We developed nitrate and nitrite actinometers to determine radiant fluxes from 290 to 410 nm. These actinometers are based on the reaction of the photochemically generated OH radical with benzoic acid to form salicylic acid (SA) and p‐hydroxybenzoic acid (pHBA). Actinom‐eter development included determination of the temperature and wavelength dependence of the quantum yield for formation of SA and pHBA from nitrate and nitrite photolysis in air‐saturated solutions. Quantum yields (at 25°C) for SA production from nitrate photolysis ranged from 0.00146 to 0.00418 between 290 and 350 nm, and from 0.00185 to 0.00633 for nitrite photolysis between 290 and 405 nm. The quantum yields for SA production were approximately 50–60% greater than quantum yields for pHBA production from nitrate and nitrite photolysis. For both actinometers, SA and pHBA formation was temperature dependent, increasing by approximately a factor of 2.2 from 0 to 35°C. Activation energies for SA formation varied with wavelength, ranging from 14.7 to 16.5 kj mol ‐1 between 290 and 330 nm for the nitrate actinometer and 12.3 to 17.8 kj mol‐1 between 310 and 390 nm for the nitrite actinometer. Activation energies for pHBA formation were 2–11% higher. Wavelength‐dependent changes in the quantum yield and activation energy for SA and pHBA formation from nitrate photolysis suggest multiple electronic transitions for nitrate from 290 to 350 nm. Quantum yields for OH radical formation from nitrate and nitrite photolyses were estimated from SA and pHBA quantum yields at 25°C. Wavelength‐dependent OH quantum yields ranged from 0.007 to 0.014 for nitrate photolysis between 290 and 330 nm and from 0.024 to 0.078 for nitrite photolysis between 298 and 390 nm. The nitrate and nitrite actinometers can maintain initial rate conditions for hours, are insensitive to laboratory lighting, easy to use and extremely sensitive; the minimum radiant energy that can be detected in our irradiation system is approximately 10‐9 einsteins.
Excellent agreement was also found between the nitrite actinometer and the OL-754, with a slope (95% CI) of 1.00 ؎ 0.01 using SA production and 1.00 ؎ 0.02 using pHBA production. These actinometers are well suited for use in the water column and are sufficiently sensitive to determine photon exposures below the 0.1% UV lightlevel.
We developed nitrate and nitrite actinometers to determine radiant fluxes from 290 to 410 nm. These actinometers are based on the reaction of the photochemically generated OH radical with benzoic acid to form salicylic acid (SA) and p-hydroxybenzoic acid (pHBA). Actinometer development included determination of the temperature and wavelength dependence of the quantum yield for formation of SA and pHBA from nitrate and nitrite photolysis in air-saturated solutions. Quantum yields (at 25°C) for SA production from nitrate photolysis ranged from 0.00146 to 0.00418 between 290 and 350 nm, and from 0.00185 to 0.00633 for nitrite photolysis between 290 and 405 nm. The quantum yields for SA production were approximately 50-60% greater than quantum yields for pHBA production from nitrate and nitrite photolysis. For both actinometers, SA and pHBA formation was temperature dependent, increasing by approximately a factor of 2.2 from 0 to 35°C. Activation energies for SA formation varied with wavelength, ranging from 14.7 to 16.5 kJ mol-' between 290 and 330 nm for the nitrate actinometer and 12.3 to 17.8 kJ mol-1 between 310 and 390 nm for the nitrite actinometer. Activation energies for pHBA formation were 211% higher. Wavelengthdependent changes in the quantum yield and activation energy for SA and pHBA formation from nitrate photolysis suggest multiple electronic transitions for nitrate from 290 to 350 nm. Quantum yields for OH radical formation from nitrate and nitrite photolyses were estimated from SA and pHBA quantum yields at 25°C. Wavelength-dependent OH quantum yields ranged from 0.007 to 0.014 for nitrate photolysis between 290 and 330 nm and from 0.024 to 0.078 for nitrite photolysis between 298 and 390 nm. The nitrate and nitrite actinometers can maintain initial rate conditions for hours, are insensitive to laboratory lighting, easy to use and extremely sensitive; the minimum radiant energy that can be detected in our irradiation system is approximately einsteins.(4) occurring in the far UV ( E , , , = 9500 M-l cm-I at 201 nm), while the second is a weak absorption band centered at 302 nm (emax = 7.14 M-
Nitrate and nitrite solar actinometers or chemical "light meters" were used to quantify light doses in photochemical and photobiological experiments involving dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycling. Light doses were calculated based on the photochemical production of salicylic acid (SA) from benzoic acid in these actinometers, with SA quantified by either spectrofluorometry or high performance liquid chromatography. Nitrate and nitrite actinometers were modified for deployment at low temperatures in Antarctic waters by addition of sodium chloride as a freezing point depressant. The addition of salt did not affect the solar response of the actinometers; however, the solar response did change slightly with latitude. In the Antarctic, peak response wavelengths (and bandwidths) for the Mylar D-wrapped actinometers in quartz tubing were 326 nm (319 -333 nm) and 353 nm (325 -380 nm) for nitrate and nitrite, respectively, and these were 2 -5 nm blue shifted compared to the peak response wavelengths and bandwidths observed in the Sargasso Sea. Excellent agreement was observed when comparing the integrated irradiance determined with the actinometers to that determined with a spectroradiometer. Likewise, diffuse attenuation coefficients for downwelling irradiance (K d (l)) calculated from water column actinometer measurements agreed well with K d (l) values calculated from irradiance measurements determined with a Biospherical PUV-511 profiling radiometer. Actinometers were used to measure light doses in experiments involving DMS and DMSP transformations during several field campaigns in the Ross Sea, Antarctica and the Sargasso Sea. Based on actinometer measurements, it was determined that DMS photolysis was dependent on UV irradiation between approximately 325 -380 nm, while biological consumption rates of DMS and DMSP were inhibited by radiation at wavelengths less than approximately 333 nm. When DMS photolysis rate constants were expressed in terms of light dose rather than time, it was possible to 1) directly determine photolysis rate constants in the water column and 2) directly compare photolysis rate constants across diverse oceanographic regions.
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