A PM<sub>2.5</sub>-capable aerosol chemical speciation monitor (ACSM) was deployed in urban Nanjing, China for the first time to measure in-situ non-refractory fine particle (NR-PM<sub>2.5</sub>) composition from October 20 to November 19, 2015 along with parallel measurements of submicron aerosol (PM<sub>1</sub>) species by a standard ACSM. Our results show that the NR-PM<sub>2.5</sub> species (organics, sulfate, nitrate, and ammonium) measured by the PM<sub>2.5</sub>-ACSM are highly correlated (<i>r</i><sup>2</sup> > 0.9) with those measured by a Sunset Lab OC/EC Analyzer and a Monitor for AeRosols and GAses (MARGA). The comparisons between the two ACSMs illustrated similar temporal variations in all NR species between PM<sub>1</sub> and PM<sub>2.5</sub>, yet substantial mass fractions of aerosol species were observed in the size range of 1–2.5 μm. On average, NR-PM<sub>1–2.5</sub> contributed 53 % of the total NR-PM<sub>2.5</sub> with sulfate and secondary organic aerosols (SOA) being the two largest contributors (26 % and 27 %, respectively). Rapid formation and thereafter growth of secondary inorganic aerosols (SIA) were observed under fog processing in NH<sub>3</sub>-rich environments. Positive matrix factorization of organic aerosol showed similar temporal variations in both primary and secondary OA between PM<sub>1</sub> and PM<sub>2.5</sub> although the mass spectra were slightly different due to more thermal decomposition on the capture vaporizer of PM<sub>2.5</sub>-ACSM. We observed an enhancement of SOA under high relative humidity conditions, which is associated with simultaneous increases in particle surface area, gas-phase species (NO<sub>2</sub>, SO<sub>2</sub>, and NH<sub>3</sub>) concentrations and aerosol water content driven by anthropogenic SIA. These results likely indicate an enhanced reactive uptake of SOA precursors upon aqueous particles. Therefore, reducing anthropogenic NO<sub>x</sub>, SO<sub>2</sub>, and NH<sub>3</sub> emissions might not only reduce SIA but also SOA burden during haze episodes in China.