Transcript AGU2010_poster2.pptx

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
[email protected]
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.
WS/WD
Tower Measurement Setup
aspirated T/RH
Tower
N
w
20-m gradient
60 m
Sonic
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
sycamore
b
study area
isoprene/
benzene
isoprene/
toluene
mg km-1
reference
comments
Sweden, 1995
~ 0.03
~ 0.01
NA
Björkqvist et al., 1997
ambient air
California, 1994-1997
0.0 – 0.01
0.0 – 0.006
NA
Kirchstetter et al., 1999
tunnel air
Austria, 1997
0.2 – 0.3
0.1 – 0.2
NA
Holzinger et al., 1999
ambient air
France, 1997-1999
0.15 ± 5% a
0.01 ± 5% b
0.29±0.12 c Borbon et al., 2001
ambient air
Switzerland, 1997
0.23 ± 10% a
NA
Reimann et al., 2000
ambient air
Germany, 1997-2003
0.1 – 0.2
0.05 – 0.1
NA
Niedojadlo et al., 2007
ambient air, tunnels
Seoul, S. Korea
~ 0.27
~ 0.12
NA
K. Na, 2008
tunnel air
Barcelona, Spain
0.25 – 0.3
0.05 – 0.1
NA
Filella & Peñuelas, 2006 ambient air
France, 2002-2003
0.06 ± ?%
0.02 ± ?%
NA
Badol et al., 2008ab
calculated EFs
Houston, TX, 2008 d
~ 0.9
~ 0.7
NA
this work
ambient air
Houston, TX, 2008 d
0.6 ± 0.2 (0.3) 0.4 ± 0.2 (0.1) NA
this work
fluxes / emissions
Europe, 2008-2010
0.11 ± 0.03
0.08 ± 0.03
0.05 – 0.2 c Montero et al., 2010
laboratory
a
determined from estimated butadiene to benzene mass emission ratio (approx. 1:2)
b determined from acetylene to toluene mass emission ratio (approx. 1:2)
c catalyst-equipped cars only
d isoprene data likely includes 2-pentenes; concentration and flux ratio from simultaneous increase during morning
rush-hours or rush-hour correlation, respectively ; nighttime ratio in parentheses
oaks
Table 2: Biogenic isoprene emission model parameters and inputs.
model parameter
input data
tree cover (% of surface area)
20 – 40 %, average 30%; value adjusted using footprint overlays
isoprene emitter contribution 15 – 25 %, average 20%
to tree leaf biomass (%)
LAI, leaf angle distribution
5 ± 1 m2 m-2, spherical
old pavement
emission algorithms
updated G93 model (Figure 4): above canopy PAR, split into direct plus diffuse;
T equal to air temperature at 13 m agl.; specific leaf area of 120 ± 20 cm2 g-1
PC
49 m
DL
40 m
Hardy (south bound)
Elysian (north bound)
Figure 3: (a) A typical residential neighborhood view south of the Hays Street tower. The isoprene emitting trees include water oak (Quercus nigra), post oak
(Quercus stellata), live oak (Quercus virginiana), and sycamore (Platanus occidentalis). (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.
PAR pyranometer
net radiation
Table 1: Estimated isoprene (here: likely including 2-pentenes) emissions from cars in relative and absolute terms.
a
Wind data (10 Hz)
b
3/8’’ and 1/4“
OD PFA Tubes
20 m
13 m
Base
Building
Relaxed Eddy
Accumulation
EC
gradient
Lag time ≈ 9 s
GCFID
CO2 /
H2O
slow: CO, NOx, O3
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 GCFID 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 measured T and PAR and the functional relationship, respectively. Only data outside the main traffic hours was used.
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.