2.2. FLEXPART-WRF model

The WRF model provides the fine meteorological fields driving the FLEXPART model during pollution events. In this study, the WRF model was configured with two nested domains, coarse and fine. The coarse domain covered the entirety of Asia with a 30 × 30 km horizontal resolution, and the nested fine domain included most of China and its surrounding regions with a 10 × 10 km horizontal resolution. The physical parameterizations used in WRF modeling were the Morrison microphysics scheme (^{Morrison et al., 2009}), Rapid Radiative Transfer Model (RRTM) scheme for long- and short-wave radiation (^{Mlawer et al., 1997}), Yonsei University (YSU) boundary layer scheme (^{Hong et al., 2006}), Grell 3D cumulus parameterization, and Noah land surface scheme (^{Grell et al., 2005}). Using the reanalysis meteorological data in the horizontal resolution of 1° × 1° obtained from the NCEP for initial and boundary meteorological conditions, the WRF simulation ran for 12 h each time, where the first 6 h of simulations constituted spin-up time.

A Lagrangian particle dispersion model, FLEXPART-WRF version 3.1 (^{Brioude et al., 2013; Stohl et al., 2005; Fast and Easter, 2006}), was used to determine the origin and transport pathways of particles arriving at the receptor site. In this model, the trajectories of a large number of particles released from a source are simulated, considering the processes of tracer transport, turbulent diffusion, and wet and dry deposition in the atmosphere (^{Brioude et al., 2013}); thus, it simulates the transport and dispersion of tracers by calculating the backward trajectories of multitudinous particles, which are termed plume backward trajectories, reflecting the distribution of potential source regions that may have impacts on a target point or receptor region (^{Chen et al., 2017; Seibert and Frank, 2004; Zhai et al., 2016}).

To improve the accuracy of the trajectory calculation, we used high-resolution WRF simulation domain 2 outputs as the input meteorological conditions for the FLEXPART model, which has been widely used to investigate the potential sources of air pollutants in relation to environmental change (^{Stohl, 2003; Gadhavi et al., 2015; Sauvage et al., 2017; Zhu et al., 2018}). In this study, the FLEXPART-WRF simulation was conducted for a 24-hour backward trajectory with the release of 10,000 air particles in the first hour from Xiangyang (XY, 32.0°N, 112.1°E), Wuhan (WH, 30.6°N, 114.3°E), and Jingzhou (JZ, 30.3°N, 112.2°E), respectively, for six pollution events during the lockdown. The output domain in FLEXPART-WRF was set up with six vertical levels (10, 100, 500, 1000, 2000, and 4000 m) with a 10–1000 km release height and a horizontal resolution of 0.1° × 0.1°.