Photoproduction of nonmethane hydrocarbons (NMHCs) in seawater

Nonmethane hydrocarbons (NMHCs) are emitted into the atmosphere from the oceanʹs surface. Although the oceanic contribution to the global NMHC budget is considered minor relative to other sources, it is one of the least well measured or understood. The major compounds emitted are a series of C2–C4 alkenes. Due to their ubiquitous presence in seawater, their high reactivity in the atmosphere, and the important role they play in the atmospheric chemistry in remote regions of the globe, the processes responsible for their formation and fate warrant examination. To ascertain the role of photoprocesses in NMHC formation in surface waters, laboratory experiments were conducted to determine the magnitude and wavelength dependence of NMHC photoproduction for a range of water types. Photochemical transformation of dissolved organic matter (DOM) present in surface seawater results in the production of the C2–C4 alkenes in the surface ocean with the UV portion (295–340 nm) of the solar spectrum controlling the bulk of this production. Under summer sunlight intensities in Miami, FL, net photoproduction rates for ethene, the dominant photoproduced NMHC in surface seawater, varied from 0.220 pmol l−1 min−1 for transparent oligotrophic waters to 6.11 pmol l−1 min−1 for highly colored estuarine waters. Apparent quantum yields of photoproduction varied from 10−9 to 10−7 depending on water type, wavelength of light and individual compound.