Nonmethane hydrocarbon chemistry in the remote marine boundary layer

We present a photochemical model of the remote marine boundary layer, focusing on the role of reactive nonmethane hydrocarbons (NMHCs). Current observations constrain total NMHC atmospheric concentrations in the remote marine boundary layer only to within a factor of 25, with the major uncertainty lying in the heavier alkenes. Using our model, we deduce air‐sea fluxes of NMHCs from these observations by demanding a balance between oceanic evasion fluxes and atmospheric loss rates for these species. The total NMHC air‐sea flux so deduced lies between 1×10 10 and 2×10 11 molec cm −2 s −1 , with the range representing either true variability or uncertainty. This is inconsistent with the total NMHC air‐sea flux ((1‐6) × 10 9 molec cm −2 s −1 ) derived using a diffusive microlayer model together with the few existing oceanic NMHC observations. Diurnally averaged OH concentrations in our model range between 1.2×10 6 and 4.0×10 5 molec cm −3 ; our preferred model run, with a total NMHC flux of 5.4×10 10 molec cm −2 s −1 , produces a diurnally averaged OH concentration of 8.4×10 5 molec cm −3 . This agrees well with OH concentrations derived from inversions of methyl chloroform data. Our model results imply a possibly significant role for NMHCs in determining OH concentrations in the remote marine boundary layer. Definitive conclusions about the role of NMHCs are prevented, however, both by inconclusive data on the NMHCs themselves and by large chemical uncertainties, particularly that in the O( 1 D ) quantum yield.