Ambient measurements and source apportionment of fossil fuel and biomass burning black carbon in Ontario

R.M. Healy, U. Sofowote, Y. Su, J. Debosz, M. Noble, C.-H. Jeong, J.M. Wang, N. Hilker, G.J. Evans, G. Doerksen, K. Jones, A. Munoz
Atmospheric Environment, Volume 161, 34-47, 2017

Black carbon (BC) is of significant interest from a human exposure perspective but also due to its impacts as a short-lived climate pollutant. In this study, sources of BC influencing air quality in Ontario, Canada were investigated using nine concurrent Aethalometer datasets collected between June 2015 and May 2016. The sampling sites represent a mix of background and near-road locations. An optical model was used to estimate the relative contributions of fossil fuel combustion and biomass burning to ambient concentrations of BC at every site. The highest annual mean BC concentration was observed at a Toronto highway site, where vehicular traffic was found to be the dominant source. Fossil fuel combustion was the dominant contributor to ambient BC at all sites in every season, while the highest seasonal biomass burning mass contribution (35%) was observed in the winter at a background site with minimal traffic contributions. The mass absorption cross-section of BC was also investigated at two sites, where concurrent thermal/optical elemental carbon data were available, and was found to be similar at both locations. These results are expected to be useful for comparing the optical properties of BC at other near-road environments globally. A strong seasonal dependence was observed for fossil fuel BC at every Ontario site, with mean summer mass concentrations higher than their respective mean winter mass concentrations by up to a factor of two. An increased influence from transboundary fossil fuel BC emissions originating in Michigan, Ohio, Pennsylvania and New York was identified for the summer months. The findings reported here indicate that BC should not be considered as an exclusively local pollutant in future air quality policy decisions. The highest seasonal difference was observed at the highway site, however, suggesting that changes in fuel composition may also play an important role in the seasonality of BC mass concentrations in the near-road environment. This finding has implications for future policies aiming to improve air quality in urban environments where fuel composition changes as a function of season.

Real-World Emission of Particles from Vehicles: Volatility and the Effects of Ambient Temperature

J. M. Wang, C.-H. Jeong, N. Zimmerman, R. M. Healy, N. Hilker, and G. J. Evans.
Environmental Science & Technology, Volume 51 (7), 4081–4090, 2017

A majority of the ultrafine particles observed in real-world conditions are systematically excluded from many measurements that help to guide regulation of vehicle emissions. To investigate the impact of this exclusion, coincident near-road particle number (PN) emission factors were quantified up- and downstream of a thermodenuder during two seasonal month-long campaigns with wide-ranging ambient temperatures (−19 to +30 °C) to determine the volatile fraction of particles. During colder temperatures (<0 °C), the volatile fraction of particles was 94%, but decreased to 85% during warmer periods (>20 °C). Additionally, mean PN emission factors were a factor of 3.8 higher during cold compared to warm periods. On the basis of 130 000 vehicle plumes including three additional campaigns, fleet mean emission factors were calculated for PN (8.5 × 1014 kg-fuel–1), black carbon (37 mg kg-fuel–1), organic aerosol (51 mg kg-fuel–1), and particle-bound polycyclic aromatic hydrocarbons (0.7 mg kg-fuel–1). These findings demonstrate that significant differences exist between particles in thermally treated vehicle exhaust as compared to in real-world vehicle plumes to which populations in near-road environments are actually exposed. Furthermore, the magnitude of these differences are dependent upon season and may be more extreme in colder climates.

Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics

D. S. Josey, S. R. Nyikos, R. K. Garner, A. Dovijarski, J. S. Castrucci, J. M. Wang, G. J. Evans, and T. P. Bender
ACS Energy Letters, Volume 2, 726-732, 2017

The outdoor lifetime and performance of organic photovoltaics (OPVs) using boron subphthalocyanine (BsubPc) derivatives as electron-accepting materials is presented. The protocols followed are based on the most advanced level of outdoor testing established by the International Summit on OPV Stability (ISOS). The stability of each BsubPc is compared using three different sets of encapsulated planar heterojunction OPVs, with each set containing a different BsubPc as the electron-accepting layer. The performance and stability of each set is tested outdoors using an epoxy glue and a glass coverslip as protection from the ambient environment. Outdoor testing continued until the OPVs reached 80 or 50% of their original power conversion efficiency, as determined by frequent indoor characterization. OPVs utilizing chloro-BsubPc are shown to exhibit the highest stability and performance, while the stability of the other two BsubPc derivatives is reduced presumably as a result of their phenoxy or phenyl functionalization in the molecular axial positions. The established structure–property relationship and guidance for the design of future compounds for application in planar heterojunction OPVs are contrary to, and could not have been anticipated from, time zero laboratory testing.

Assessing the Climate Trade-Offs of Gasoline Direct Injection Engines

N. Zimmerman, J. M. Wang, C.-H. Jeong, J. S. Wallace, and G. J. Evans
Environmental Science & Technology, Volume 50 (15), 8385-8392, 2016

Compared to port fuel injection (PFI) engine exhaust, gasoline direct injection (GDI) engine exhaust has higher emissions of black carbon (BC), a climate-warming pollutant. However, the relative increase in BC emissions and climate trade-offs of replacing PFI vehicles with more fuel efficient GDI vehicles remain uncertain. In this study, BC emissions from GDI and PFI vehicles were compiled and BC emissions scenarios were developed to evaluate the climate impact of GDI vehicles using global warming potential (GWP) and global temperature potential (GTP) metrics. From a 20 year time horizon GWP analysis, average fuel economy improvements ranging from 0.14 to 14% with GDI vehicles are required to offset BC-induced warming. For all but the lowest BC scenario, installing a gasoline particulate filter with an 80% BC removal efficiency and <1% fuel penalty is climate beneficial. From the GTP-based analysis, it was also determined that GDI vehicles are climate beneficial within <1–20 years; longer time horizons were associated with higher BC scenarios. The GDI BC emissions spanned 2 orders of magnitude and varied by ambient temperature, engine operation, and fuel composition. More work is needed to understand BC formation mechanisms in GDI engines to ensure that the climate impacts of this engine technology are minimal.

Single-particle characterization of biomass burning organic aerosol (BBOA): evidence for non-uniform mixing of high molecular weight organics and potassium.

A. K. Y. Lee, M. D. Willis, R. M. Healy, J. M. Wang, C.-H. Jeong, J. C. Wenger, G. J. Evans, and J. P. D. Abbatt
Atmospheric Chemistry and Physics, Volume 16, 5561-5572, 2016

Biomass burning organic aerosol (BBOA) can be emitted from natural forest fires and human activities such as agricultural burning and domestic energy generation. BBOA is strongly associated with atmospheric brown carbon (BrC) that absorbs near-ultraviolet and visible light, resulting in significant impacts on regional visibility degradation and radiative forcing. The mixing state of BBOA can play a critical role in the prediction of aerosol optical properties. In this work, single-particle measurements from a Soot-Particle Aerosol Mass Spectrometer coupled with a light scattering module (LS-SP-AMS) were performed to examine the mixing state of BBOA, refractory black carbon (rBC), and potassium (K, a tracer for biomass burning aerosol) in an air mass influenced by wildfire emissions transported from northern Québec to Toronto, representing aged biomass burning plumes. Cluster analysis of single-particle measurements identified five BBOA-related particle types. rBC accounted for 3–14 wt % of these particle types on average. Only one particle type exhibited a strong ion signal for K+, with mass spectra characterized by low molecular weight organic species. The remaining four particle types were classified based on the apparent molecular weight of the BBOA constituents. Two particle types were associated with low potassium content and significant amounts of high molecular weight (HMW) organic compounds. Our observations indicate non-uniform mixing of particles within a biomass burning plume in terms of molecular weight and illustrate that HMW BBOA can be a key contributor to low-volatility BrC observed in BBOA particles. The average mass absorption efficiency of low-volatility BBOA is about 0.8–1.1 m2 g−1 based on a theoretical closure calculation. Our estimates indicate that low-volatility BBOA contributes ∼ 33–44 % of thermo-processed particle absorption at 405 nm; and almost all of the BBOA absorption was associated with low-volatility organics.

