Accurate measurements of atmospheric reactive mercury (RM or HgII) including gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM) are crucial to improving understanding of mercury (Hg) behavior in the ambient air and evaluating the effectiveness of the Minamata Convention. As part of the Speciation and Transformation of Atmospheric Mercury in a Polluted region (STAMP) campaign in eastern China, comparison of Tekran, the reactive mercury active system (RMAS) and the micro-orifice uniform deposit impactors (MOUDI) system for RM measurements was conducted in this study. The ratio of GOMTekran/GOMRMAS was found to be positively correlated with the ratio of PBM/GOM and the proportion of -Br/Cl in RM based on deconvolution of the RMAS thermal desorption profiles. HgII reduction by the aqueous HO2 radicals was found to be the most likely cause of GOM underestimation by denuder-based methods. PBM acting as a substitute for GOM in HgII reduction could limit the underestimation. High particulate matter (PM) environment could cause PBM breakthrough in RMAS sampling, resulting in PBM underestimation and GOM overestimation. The chemical compound characteristics of RM and HgII distribution on particles by size provide evidences for the sources of the discrepancies in PBM measurements by different methods. Particle-size-resolved compound profiles are useful approaches for accurate RM quantification. Display omitted •Intercomparison for RM measurements was performed in the STAMP campaign.•A theory of mechanism for GOM underestimation by KCl denuders was proposed.•High PM environment could cause PBM breakthrough in the RMAS method.•Particle-size-resolved compound profiles are useful tools for RM quantification.