Environ. Sci. Technol. 2006, 40, 7771-7777
Factors Affecting the Efficiency of Carbon Monoxide Photoproduction in the St. Lawrence Estuarine System (Canada) Y O N G Z H A N G , †,‡ H U I X I A N G X I E * ,‡ A N D GUOHUA CHEN† College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, China, 266003; Institut des sciences de la mer, Universite´ du Que´bec a` Rimouski, Que´bec, Canada G5L3A1
This study examined the effects of water temperature and the origin (terrestrial vs marine) and light history of chromophoric dissolved organic matter (CDOM) on the apparent quantum yields of carbon monoxide (CO) photoproduction for water samples collected along a salinity gradient (salinity range: 0-33) in the St. Lawrence estuarine system (Canada). The solar insolation-weighted mean apparent quantum yield of CO (Φ h CO) decreased as much as fourfold with increasing salinity and showed a strong positive correlation with the dissolved organic carbonspecific absorption coefficient at 254 nm. This suggests that terrestrial CDOM is more efficient at photochemically producing CO than is marine algae-derived CDOM and that aromatic moieties are likely involved in this photoprocess. CDOM photobleaching, mainly at the very early stage, dramatically decreased Φ h CO (by up to 6.4 times) for lowsalinity samples, but photobleaching had little effect on the most marine sample. For a 20 °C increase in temperature, Φ h CO increased by ∼70% for low-salinity samples and 3040% for saline samples. This study demonstrates that water temperature, as well as the CDOM’s origin and light history, strongly affect the efficiency of CO photoproduction. These factors should be taken into account in modeling the photochemical fluxes of CO and other related CDOM photoproducts on varying spatiotemporal scales.
Introduction Carbon monoxide (CO) in the surface ocean is primarily produced from the photolysis of chromophoric dissolved organic matter (CDOM) and is lost by microbial consumption and outgassing (1-3). The primary motivation of early studies of seawater CO arose from the observation that the ocean is a net source of atmospheric CO (4, 5), which regulates the oxidizing capacity of the atmosphere (6). Recently, interest in the marine CO cycle has expanded and diversified. The strong diel variation of the CO concentration in the surface ocean (1), imposed by the diurnal fluctuation of the solar insolation and modulated by microbial removal, outgassing, and vertical mixing, renders CO a suitable probe for upperocean mixing dynamics, photochemistry, optics, biology, and * Corresponding author phone: (418) 724-1767; fax: (418) 7241842; e-mail:
[email protected]. † Ocean University of China. ‡ Universite ´ du Que´bec. 10.1021/es0615268 CCC: $33.50 Published on Web 11/01/2006
2006 American Chemical Society
air-sea gas exchange (7). As the second most abundant inorganic carbon-containing product of CDOM photochemistry, CO is of significance to marine carbon cycling (8). CO is also considered a useful proxy for general CDOM photoreactivity (9) and for the difficult-to-measure photoproduction of dissolved inorganic carbon (DIC) (8, 10) and biolabile carbon (11), which together have been proposed to be one of the major terms in the ocean carbon cycle (12). Therefore, any significant advances or modifications in our knowledge of oceanic CO would affect our view on other major marine biogeochemical cycles. To quantitatively assess the role of CDOM photooxidation in the fate of organic carbon in the ocean (10, 13, 14), two approaches have been employed most frequently: in situ incubations (15) and optical-photochemical coupled modeling based on apparent quantum yield (AQY) (13, 16-18). The former determines water column photochemical fluxes by directly incubating water samples at varying depths in the photic zone; it requires laborious fieldwork, but is thought to closely simulate the natural photochemistry and the in situ light field. The latter calculates photochemical rates by combining experimentally determined AQY spectra with CDOM absorption coefficient spectra and underwater irradiance. As CDOM absorption coefficients can be retrieved from satellite ocean color measurements (19), the modeling approach appears promising for large-scale investigations (12, 17). The reliability of this approach depends, to a large extent, on the reliability of the AQY spectra used in the model. Potentially large uncertainties in published AQY spectra are partly associated with lack of quantitative knowledge of the influences of CDOM quality and environmental conditions on the related photoprocesses, including CO photoproduction. This study determined CO AQY (ΦCO) spectra on water samples from the estuary and Gulf of St. Lawrence (Canada) and evaluated the effects of water temperature as well as the CDOM’s origin (terrestrial vs marine) and light history on ΦCO. The implications of these influences for the mechanisms of CO photoproduction and for modeling the photochemical fluxes of CO and other related CDOM photoproducts are discussed.
Experimental Section Sampling. Sampling stations were dispersed along a salinity gradient from the upstream limit of the St. Lawrence estuary near Quebec City through the Gulf of St. Lawrence and to the open Atlantic off Cabot Strait. Thirteen stations were sampled for absorbance and DOC measurements and six for the AQY study (Figure 1). Water samples (2 m deep) were taken in late July 2004 for Stations 1-12 and in mid-June 2005 for Station 13 using 12-L Niskin bottles attached to a CTD rosette. Samples were gravity-filtered upon collection through Pall AcroPak 1000 capsules sequentially containing 0.8 µm and 0.2 µm polyethersulfone membrane filters. The filtered water was transferred in darkness into acid-cleaned, 4 L clear glass bottles, stored in darkness at 4 °C, and brought back to the laboratory at Rimouski. Samples were re-filtered with 0.22 µm polycarbonate membranes (Millipore) immediately prior to irradiations, which were carried out within 2 months of sample collection. Photobleaching. In order to evaluate the effect of the CDOM’s light history on ΦCO (i.e., dose dependence), filtered samples, placed in a clear glass container covered with a quartz plate, kept at 15 °C and continuously stirred, were irradiated with a SUNTEST XLS+ solar simulator equipped with a 1.5 kW xenon lamp. Radiations emitted from the xenon lamp were screened by a Suprax long band-pass cutoff filter VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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by CDOM at wavelength λ. A Matlab-coded iterative curvefit method (20, 23) was employed to derive ΦCO(λ). Briefly, this method assumes an appropriate mathematical form with unknown parameters to express the change in ΦCO as a function of wavelength. Decreasing exponential functions are usually chosen for AQY spectra of CDOM photoprocesses (20, 23, 24). The amount of CO produced in an irradiation cell over the exposure time can then be predicted as the product of the assumed ΦCO(λ) function and the number of photons absorbed by CDOM integrated over the 250-600 nm wavelength range, assuming no CO photoproduction above 600 nm. We followed Hu et al.’s (25) recommendations to calculate the number of photons absorbed by CDOM. The optimum values of the unknown parameters in the assumed ΦCO(λ) function are obtained by varying these parameters from initial estimates until the minimum difference between the measured and predicted CO production is achieved. The following quasi-exponential form was adopted to fit the data: FIGURE 1. Sampling locations in the St. Lawrence estuarine system and in the Atlantic Ocean off Cabot Strait. Triangles represent stations that were sampled for the CO quantum yield study. All stations were sampled for CDOM absorbance and DOC measurements. to minimize radiations
