Variability of carbon monoxide and carbon dioxide apparent quantum yield spectra in three coastal estuaries of the South Atlantic Bight

Abstract

Abstract. The photochemical oxidation of oceanic dissolved organic carbon (DOC) to carbon monoxide (CO) and carbon dioxide (CO 2 ) has been estimated to be a significant process with global photoproduction transforming petagrams of DOC to inorganic carbon annually. To further quantify the importance of these two photoproducts in coastal DOC cycling, 38 paired apparent quantum yield (AQY) spectra for CO and CO 2 were determined at three locations along the coast of Georgia, USA over the course of one year. The AQY spectra for CO 2 were considerably more varied than CO. CO AQY spectra exhibited a seasonal shift in spectrally integrated (260 nm-490 nm) AQY from higher efficiencies in the autumn to less efficient photoproduction in the summer. While full-spectrum photoproduction rates for both products showed positive correlation with pre-irradiation UV-B sample absorption (i.e. chromophoric dissolved organic matter, CDOM) as expected, we found no correlation between AQY and CDOM for either product at any site. Molecular size, approximated with pre-irradiation spectral slope coefficients, and aromatic content, approximated by the specific ultraviolet absorption of the pre-irradiated samples, were also not correlated with AQY in either data set. The ratios of CO 2 to CO photoproduction determined using both an AQY model and direct production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0, respectively. Combined, both products represent a loss of 2.9 to 3.2 % of the DOC delivered to the estuaries and inner shelf of the South Atlantic Bight yearly, and 6.4 to 7.3 % of the total annual degassing of CO 2 to the atmosphere. This result suggests that direct photochemical production of CO and CO 2 is a small, yet significant contributor to both DOC cycling and CO 2 gas exchange in this coastal system.

The efficiency and spectral photon dose dependence of photochemically induced changes to the bioavailability of dissolved organic carbon

“…The AQY spectrum of a single photochemical product, such as CO or CO 2 , which undergoes no subsequent photochemical reactions once formed, can be usually be mathematically modeled using a decreasing exponential shape with increasing wavelength (Zafiriou et al 2003;Reader and Miller 2012). In the present study, the resulting labile ''photoproduct'' (BLP) is not a single molecule, but rather a whole host of compounds that may or may not undergo subsequent photochemical reactions.…”

“…The length and intensity of this irradiation provide a total photon dose equivalent to just over one day's worth of sunlight at 30uN latitude (same latitude as the study) during midsummer. This irradiation dose was chosen based on previous work in the same geographical region, which examined carbon monoxide photochemistry (Reader and Miller 2012) and established that this photon dose limited photochemical fading of the sample, while simultaneously producing measurable photochemical effects. The ultraviolet-visible absorbance (250-700 nm, 1 nm resolution) of the samples was measured directly in the sample cell before and after irradiation (Perkin Elmer Lambda 40).…”

“…Characteristics of the seasonal study samples. Dark respiration is the respiration in the dark control over the incubation period of 10 d. Spectral slope is the slope coefficient of the exponential fit to the CDOM absorption curve, and slope ratio is the ratio between the two slope values (for details on calculations and the motivation for choosing these wavelengths, see Helms et al 2008;Reader and Miller 2012). Specific ultraviolet absorption (SUVA 254 ) was calculated according to Weishaar et al (2003 was constant over the range of O 2 concentrations seen in all experiments and was accounted for in our calculations.…”

“…We attempted to determine AQYs using nlinfit, a nonlinear fitting routine in MATLABH as per the methods described in Johannessen and Miller (2001) and Reader and Miller (2012). The nlinfit routine is an iterative fitting routine that requires a defined equation a priori to fit the spectral data, as well as initial estimates for the fitting parameters of the equation.…”

“…Photochemical methods-Upon return to the laboratory, samples were filtered (0.2 mm, acid-washed Whatman Polycap AS75) directly into 15 quartz spectrophotometric cells (10 cm path length, ,30 mL; Spectrocell), sealed without headspace using Teflon-faced butyl rubber septa (Specialty Silicone Products, SSP), and placed in a chilled aluminum block (held at 15uC) for irradiation under the solar simulator (Suntest Atlas CPS; Ziolkowski and Miller 2007;Reader and Miller 2012). Duplicates of seven different Schott-glass cutoff filters (WG280, WG295, WG305, WG320, GG385, GG420, GG475) were placed over the cells in order to create 14 distinct light fields, and the 15 th cell was covered with an opaque disk to create a dark treatment.…”