Nitrate photolysis in seawater by sunlight

The photolysis of nitrate in seawater by sunlight has been re-examined using abiotic seawater and naturally occurring concentrations. Photochemical formation of nitrite from nitrate was observed. First-order nitrate photolysis rate coefficients calculated from nitrite appearance (corrected for concomitant nitrite photolysis) ranged from 0 to 2.3 yr−1, median 0.7 yr−1. The coefficients did not correlate well with water chemistry, but decreased with increasing light dose. A first-order rate coefficient of 0.4 yr−1 was calculated for the primary photochemical process NO3− + hυ = NO2− + O(3P) under sea surface equatorial insolation and cloudiness conditions. However, no significant nitrate concentration decreases could be detected, suggesting an upper limit for the net first-order nitrate loss rate coefficient of 0.3 yr−1. The data thus imply some conversion in the reverse sense: NO2− + hυ →→ NO3−. If our median rate estimate applies to surface oceanic conditions, nitrate photolysis proceeds at roughly 0.02–0.5% of the rate of N incorporation during primary production. It is thus not a significant NO3-N sink. Since such reactive species as oxygen atoms, nitrogen dioxide, and hydroxyl radicals are produced, the reaction may have significant consequences in seawater. However, nitrite photolysis is almost certainly a more significant process. The results show internal inconsistencies and our rates are markedly different from those calculated using data from other studies. Nitrate photolysis rates are theoretically concentration- and light dose-dependent. Whether these dependencies explain the apparent discrepancies is unclear, as methodological effects may also be involved. The system requires further study.