Controls on gas transfer velocities in a large river

Abstract

[1] The emission of biogenic gases from large rivers can be an important component of regional greenhouse gas budgets. However, emission rate estimates are often poorly constrained due to uncertainties in the air-water gas exchange rate. We used the floating chamber method to estimate the gas transfer velocity (k) of carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) in the Markland Pool of the Ohio River, a large tributary of the Mississippi River (U.S.A). We measured k every two weeks for a year at one site and at 15 additional sites distributed across the length of the pool during two summer surveys. We found that k was positively related to both water currents and wind speeds, with 46% of the gas transfer attributable to water currents at low wind speeds (e.g., 0.5 m s À1 ) and 11% at higher wind speeds (e.g., >2.0 m s À1 ). Gas transfer velocity was highly sensitive to wind, possibly because the direction of river flow was often directly opposed to the wind direction. Gas transfer velocity values derived for CH 4 were consistently greater than those derived for CO 2 when standardized to a Schmidt number of 600 (k 600 ), possibly because the transfer of CH 4 , a poorly soluble gas, was enhanced by surfacing microbubbles. Additional research to determine the conditions that support microbubble enhanced gas transfer is merited.

Carbon Dioxide Emissions along the Lower Amazon River

“…The factors controlling k in large river basins is a mixture of all those described above. Wind are generally higher than those in headwater streams and current velocities are typically faster than in estuaries (Beaulieu et al, 2012). Additionally, the effect of wind on the water turbulence can be related to the water body orientation, shape, size, and direction of wind and water current.…”

“…Additionally, the effect of wind on the water turbulence can be related to the water body orientation, shape, size, and direction of wind and water current. When wind and water currents are directionally opposed they can interact synergistically to produce unusually high k-values for any given wind speed (Zappa et al, 2007;Beaulieu et al, 2012). Prevailing winds in the lower Amazon are from the NE, which is the opposite direction of river outflow from Almeirim and Macapá (during an outgoing tide when net discharge is positive).…”

Dynamics of groundwater‐derived nitrate and nitrous oxide in a tidal estuary from radon mass balance modeling

“…Although there are many different equations used to estimate k values, the approach by Kremer et al (2003) was considered the most appropriate in this study based on the similar dimension of the studied estuaries (shallow coastal systems with little fetch). A linear relationship between the slope of the regression of k 600 vs. u 10 and the surface area of disparate waterbodies (Beaulieu et al 2012) shows that changes of k 600 were strongly affected by the surface area that represents the fetch of the estuary. From this relationship, the slope was predicted to be , 1.0 cm h 21 m 21 s for a surface area of , 0.5 km 2 , and this aligns with the estimated slope (0.7 cm h 21 m 21 s); this justifies the use of the approach.…”

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order

“…However, recent studies (17)(18)(19)(20) suggesting riverine N 2 O loss is underestimated by up to threefold notably contradict the EF 5r reduction. Uncertainty in the EF 5r can be attributed to a scarcity of studies (21,22), poorly constrained water-air gaseous exchange relationships (23,24), and high variability in river morphology (25,26). Further, the EF 5r assumes a linear relation between nitrate in water and N 2 O emissions (14), the validity of which is the subject of considerable debate (27)(28)(29)(30).…”