Photochemically generated nitric oxide in seawater: The peroxynitrite sink and its implications for daytime air quality.

In this study, we experimentally investigated the magnitude of the peroxynitrite sink: a radical-radical consumption mechanism for photochemically generated nitric oxide (NO) in surface seawater that describes NO reactions with co-generated superoxide (O) to yield peroxynitrite (ONOO). Measurements of photochemically generated NO, O and ONOO were conducted on seawater samples obtained from the Seto Inland Sea, Japan. Nitrite, dissolved organic carbon, chromophoric dissolved organic matter and pH were also measured in the same samples using standard analytical methods. The average photoformation rates of NO, O and ONOO were: 1.78 × 10 M s, 7.19 × 10 M s and 9.0 × 10 M s, respectively, and the average steady-state concentrations were: 67.28 × 10 M, 2.69 × 10 M and 2.26 × 10 M, respectively. Further evaluation of the experimental data indicated that the existence of ONOO in seawater strongly depends on, and is limited by, photoformed NO. Seawater alkalinity favored the consumption of photoformed NO· via the peroxynitrite sink. The magnitude of average sinks (%) calculated from kinetic estimates and experimental data were: 0.17% and 0.11%, respectively. These results show that the consumption of photochemically generated NO· via the peroxynitrite sink is not significant in surface seawater. Therefore, we propose that sea-to-air efflux across the marine boundary layer is the major sink of photochemical NO· and can be regarded as a non-anthropogenic contributor to daytime atmospheric NO concentrations.