Analyzing the Characteristics of Cloud Condensation Nuclei (CCN) in Hebei, China Using Multi-Year Observation and Reanalysis Data

The study of the characteristics and variations of cloud condensation nuclei (CCN) plays an important role in understanding the effects of aerosol–cloud interactions. This paper selected observation data in a city region of Shijiazhuang in North China from 2005 to 2007, along with the corresponding MERRA-2 and ERA5 data, to analyze the characteristics of CCN, sort the factors affecting the diurnal variation of CCN number concentration (${\mathrm{N}}_{\mathrm{CCN}}$) according to their importance, and build the relationship between ${\mathrm{N}}_{\mathrm{CCN}}$ and supersaturation ($\mathrm{SS}$) in the heavily polluted region. The results show that there was a bimodal distribution of a daily time series for ${\mathrm{N}}_{\mathrm{CCN}}$ in Shijiazhuang, China. By calculating the correlation between CCN and pollutants observed in winter 2007, we identified that the dominant factor for peaks of ${\mathrm{N}}_{\mathrm{CCN}}$ is ${\mathrm{SO}}_2$ in the morning but ${\mathrm{NO}}_2$ in the evening. We also ranked the factors affecting the diurnal variation of ${\mathrm{N}}_{\mathrm{CCN}}$ by using observation and reanalysis data and found that the concentration of pollutants is the greatest impact factor in summer, but the atmospheric stability is the greatest factor in winter. Finally, we determined the relationship between ${\mathrm{N}}_{\mathrm{CCN}}$ and $\mathrm{SS}$ according to the Twomey formula (${\mathrm{N}}_{\mathrm{CCN}}={\mathrm{cSS}}^{\mathrm{k}}$) and found there was a reasonable value range (i.e., 0.5~0.7) for the parameter $\mathrm{k}$ in East and North China. Specifically, it is more reasonable for $\mathrm{k}$ to be 0.5 in summer and 0.7 in winter.