Quantification of black carbon mixing state from traffic: implications for aerosol optical properties

Megan D. Willis, Robert M. Healy, Nicole Riemer, Matthew West, Jon M. Wang, Cheol-Heon Jeong, John C. Wenger, Greg J. Evans, Jonathan P. D. Abbatt, and Alex K. Y. Lee
Atmospheric Chemistry and Physics, Volume 16, 4693-4706, 2016

The climatic impacts of black carbon (BC) aerosol, an important absorber of solar radiation in the atmosphere, remain poorly constrained and are intimately related to its particle-scale physical and chemical properties. Using particle-resolved modelling informed by quantitative measurements from a soot-particle aerosol mass spectrometer, we confirm that the mixing state (the distribution of co-emitted aerosol amongst fresh BC-containing particles) at the time of emission significantly affects BC-aerosol optical properties even after a day of atmospheric processing. Both single particle and ensemble aerosol mass spectrometry observations indicate that BC near the point of emission co-exists with hydrocarbon-like organic aerosol (HOA) in two distinct particle types: HOA-rich and BC-rich particles. The average mass fraction of black carbon in HOA-rich and BC-rich particle classes was  < 0.1 and 0.8, respectively. Notably, approximately 90 % of BC mass resides in BC-rich particles. This new measurement capability provides quantitative insight into the physical and chemical nature of BC-containing particles and is used to drive a particle-resolved aerosol box model. Significant differences in calculated single scattering albedo (an increase of 0.1) arise from accurate treatment of initial particle mixing state as compared to the assumption of uniform aerosol composition at the point of BC injection into the atmosphere.

Field measurements of gasoline direct injection emission factors: spatial and seasonal variability

Naomi Zimmerman, Jonathan M. Wang, Cheol-Heon Jeong, Manuel Ramos, Nathan Hilker, Robert M. Healy, Kelly Sabaliauskas, James S. Wallace, and Greg J. Evans
Environmental Science & Technology, Volume 50 (4), 2035–2043, 2016

Four field campaigns were conducted between February 2014 and January 2015 to measure emissions from light-duty gasoline direct injection (GDI) vehicles (2013 Ford Focus) in an urban near-road environment in Toronto, Canada. Measurements of CO2, CO, NOx, black carbon (BC), benzene, toluene, ethylbenzene-xylenes (BTEX), and size-resolved particle number (PN) were recorded 15 m from the roadway and converted to fuel-based emission factors (EFs). Other than for NOx and CO, the GDI engine had elevated emissions compared to the Toronto fleet, with BC and BTEX EFs in the 80-90th percentile, and PN EFs in the 75th percentile during wintertime measurements. Additionally, for three campaigns, a second platform for measuring PN and CO2 was placed 1.5-3 m from the roadway to quantify changes in PN with distance from point of emission. GDI vehicle PN EFs were found to increase by up to 240% with increasing distance from the roadway, predominantly due to an increasing fraction of sub-40 nm particles. PN and BC EFs from the same engine technology were also measured in the laboratory. BC EFs agreed within 20% between the laboratory and real-world measurements; however, laboratory PN EFs were an order of magnitude lower due to exhaust conditioning.

Plume-based analysis of vehicle fleet air pollutant emissions and the contribution from high emitters

Jonathan M. Wang, Cheol-Heon Jeong, Naomi Zimmerman, Robert M. Healy, Daniel K. Wang, Fu Ke, Greg J. Evans
Atmospheric Measurement Techniques, Volume 8, 3263-3275, 2015

