An integrated synchronous online analyzer for gaseous and particulate reactive oxygen species (ROS): development, characterization and field observations

An integrated online analyzer was developed for in situ, synchronous quantification of gaseous and particulate reactive oxygen species (ROS), with concentrations reported as H 2 O 2 equivalents. 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 reproducibility (RSD 0.57 %, n = 10), and robust linearity ( y ≈0.1 x , R 2 = 0.99) with detection limits of 0.07 ppbv (ROS g ) and 0.007 µg m −3 (ROS p ) expressed as H 2 O 2 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 p declined coherently during pollution clean-up, linking ROS variability to coupled changes in oxidation, partitioning, and removal.