Abstract.Hydrogen peroxide (H 2 O 2 ) and organic peroxides play important roles in the cycle of oxidants and the formation of secondary aerosols in the atmosphere. Recent field observations have suggested that the budget of peroxyacetic acid (PAA, CH 3 C(O)OOH) is potentially related to the aerosol phase processes, especially to secondary aerosol formation. Here, we present the first laboratory measurements of the uptake coefficient of gaseous PAA and H 2 O 2 onto ambient fine particulate matter (PM 2.5 ) as a function of relative humidity (RH) at 298 K. The results show that the PM 2.5 , which was collected in an urban area, can take up PAA and H 2 O 2 at the uptake coefficient (Îł ) of 10 â4 , and both Îł PAA and Îł H 2 O 2 increase with increasing RH. The value of Îł PAA at 90 % RH is 5.4 ± 1.9 times that at 3 % RH, whereas Îł H 2 O 2 at 90 % RH is 2.4 ± 0.5 times that at 3 % RH, which suggests that PAA is more sensitive to the RH variation than H 2 O 2 is. Considering the larger Henry's law constant of H 2 O 2 than that of PAA, the smaller RH sensitivity of the H 2 O 2 uptake coefficient suggests that the enhanced uptake of peroxide compounds on PM 2.5 under humid conditions is dominated by chemical processes rather than dissolution. Considering that mineral dust is one of the main components of PM 2.5 in Beijing, we also determined the uptake coefficients of gaseous PAA and H 2 O 2 on authentic Asian Dust storm (ADS) and Arizona Test Dust (ATD) particles. Compared to ambient PM 2.5 , ADS shows a similar Îł value and RH dependence in its uptake coefficient for PAA and H 2 O 2 , while ATD gives a negative dependence on RH. The present study indicates that, in addition to the mineral dust in PM 2.5 , other components (e.g., soluble inorganic salts) are also important to the uptake of peroxide compounds. When the heterogeneous reaction of PAA on PM 2.5 is considered, its atmospheric lifetime is estimated to be 3.0 h on haze days and 7.1 h on non-haze days, values that are in good agreement with the field observations.