Flux Measurements of Volatile Organic Compounds by Relaxed Eddy Accumulation method combined with GC-FID system in north of downtown Houston, Texas

A tall tower flux measurement setup was established in metropolitan Houston, Texas, to measure energy and trace gas fluxes from both anthropogenic and biogenic emission sources in the urban surface layer. We describe a new relaxed eddy accumulation (REA) system combined with a dual-channel GC-FID used for VOC flux measurements, focusing on benzene, toluene, ethylbenzene and xylene (BTEX) results. Ambient air sampled from 60 m above the ground next to a sonic anemometer is subsampled by a membrane pump and pushed into an REA valve system with two Teflon bag reservoirs, then transferred to the GC preconcentration units for thermal desorption. We discuss the performance of our system and the selected BTEX measurement results using approximately 7 weeks of data (June 1 – July 22, 2008), presenting diurnal variations of concentrations and fluxes of these traffic tracers. The measured values exhibited diurnal cycles with a dominant morning peak during weekdays related to rush-hour traffic and additional workday daytime flux maxima for toluene and xylenes. A two-way comparison with EPA National Emission Inventories (NEI) suggests potential underestimates in the NEI.