6 research outputs found
Heterogeneous OH Initiated Oxidation: A Possible Explanation for the Persistence of Organophosphate Flame Retardants in Air
Heterogeneous reactions between OH
radicals and emerging flame
retardant compounds coated on inert particles have been investigated.
Organophosphate esters (OPEs) including triphenyl phosphate (TPhP),
tris-2-ethylhexyl phosphate (TEHP), and tris-1,3-dichloro-2-propyl
phosphate (TDCPP) were coated on (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> particles and exposed to OH radicals in a photochemical flow
tube at 298 K and (38.0 ± 2.0) % RH. The degradation of these
particle-bound OPEs was observed as a result of OH exposure, as measured
using a Time-of-Flight Aerosol Mass Spectrometer. The derived second-order
rate constants for the heterogeneous loss of TPhP, TEHP, and TDCPP
were (2.1 ± 0.19) × 10<sup>–12</sup>, (2.7 ±
0.63) × 10<sup>–12</sup>, and (9.2 ± 0.92) ×
10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, respectively, from which approximate atmospheric
lifetimes are estimated to be 5.6 (5.2–6.0), 4.3 (3.5–5.6),
and 13 (11–14) days. Additional coating of the OPE coated particles
with an OH radical active species further increased the lifetimes
of these OPEs. These results represent the first reported estimates
of heterogeneous reaction rate constants for these species. The results
demonstrate that particle bound OPEs are highly persistent in the
atmosphere with regard to OH radical oxidation, consistent with the
assumption that OPEs can undergo medium or long-range transport, as
previously proposed on the basis of field measurements. Finally, these
results indicate that future risk assessment and transport modeling
of emerging priority chemicals with semi- to low-volatility must consider
particle phase heterogeneous loss processes when evaluating environmental
persistence
Filterable Redox Cycling Activity: A Comparison between Diesel Exhaust Particles and Secondary Organic Aerosol Constituents
The
redox activity of diesel exhaust particles (DEP) collected
from a light-duty diesel passenger car engine was examined using the
dithiothreitol (DTT) assay. DEP was highly redox-active, causing DTT
to decay at a rate of 23–61 pmol min<sup>–1</sup> μg<sup>–1</sup> of particle used in the assay, which was an order
of magnitude higher than ambient coarse and fine particulate matter
(PM) collected from downtown Toronto. Only 2–11% of the redox
activity was in the water-soluble portion, while the remainder occurred
at the black carbon surface. This is in contrast to redox-active secondary
organic aerosol constituents, in which upward of 90% of the activity
occurs in the water-soluble fraction. The redox activity of DEP is
not extractable by moderately polar (methanol) and nonpolar (dichloromethane)
organic solvents, and is hypothesized to arise from redox-active moieties
contiguous with the black carbon portion of the particles. These measurements
illustrate that “Filterable Redox Cycling Activity”
may therefore be useful to distinguish black carbon-based oxidative
capacity from water-soluble organic-based activity. The difference
in chemical environment leading to redox activity highlights the need
to further examine the relationship between activity in the DTT assay
and toxicology measurements across particles of different origins
and composition
Formation of Light Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal and Ammonium Sulfate
In the atmosphere, volatile organic
compounds such as glyoxal can
partition into aqueous droplets containing significant levels of inorganic
salts. Upon droplet evaporation, both the organics and inorganic ions
become highly concentrated, accelerating reactions between them. To
demonstrate this process, we investigated the formation of organo-nitrogen
and light absorbing materials in evaporating droplets containing glyoxal
and different ammonium salts including (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub>, and NH<sub>4</sub>Cl.
