Spatiotemporal variability of NO2 and PM2.5 over Eastern China: observational and model analyses with a novel statistical method
Mengyao Liu, Jintai Lin, Yuchen Wang, Yang Sun, Bo Zheng, Jingyuan Shao, Lulu Chen, Yixuan Zheng, Jinxuan Chen, May Fu, Yingying Yan, Qiang Zhang
Atmospheric Chemistry and Physics Discussions
Eastern China (27°&thinsp;N&ndash;41°&thinsp;N, 110°&thinsp;E&ndash;123°&thinsp;E) is heavily polluted by nitrogen dioxide (NO<sub>2</sub>), particulate matter with aerodynamic diameter below 2.5&thinsp;μm (PM<sub>2.5</sub>) and other air pollutants. These pollutants vary in a variety of temporal and spatial scales, with many temporal scales non-periodic and non-stationary, challenging proper quantitative characterization and visualization. This study uses a newly
... piled EOF-EEMD analysis-visualization package to evaluate the spatiotemporal variability of ground-level NO<sub>2</sub>, PM<sub>2.5</sub>, and their associations with meteorological processes over Eastern China in Fall&ndash;Winter 2013. Applying the package to observed hourly pollutant data reveals a primary spatial pattern representing Eastern China-wide synchronous variation in time, which is dominated by diurnal variability with a much weaker day-to-day signal. A secondary spatial mode, representing north-south opposing changes in time with no constant period, is characterized by wind-related dilution or buildup of pollutants from one day to another. <br><br> We further evaluate simulations of GEOS-Chem and WRF/CMAQ in capturing the spatiotemporal variability of pollutants. GEOS-Chem underestimates NO<sub>2</sub> by about 17&thinsp;μg/m<sup>3</sup> and PM<sub>2.5</sub> by 35&thinsp;μg/m<sup>3</sup> on average. It reproduces the diurnal variability for both pollutants. For the day-to-day variation, GEOS-Chem reproduces the observed north-south contrasting mode for both pollutants but not the Eastern China-synchronous mode (especially for NO<sub>2</sub>). The model errors are due to a first model layer too thick (about 130&thinsp;m) to capture the near-surface vertical gradient, deficiencies in the nighttime nitrogen chemistry in the first layer, and missing secondary organic aerosols and anthropogenic dusts. CMAQ overestimates the diurnal cycle of pollutants due to too weak boundary layer mixing &ndash; especially in the nighttime, CMAQ overestimates NO<sub>2</sub> by about 30&thinsp;μg/m<sup>3</sup> and PM<sub>2.5</sub> by 60&thinsp;μg/m<sup>3</sup>. For the day-to-day variability, CMAQ reproduces the observed Eastern-China synchronous mode but not the north-south opposing mode of NO<sub>2</sub>. Both models capture the day-to-day variability of PM<sub>2.5</sub> better than that of NO<sub>2</sub>. These results shed light on model improvement. The EOF-EEMD package is freely accessible.