Impact of inter-annual meteorological variation from 2001 to 2015 on the contribution of regional transport to PM2.5 in Beijing, China

Abstract

Abstract Particles with an aerodynamic diameter of 2.5 μm or less (PM2.5) in the ambient air are affected not only by local emissions but also by regional transport of the pollutant and its precursors, which may vary considerably under different meteorological conditions. This issue has been extensively investigated in terms of the contribution of regional transport to PM2.5 at different seasons in a specific year. However, the literature yields conflicting results, and the extent to which inter-annual meteorological variation affects the contribution of regional transport is not fully understood, especially in the long-term. Therefore, in this study, the WRF/CMAQ model was employed to investigate inter-annual variations in the contribution of regional transport to PM2.5 concentration in Beijing from 2001 to 2015. The emission inventory and model configurations were kept the same for the 15-year simulation to highlight the impact of inter-annual meteorological variations. The major findings can be summarized as follows: (1) inter-annual meteorological variation has an impact on both the PM2.5 concentration in Beijing and the contribution of regional transport; (2) the annual average PM2.5 concentrations in Beijing varied during the study period, from 75.3 μg/m3 in 2002 to 108.1 μg/m3 in 2013; (3) the years with high annual anomalies of PM2.5 was found during periods of low wind speed, high relative humidity, and low planetary boundary layer (PBL) height; (4) the annual variability was relatively small compared to the changes in seasonal because some seasons in a year are higher than their historical average values, and some are lower, which often negate each other, making the annual average close to the historical average; (5) the contribution of regional transport averaged 44.5% and ranged from 40.7% in 2007 to 48.5% in 2001. On a seasonal basis, the inter-annual fluctuation was more significant, with fluctuation rates of 43.0%, 25.5%, 25.0%, and 20.6% in winter, fall, spring, and summer, respectively; and (6) in general, there is a good positive correlation between the regional contribution and PM2.5 concentration in Beijing. Specifically, when the regional transport contribution is large, the PM2.5 concentration tends to be higher. This is especially true during years when both the concentration and contribution deviate significantly from the historical average.