Measurement and modeling of the dry deposition of peroxides

easurements of the dry deposition velocity (Vd) of hydrogen peroxide (H2O2) and total organic peroxides (ROOH) were made during four experiments at three forested sites. Details and uncertainties associated with the measurement of peroxide fluxes by the flux-gradient method are discussed. The results are compared to those predicted using a bulk-resistance model of the type commonly used in regional photochemical models. Good agreement between the H2O2 Vd measurements and a bulk resistance model is obtained when the model contains a zero surface resistance (Rc) and a common form for the laminar leaf-layer resistance (Rb) based on Schmidt and Prandtl numbers. In this case, a near-zero (<5 s m-1) surface resistance is confirmed for H2O2 within experimental uncertainties. Surface resistances for ROOH were determined to be about 10–15 s m-1 over a coniferous forest and 20–40 s m-1 over a deciduous forest. Higher uncertainties for ROOH prevent a detailed analysis of the differences in Rc among forest types. However, the ratio of deposition velocities (ROOH/H2O2), computed from normalized concentration gradients, ranged from 0.28 to 0.61 (geometric mean) at the three sites. Differences in molecular diffusivities between H2O2 and ROOH can only account for an estimated 16% difference in Vd. Thus, the major constituent of ROOH must also be less soluble and/or less reactive than H2O2, which is consistent with the characteristics of methylhydroperoxide (MHP).