Evidence for significant C-5 alkene emissions from car traffic

1. WS/WD aspirated T/RH Evidence for significant C-5 alkene emissions from car traffic Gunnar W. Schade, and Changhyoun Park Atmospheric Sciences, Texas A&M University, 3150 TAMU, College Station gws@geos.tamu.edu Tower PAR pyranometer N net radiation Sonic Summary We present evidence from urban flux tower measurements in Houston, Texas, (Figures 1&2) that a C5-alkenes, including isoprene, are emitted from car traffic in larger amounts than expected. Our GC-dual FID instrument setup measures VOC concentrations at 60 m above ground level and determines fluxes via a novel relaxed eddy accumulation technique. C-5 2-alkenes and isoprene, 2-methyl-1,3-butadiene, are not chromatographically separated, but past VOC measurements suggest that isoprene, a biogenic hydrocarbon, generally dominates during the growing season (Figure 3). Our measured summertime C-5 alkene fluxes in 2008 generally followed the expected, light and temperature driven emission pattern of isoprene from a significant density of oak trees in the tower’s footprint area (Figure 4). However, nighttime fluxes were significantly different from an expected zero biogenic flux, and daytime, particularly morning rush hour fluxes were significantly higher than modeled biogenic fluxes (Table 2) using literature data for basal emission and an onsite and GIS biomass survey as inputs. Wintertime measurements in January 2009 confirmed a small ‘isoprene’ flux, probably either isoprene, C-5 2-alkene, or mixed emissions from car exhaust. Isoprene emissions from car traffic have been described several times before (Table 1), but emission rates have generally been considered unimportant as compared to biogenic emissions. A quantitative comparison of our data to simultaneously measured toluene and benzene emissions however suggests that these C-5 alkene emissions may have increased relative to aromatics since the 1990s. This possible importance of traffic emissions is supported by recent direct car exhaust measurements in Europe and Japan, and elevated airborne isoprene measurements over Houston. Car exhaust measurements show that (i) the pentenes to benzene emission ratio for the newest car models is between 1:4 and 1:3, somewhat lower than the ratio obtained from our data, and (ii) cold start alkene emissions can be an order of magnitude higher than ‘regular’ emissions. Assuming our isoprene peak includes cis/trans-2-pentenes, traffic emissions at this site approximately double biogenic isoprene emissions (Figure 5), and, due to high reactivity, therefore ought to be included in emissions inventories. Table 1: Estimated isoprene (here: likely including 2-pentenes) emissions from cars in relative and absolute terms. Hardy (south bound) Elysian (north bound) Quitman Road (east/west bound) Figure 2: Land cover, summertime wind rose, and commuter axes (light blue & red) with mean weekday (black) and weekend (gray) (both Jan ‘08) vehicle density. Note that most data are from SE to S wind directions with footprints overlying a tree-rich residential area and the commuter axes. Rush hours are unusually long, possibly affected by local school traffic, and midday and evening vehicle counts hardly differ on the weekend. a b sycamore oaks Table 2: Biogenic isoprene emission model parameters and inputs. old pavement Figure 3: (a) A typical residential neighborhood view south of the Hays Street tower. The isoprene emitting trees include water oak (Quercusnigra), post oak (Quercusstellata), live oak (Quercusvirginiana), and sycamore (Platanusoccidentalis). (b) The diurnal cycle of ambient isoprene mixing ratios ( = all data,  = weekdays,  = weekends) was similar to findings at other locations with traffic influences (Qin et al., 2007, Reimann et al., 2000), showing a morning maximum right after the rush-hour traffic density maximum and non-diminishing abundances at night. Tower Measurement Setup w 60 m 49 m 40 m 20 m 13 m a b PC 20-m gradient DL Wind data (10 Hz) 3/8’’ and 1/4“ OD PFA Tubes Relaxed Eddy Accumulation Base Building GC- FID EC CO2 / H2O gradient Lag time ≈ 9 s slow: CO, NOx, O3 Figure 5: Box plot of measured isoprene emissions (bisque, likely including 2-pentenes) and modeled biogenic emissions (green) (a). The residuals (b), as compared to Figure 2 (left), suggest that traffic is a major anthropogenic contributor to the ‘excess’ flux. However, we also suspect that biogenic emissions could be underestimated, because diffuse radiation is likely higher in this urban area. In (b) we included a model (mean in red, min/max in orange lines) that uses an emission factor of 0.05 to 0.2 mg km-1 traffic counts, times the car counts per hour, times median road impact on footprint (%), times car travel through footprint (~1 km car-1), and divided by individual plume size at canopy height (~100 m2) to calculate emissions. It scales surprisingly well with the residuals, suggesting that current emission factors maybe correct within a factor of two. Figure 1: Our site’s strength is an extensive setup incorporating both criteria pollutant and carbon flux measurements. The former are assessed via a flux gradient method, the latter via eddy covariance and REA measurements. The REA GC- FID setup and its results are described by Park et al., Atmos. Environ. 44, 2010. Figure 4: Measured isoprene emissions displayed a PAR (a) and air temperature (b) response that was matching the expected responses relatively well. The open squares with standard error bars depict the data; open circles and dashed lines depict model output using the G93 algorithms using actually measuredT and PAR and the functional relationship, respectively. Only data outside the main traffic hours was used.