Size-fractioned aerosol samples were collected by an eight-stage Anderson sampler for four seasons from November 2014 to August 2015 at a coastal and suburban site in Ningbo, China, with a total of 270 samples were obtained. The seasonal variations and size distributions of water-soluble inorganic ions (WSIIs), carbonaceous aerosols (OC and EC), which consist of four organic carbon (OC1-OC4), pyrolyzed carbon (OP) and three elemental carbon fraction (EC1-EC3), were investigated. For the sampling periods, the average total concentration of WSIIs, OC and EC in PM_1.1, PM_1.1–2.1 and PM_2.1–9.0 were 21.3 ± 7 μg/m³, 6.7 ± 2.7 μg/m³ and 12.8 ± 1.9 μg/m³, constituting 75.5%, 62.7% and 43.2% of the different size particle mass, respectively. The predominant chemical species were SO₄^2-, NO3, and OC. WSIIs, OC and EC all exhibited significant difference between PM_2.1 and PM_2.1–9.0, reflecting their different sources. Ion balance calculations showed that the acidity of aerosols increased with a decrease in size, with the maximum of 1.07 in 1.1-2.1 μm and the minimum of 0.47 in 2.1–9 μm. It showed that size distributions of high-temperature carbon fraction such as OC4, OP and EC1 were almost unimodal during all seasons as well as SO₄^2- and NH₄⁺, in contrast, that of lower temperature carbon fraction (OC1-OC3), Mg²⁺, and Ca²⁺ appear like bimodal. Furthermore, the high consistency between the size distribution of OC4, OP and SO₄^2-, NH₄⁺ in all seasons suggests that the similar or related generation process for the secondary organic and inorganic/ionic species, which contribute the most significant component of the particulate matter. Besides the secondary aerosols, primary carbonaceous aerosols (PC), which may originate in emissions from mixed combustion or natural source, also contributed a significant fraction of haze pollution, especially in autumn, spring and summer.

Meteorological factors and PM during the four sampling periods: (a) wind speed (WS) and wind direction (WD); (b) ambient temperature (T) and relative humidity (RH); (c) PM2.5 and PM10 mass concentration.