Anthropogenic and Biogenic Carbon Dioxide Fluxes From Typical Land Uses in Houston, Texas

Abstract

A flux measurement setup was established at a communications tower north of downtown Houston, Texas, to measure energy and trace gas fluxes from a variety of emission sources in the urban surface layer. The first part of this study details the development of a correction for latent heat and carbon dioxide (CO2) fluxes due to a low-pass filtering of the true water vapor and CO2 atmospheric signals. A method of spectral analysis was used to develop a correction scheme for this flux underestimation through the observation that the low-pass filtering is a strong function of atmospheric relative humidity and wind speeds. The determined flux corrections for latent heat fluxes (average correction of 34%) were significantly larger than CO2 fluxes (3-4%), suggesting the low-pass filtering had a more dominant effect on the water vapor signal.

For the second part of this study, we describe a quadrant analysis technique for separating measured net fluxes into their biogenic (c’ < 0 and q’ > 0 signals) and anthropogenic (c’ > 0 and T’ > 0 signals) components, and quantify these fluxes through the use of a refined relaxed eddy accumulation model. A method of minimizing a defined net – (biogenic + anthropogenic) residual was used to determine an accurate “hole” around the origin to exclude points in the desired quadrants that appear there randomly.

The magnitude of the biogenic flux contribution showed expected relationships with temperature and irradiance. The anthropogenic flux contribution showed a strong correlation with measured traffic counts collected on local thoroughfares. Due to a presumed small bias in the flux calculation methodology, neither flux contribution truly measured zero, so anthropogenic and biogenic “background” fluxes were calculated (0.01 µmol mo^-1 m s^-1 and 0.01 – 0.02 µmol mol^-1 m s^-1 respectively).

Future work involves perfecting the model, with the goal being widespread use of the theory at urban flux sites. The accurate separation of the net flux gives a more complete picture of the carbon cycle and allows for more accurate analysis of the effects of increased temperature and CO2 concentration on urban vegetation as compared to non-urban vegetation.