An integrated online analyzer was developed for in situ, synchronous quantification of reactive oxygen species (ROS) in gaseous and particulate phases. Gaseous ROS (ROS_g) are absorbed by a glass spiral absorption tube, whereas particulate ROS (ROS_p) are collected at ambient temperature using a rotating wet annular denuder (WAD) for gas removal followed by a spray growth collection chamber. The collected solutions are analyzed using a fluorescence probe method, and the resulting fluorescent signal is recorded using a compact LED-PMT module (470/520 nm) and LabVIEW-based acquisition. The system achieved high stability (RSD 0.37 % over 10 h), fast tracking (7 min response), good repeatability (RSD 0.57 %, n = 10), and robust linearity (y ≈ 0.1x, R²= 0.99) with detection limits of 0.07 ppbv (ROS_g) and 0.006 µg m^-3 (ROS_p) expressed as H₂O₂ equivalents. Field deployment in Beijing across four seasons revealed pronounced seasonal, diurnal, and pollution-regime dependence. ROS_g and ROS_p were highest in spring, while autumn exhibited the lowest levels despite severe PM_2.5 pollution. During humid autumn haze, enhanced aerosol water and secondary inorganic accumulation coincided with only modest ROS_g growth and constrained ROS_p, indicating rapid multiphase turnover and efficient condensed-phase loss. In contrast, ozone-driven pollution in spring and summer strengthened photochemical production and gas-particle coupling, increasing ROS in both phases. Both ROS_g and ROS_pdeclined coherently during pollution clean-up, linking ROS variability to coupled changes in oxidation, partitioning, and removal.