Quantification of green roof carbon dioxide, heat, and water fluxes using the gradient flux technique

Green roofs address several important problems associated with urbanization, but there has been limited quantification of their benefits. On the small scales necessary on a rooftop, direct eddy covariance cannot be used to measure the carbon dioxide, sensible and latent heat, and water fluxes between the plant canopy and the atmosphere. Thus, the gradient flux technique was used to calculate these fluxes between a green roof and the atmosphere over 18 days in summer, 2008. Measurements of atmospheric CO2 and H2O concentration and meteorological variables were taken in the atmospheric surface boundary layer within 35 cm of the green roof plant canopy:

CO2 and H2O concentrations were sampled with one sensor at five different heights for five minutes each over the course of thirty minute intervals to construct a vertical profile, as well as a being sampled continuously with another sensor at the median height to quantify time variability; temperature and relative humidity were sampled at the highest, lowest, and median heights; wind velocity was recorded at two heights to measure shear velocity, a parameter describing atmospheric mixing; canopy temperature and moisture, and solar radiation were also measured.

Results show the plant canopy was a net sink of carbon dioxide during the day and a net source of carbon dioxide during the night, as expected. Average carbon dioxide flux was approximately 0.135 μ mol m-2 s-1 into the canopy over the experiment, or about 51.1 g C m-2 year-1 into the canopy if seasonality is neglected. Long-term studies coupling this method with detailed mass and heat budgets will allow more complete quantification of carbon dioxide, water and heat fluxes on green roofs. This will aid in improving quantification of both known benefits of green roofs, as well as more speculative benefits such as carbon sequestration.