A PM_2.5-capable aerosol chemical speciation monitor (Q-ACSM) was deployed in urban Nanjing, China, for the first time to measure in situ non-refractory fine particle (NR-PM_2.5) composition from 20 October to 19 November 2015, along with parallel measurements of submicron aerosol (PM₁) species by a standard Q-ACSM. Our results show that the NR-PM_2.5 species (organics, sulfate, nitrate, and ammonium) measured by the PM_2.5-Q-ACSM are highly correlated (r² > 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 Q-ACSMs illustrated similar temporal variations in all NR species between PM₁ and PM_2.5, yet substantial mass fractions of aerosol species were observed in the size range of 1–2.5 µm. On average, NR-PM_1−2.5 contributed 53 % of the total NR-PM_2.5, with sulfate and secondary organic aerosols (SOAs) being the two largest contributors (26 and 27 %, respectively). Positive matrix factorization of organic aerosol showed similar temporal variations in both primary and secondary OAs between PM₁ and PM_2.5, although the mass spectra were slightly different due to more thermal decomposition on the capture vaporizer of the PM_2.5-Q-ACSM. We observed an enhancement of SOA under high relative humidity conditions, which is associated with simultaneous increases in aerosol pH, gas-phase species (NO₂, SO₂, and NH₃) concentrations and aerosol water content driven by secondary inorganic aerosols. These results likely indicate an enhanced reactive uptake of SOA precursors upon aqueous particles. Therefore, reducing anthropogenic NO_x, SO₂, and NH₃ emissions might not only reduce secondary inorganic aerosols but also the SOA burden during haze episodes in China.