The implementation of strict emission control during the G20 summit in China and the occurrence of Typhoon Malarkas in September 2016 provided a valuable opportunity to examine the role of control measures and special weather condition in the formation of PM_2.5. Water-soluble inorganic ions (WSII) were measured hourly in addition to PM_2.5 mass concentration and black carbon (BC) and gaseous pollutants in Ningbo located in the Yangtze River Delta (YRD) region of China. Three distinct cases, i.e., a control case, a normal case, and a typhoon case, were investigated during the study period. PM_2.5 was higher in the control case (37.5 μg m^-3) than in the normal case (29.8 μg m^-3), whereas the lowest PM_2.5 (14.2 μg m^-3) was observed in the typhoon case. The analyses of meteorology and backward trajectory suggested that stable weather and regional transport from inland regions accounted for the high PM_2.5 under strict control. Only the concentrations of Ca²⁺ and NO₃^- decreased in the control case, while those of all water-soluble inorganic components (except Na⁺ and Mg²⁺) decreased substantially in the typhoon case. SO₄^2- dominated the WSII, with the highest contribution, 62%, in the control case. This result was attributed to a stagnant atmosphere with a high relative humidity (RH), which was beneficial for the transformation of SO2 to SO₄^2-. The control and typhoon cases both resulted in extremely low NO2, but the contribution of NO₃^- to PM_2.5 was reduced in the control case and increased in the typhoon case. The close correlation of the NOR (nitrate oxidation ratio) with Na⁺ and Mg²⁺ in the typhoon case was indicative of sea salt associated with the typhoon which provided a surface for the heterogeneous formation of NO₃^-. The presented results facilitate a better understanding of the characteristics and formation of PM_2.5 under the influence of artificial control and natural intervention.