An automated identification and integration method has been developed for in-use vehicle emissions under real-world conditions. This technique was applied to high-time-resolution air pollutant measurements of in-use vehicle emissions performed under real-world conditions at a near-road monitoring station in Toronto, Canada, during four seasons, through month-long campaigns in 2013–2014. Based on carbon dioxide measurements, over 100 000 vehicle-related plumes were automatically identified and fuel-based emission factors for nitrogen oxides; carbon monoxide; particle number; black carbon; benzene, toluene, ethylbenzene, and xylenes (BTEX); and methanol were determined for each plume. Thus the automated identification enabled the measurement of an unprecedented number of plumes and pollutants over an extended duration. Emission factors for volatile organic compounds were also measured roadside for the first time using a proton transfer reaction time-of-flight mass spectrometer; this instrument provided the time resolution required for the plume capture technique. Mean emission factors were characteristic of the light-duty gasoline-dominated vehicle fleet present at the measurement site, with mean black carbon and particle number emission factors of 35 mg kg fuel−1 and 7.5 × 1014 # kg fuel−1, respectively. The use of the plume-by-plume analysis enabled isolation of vehicle emissions, and the elucidation of co-emitted pollutants from similar vehicle types, variability of emissions across the fleet, and the relative contribution from heavy emitters. It was found that a small proportion of the fleet (< 25 %) contributed significantly to total fleet emissions: 100, 100, 81, and 77 % for black carbon, carbon monoxide, BTEX, and particle number, respectively. Emission factors of a single pollutant may help classify a vehicle as a high emitter; however, regulatory strategies to more efficiently target multi-pollutant mixtures may be better developed by considering the co-emitted pollutants as well.

Light-absorbing properties of ambient black carbon and brown carbon from fossil fuel and biomass burning sources

R. M. Healy, J. M. Wang, C.-H. Jeong, A. K. Y. Lee, M. D. Willis, E. Jaroudi, N. Zimmerman, N. Hilker, M. Murphy, S. Eckhardt, A. Stohl, J. P. D. Abbatt, J. C. Wenger, G. J. Evans
Journal of Geophysical Research: Atmospheres, Volume 120, Issue 13, 6619-6633, 2015

The optical properties of ambient black carbon-containing particles and the composition of their associated coatings were investigated at a downtown site in Toronto, Canada, for 2 weeks in June 2013. The objective was to assess the relationship between black carbon (BC) coating composition/thickness and absorption. The site was influenced by emissions from local vehicular traffic, wildfires in Quebec, and transboundary fossil fuel combustion emissions in the United States. Mass concentrations of BC and associated nonrefractory coatings were measured using a soot particle–aerosol mass spectrometer (SP-AMS), while aerosol absorption and scattering were measured using a photoacoustic soot spectrometer (PASS). Absorption enhancement was investigated both by comparing ambient and thermally denuded PASS absorption data and by relating absorption data to BC mass concentrations measured using the SP-AMS. Minimal absorption enhancement attributable to lensing at 781 nm was observed for BC using both approaches. However, brown carbon was detected when the site was influenced by wildfire emissions originating in Quebec. BC coating to core mass ratios were highest during this period (~7), and while direct absorption by brown carbon resulted in an absorption enhancement at 405 nm (>2.0), no enhancement attributable to lensing at 781 nm was observed. The efficiency of BC coating removal in the denuder decreased substantially when wildfire-related organics were present and may represent an obstacle for future similar studies. These findings indicate that BC absorption enhancement due to lensing is minimal for downtown Toronto, and potentially other urban locations, even when impacted by long-range transport events.

A source-independent empirical correction procedure for the fast mobility and engine exhaust particle sizers

Naomi Zimmerman, Cheol-Heon Jeong, Jonathan M. Wang, Manuel Ramos, James S. Wallace, Greg J. Evans
Atmospheric Environment, Volume 100, 178–184, 2015

The TSI Fast Mobility Particle Sizer (FMPS) and Engine Exhaust Particle Sizer (EEPS) provide size distributions for 6–560 nm particles with a time resolution suitable for characterizing transient particle sources; however, the accuracy of these instruments can be source dependent, due to influences of particle morphology. The aim of this study was to develop a source-independent correction protocol for the FMPS and EEPS. The correction protocol consists of: (1) broadening the >80 nm size range of the distribution to account for under-sizing by the FMPS and EEPS; (2) applying an existing correction protocol in the 8–93 nm size range; and (3) dividing each size bin by the ratio of total concentration measured by the FMPS or EEPS and a water-based Condensation Particle Counter (CPC) as a surrogate scaling factor to account for particle morphology. Efficacy of the correction protocol was assessed for three sources: urban ambient air, diluted gasoline direct injection engine exhaust, and diluted diesel engine exhaust. Linear regression against a reference instrument, the Scanning Mobility Particle Sizer (SMPS), before and after applying the correction protocol demonstrated that the correction ensured agreement within 20%.

