Photoproduction of nitric oxide in seawater

<p>Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle. However, our knowledge about its production and consumption pathways in oceanic environments is rudimentary. In order to decipher the major factors affecting NO photochemical production, we irradiated several artificial seawater samples as well as 31 natural surface seawater samples in laboratory experiments. The seawater samples were collected during a cruise to the western tropical North Pacific Ocean (WTNP, a N–S section from 36 to 2<span class="inline-formula">^∘</span>&thinsp;N along 146 to 143<span class="inline-formula">^∘</span>&thinsp;E with 6 and 12 stations, respectively, and a W–E section from 137 to 161<span class="inline-formula">^∘</span>&thinsp;E along the Equator with 13 stations) from November 2015 to January 2016. NO photoproduction rates from dissolved nitrite in artificial seawater showed increasing trends with decreasing pH, increasing temperature, and increasing salinity. In contrast, NO photoproduction rates (average: <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0.5</mn><mo>±</mo><mn mathvariant="normal">0.2</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">12</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="82pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8b0024d5cf7d62b1f85acc37b9872754"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-16-135-2020-ie00001.svg" width="82pt" height="14pt" src="os-16-135-2020-ie00001.png"/></svg:svg></span></span>&thinsp;mol&thinsp;L<span class="inline-formula">⁻¹</span>&thinsp;s<span class="inline-formula">⁻¹</span>) in the natural seawater samples from the WTNP did not show any correlations with pH, water temperature, salinity, or dissolved inorganic nitrite concentrations. The flux induced by NO photoproduction in the WTNP (average: <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">13</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">12</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="9ae0e0bfa9d6268e2fb2bd27ff4f362b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-16-135-2020-ie00002.svg" width="52pt" height="14pt" src="os-16-135-2020-ie00002.png"/></svg:svg></span></span>&thinsp;mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;s<span class="inline-formula">⁻¹</span>) was significantly larger than the NO air–sea flux density (average: <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1.8</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">12</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="55pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="938b59228ab2d319d2f6d76a068cb05e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-16-135-2020-ie00003.svg" width="55pt" height="14pt" src="os-16-135-2020-ie00003.png"/></svg:svg></span></span>&thinsp;mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;s<span class="inline-formula">⁻¹</span>), indicating a further NO loss process in the surface layer.</p>