The impact of nonmethane hydrocarbon compounds on tropospheric photochemistry

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Abstract
Nonmethane hydrocarbons (NMHC) play an important role in global scale tropospheric photochemistry. The representation of NMHC chemistry in three‐dimensional tropospheric chemistry transport models requires a highly parameterized description of only the most important processes, as the number of reactions and compounds involved is very large. In this paper a chemical scheme is presented, based on the Carbon Bond Mechanism 4 (CBM‐4), modified for use in the global Tracer Model 3 (TM3). The original scheme has been extended with reactions important under background conditions, and reaction rates and product distributions have been updated. Box model tests show that the modified CBM‐4 and a detailed reference mechanism agree well for a broad range of chemical conditions. Results of TM3 runs with and without NMHC chemistry are compared with observations, illustrating the effects of NMHC on key compounds of tropospheric photochemistry, such as ozone, the hydroxyl radical, carbon monoxide, and NOx. In particular, the simulation of ozone over polluted regions improves when NMHC chemistry is accounted for. Globally, the contribution of NMHC to net photochemical ozone production is estimated at about 40%, leading to a 17% increase of the tropospheric ozone column. OH is depleted over the continents owing to reactions with NMHC, which is most evident in regions with strong biogenic emissions. Although NMHC significantly influence the global OH distribution, their effect on its total tropospheric content appears to be marginal. Results of sensitivity runs with and without organic peroxy nitrates show that this N reservoir may significantly change the global NOx distribution, leading to a NOx increase over the oceans of the order of 50%. This increase improves the agreement between simulated and observed nitrate levels at remote stations, although large discrepancies remain. In general, the present treatment of NMHC reproduces the main features of tropospheric chemistry.