contributions to haze formation. The predicted temporal variations of PM_2.5 and meteorological variables are in good agreement with in-situ observations in Zhejiang. Simulated wind direction and pollution levels in upwind areas have significant effects on modeled near-surface PM_2.5 concentrations. Both the large-scale atmospheric circulations and local weather conditions contributed to the accumulation of pollutants in Zhejiang, featuring the weakened East Asia trough, the dominance of a weak high pressure system, a low wind speed and a low planetary boundary layer height (PBLH) during the heavily polluted period from 4 to 7 December. The near-surface PM_2.5 concentration was highly positively correlated with the accumulated PM_2.5 transport mass (R = 0.88 at a lag of 7 h) and moderately negatively correlated with the PBLH (R = −0.39) during this period, indicating that the horizontal transport across Zhejiang's boundary under stable meteorological condition played important roles in the high PM_2.5 concentrations. The analysis of PM_2.5 transboundary transport flux shows that the main transport pathway for outside pollutants affecting Zhejiang was mainly the northerly flow. There were strong input fluxes mainly within the planetary boundary layer on 4 and 6 December before the maximum concentration occurred in the early morning on 7 December. The contribution of PM_2.5 transboundary transport to the aerosol pollution in the study domain was determined by a receptor-oriented model based on the conservation of mass formula for PM_2.5. The PM_2.5 transboundary transport across the Zhejiang's boundary contributed 62% to the total PM_2.5 increase during the severe haze episode, and the primary emission and secondary PM_2.5 formation contributed 15% and 23%, respectively. This result suggests that the joint efforts with neighboring provinces to mitigate pollutant emissions are important to improve air quality in Zhejiang during winter.