Our results demonstrate that evaporating glyoxal-(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> droplets produce light absorbing species on
a time scale of seconds, which is orders of magnitude faster than
observed in bulk solutions. Using aerosol mass spectrometry, we show
that particle-phase organics with high N:C ratios were formed when
ammonium salts were used, and that the presence of sulfate ions promoted
this chemistry. Since sulfate can also significantly enhance the Henry’s
law partitioning of glyoxal, our results highlight the atmospheric
importance of such inorganic–organic interactions in aqueous
phase aerosol chemistry
Are Emissions of Black Carbon from Gasoline Vehicles Underestimated? Insights from Near and On-Road Measurements
Measurements of black carbon (BC) with a high-sensitivity
laser-induced
incandescence (HS-LII) instrument and a single particle soot photometer
(SP2) were conducted upwind, downwind, and while driving on a highway
dominated by gasoline vehicles. The results are used with concurrent
CO<sub>2</sub> measurements to derive fuel-based BC emission factors
for real-world average fleet and heavy-duty diesel vehicles separately.
The derived emission factors from both instruments are compared, and
a low SP2 bias (relative to the HS-LII) is found to be caused by a
BC mass mode diameter less than 75 nm, that is most prominent with
the gasoline fleet but is not present in the heavy-duty diesel vehicle
exhaust on the highway. Results from both the LII and the SP2 demonstrate
that the BC emission factors from gasoline vehicles are at least a
factor of 2 higher than previous North American measurements, and
a factor of 9 higher than currently used emission inventories in Canada,
derived with the MOBILE 6.2C model. Conversely, the measured BC emission
factor for heavy-duty diesel vehicles is in reasonable agreement with
previous measurements. The results suggest that greater attention
must be paid to black carbon from gasoline engines to obtain a full
understanding of the impact of black carbon on air quality and climate
and to devise appropriate mitigation strategies
Measurements of Gas phase Acids in Diesel Exhaust: A Relevant Source of HNCO?
Gas-phase
acids in light duty diesel (LDD) vehicle exhaust were
measured using chemical ionization mass spectrometry (CIMS). Fuel
based emission factors (EF) and NO<sub><i>x</i></sub> ratios
for these species were determined under differing steady state engine
operating conditions. The derived HONO and HNO<sub>3</sub> EFs agree
well with literature values, with HONO being the single most important
acidic emission. Of particular importance is the quantification of
the EF for the toxic species, isocyanic acid (HNCO). The emission
factors for HNCO ranged from 0.69 to 3.96 mg kg<sub>fuel</sub><sup>–1</sup>, and were significantly higher than previous biomass
burning emission estimates. Further ambient urban measurements of
HNCO demonstrated a clear relationship with the known traffic markers
of benzene and toluene, demonstrating for the first time that urban
commuter traffic is a source of HNCO. Estimates based upon the HNCO-benzene
relationship indicate that upward of 23 tonnes of HNCO are released
annually from commuter traffic in the Greater Toronto Area, far exceeding
the amount possible from LDD alone. Nationally, 250 to 770 tonnes
of HNCO may be emitted annually from on-road vehicles, likely representing
the dominant source of exposure in urban areas, and with emissions
comparable to that of biomass burning
Enhanced Light Scattering of Secondary Organic Aerosols by Multiphase Reactions
Secondary
organic aerosol (SOA) plays a pivotal role in visibility
and radiative forcing, both of which are intrinsically linked to the
refractive index (RI). While previous studies have focused on the
RI of SOA from traditional formation processes, the effect of multiphase
reactions on the RI has not been considered. Here, we investigate
the effects of multiphase processes on the RI and light-extinction
of <i>m</i>-xylene-derived SOA, a common type of anthropogenic
SOA. We find that multiphase reactions in the presence of liquid water
lead to the formation of oligomers from intermediate products such
as glyoxal and methylglyoxal, resulting in a large enhancement in
the RI and light-scattering of this SOA. These reactions will result
in increases in light-scattering efficiency and direct radiative forcing
of approximately 20%–90%. These findings improve our understanding
of SOA optical properties and have significant implications for evaluating
the impacts of SOA on the rapid formation of regional haze, global
radiative balance, and climate change