Surface Energy Budget Observed for Winter Wheat in the North China Plain During a Fog–Haze Event

In recent winters, fog–haze events have occurred frequently over the North China Plain. To understand the characteristics of conventional meteorological conditions, the near-surface radiation balance, and the surface energy budget under different pollution levels, we analyzed data collected at an observation site in Gucheng, which is located in the Hebei province in North China, based on a campaign that ran from December 1 2016 to January 31 2017. We found that meteorological conditions with a lower wind speed, weakly unstable (stable) stratification, higher relative humidity, and lower surface pressure during the daytime (night-time) are associated with fog–haze events. On heavy pollution days (defined as days with a daily mean PM _2.5 concentration > 150 μg m ⁻³ ), the decrease in downward shortwave radiation (S _↓ ) and the increase in downward longwave radiation (L _↓ ) are significant. The mean S _↓ (L _↓ ) values on clean-air days (daily mean PM _2.5 concentration < 75 μg m ⁻³ ) and heavily polluted days was 222 (222) W m ⁻² and 124 (265) W m ⁻² , respectively. Due to the negative (positive) radiative forcing of aerosols during the daytime (night-time), the daily maximum (night-time mean) net radiation (R _n ) is negatively (positively) related to the daily mean PM _2.5 concentration, the correlation coefficient between the daily maximum (night-time mean) R _n and daily mean PM _2.5 concentration being − 0.47 (0.51). Diurnal variations in sensible heat flux (H) and latent heat flux (λE) are insignificant on heavily polluted days, the mean daily maximum H (λE) is only 40 (28) W m ⁻² on heavily polluted days, but reaches 90 (42) W m ⁻² on clean-air days. Additionally, the friction velocity, standard deviation of vertical velocity, and turbulent kinetic energy on heavily polluted days are also quantified.