Hourly particulate matter mass concentrations and meteorological parameters, recorded by the China National Environmental Monitoring Center (CNEMC) and China Observatory, between June 2013 and March 2016 in Beijing, Xi’an, Shanghai and Guangzhou were examined to explore correlations. Characteristics of a rapid (abrupt) rise in PM2.5 mass concentration during early stage of a serious urban PM pollution event were examined and compared with pollution events with a gradual (accumulative) rise in PM2.5 mass concentration. The accumulative rise in air pollution is characterized by a prolonged slow PM2.5 growth rate (3–5 µg m–3 h–1), and could eventually lead to middle level pollution (ambient PM2.5 mass concentration of about 150 µg m–3), accompanied with an uncertain temporal variation in SO2, NO2, O3 and CO concentrations. The abrupt rise process is characterized by a short-term high aerosol growth rate (> 10 µg m–3 h–1), and could eventually form severe air pollution (PM2.5 mass concentration exceeds 250 µg m–3), with a constant increase in gaseous pollutants concentrations. The average relative humidity (RH) was observed to have a less impact on the rise of PM2.5 concentration, but the fluctuation in RH was found to have a strong correlation with the rise in PM2.5 concentration. Further analysis has indicated that both abrupt and accumulative rise in PM2.5 mass concentration could be due to the RH fluctuation in atmosphere. The RH variation is important for the study of the fine-particle growth and for prediction of PM pollution episodes.