The impacts of precursor reduction and meteorology on ground-level ozone in the Greater Toronto Area

Stephanie C. Pugliese, Jennifer G. Murphy, Jeff A. Geddes, Jonathan M. Wang
Atmospheric Chemistry and Physics, Volume 14, 8197-8207, 2014

Tropospheric ozone (O3) is a major component of photochemical smog and is a known human health hazard, as well as a damaging factor for vegetation. Its precursor compounds, nitrogen oxides (NOx) and volatile organic compounds (VOCs), have a variety of anthropogenic and biogenic sources and exhibit non-linear effects on ozone production. As an update to previous studies on ground-level ozone in the Greater Toronto Area (GTA), we present an analysis of NO2, VOC and O3 data from federal and provincial governmental monitoring sites in the GTA from 2000 to 2012. We show that, over the study period, summertime 24 h VOC reactivity and NO2 midday (11:00–15:00) concentrations at all sites decreased significantly; since 2000, all sites experienced a decrease in NO2 of 28–62% and in measured VOC reactivity of at least 53–71%. Comparing 2002–2003 to 2011–2012, the summed reactivity of OH towards NO2 and a suite of measured VOCs decreased from 8.6 to 4.6 s−1. Ratios of reactive VOC pairs indicate that the effective OH concentration experienced by primary pollutants in the GTA has increased significantly over the study period. Despite the continuous decrease in precursor levels, ozone concentrations are not following the same pattern at all stations; it was found that the Canada-wide Standard for ozone continues to be exceeded at all monitoring stations. Additionally, while the years 2008–2011 had consistently lower ozone levels than previous years, 2012 experienced one of the highest recorded summertime ozone concentrations and a large number of smog episodes. We demonstrate that these high ozone observations in 2012 may be a result of the number of days with high solar radiation, the number of stagnant periods and the transport of high ozone levels from upwind regions.

Comparison of three nanoparticle sizing instruments: The influence of particle morphology

Naomi Zimmerman, Krystal J. Godri Pollitt, Cheol-Heon Jeong, Jonathan M. Wang, Terry Jung, Josephine M. Cooper, James S. Wallace, Greg J. Evans
Atmospheric Environment, Volume 86, 140-147, 2014

The TSI Fast Mobility Particle Sizer (FMPS), Engine Exhaust Particle Sizer (EEPS), and Scanning Mobility Particle Sizer (SMPS) provide size distributions for 6–560 nm particles. The aim of this study was to perform comprehensive equivalence testing of these three particle sizing instruments with particles of contrasting chemical and physical characteristics (urban ambient, diesel exhaust, and laboratory-generated particulate). It was observed that the EEPS and FMPS measurements agreed to within 15% thus concluding that data from these instruments may be considered equivalent. Parallel measurements with the SMPS showed that when measuring diesel exhaust particulate during ISO8178 Mode 9 operation there is significant overestimation of particle concentrations by both the EEPS and the FMPS in the 20–120 nm size range (25–38% overestimation). This overestimation also occurred for near-road measurement of heavy emitter vehicle plumes in ambient samples (up to 75% overestimation). Laboratory-generated soot agglomerate particles, whose shape was verified by transmission electron microscopy, were also tested. The agglomerate nature of diesel soot particulate was the dominant cause of the overestimation; parallel measurements with an FMPS and an Ultrafine Condensation Particle Counter of the laboratory-generated soot particulate showed overestimation by up to a factor of three.

Methane fluxes measured by eddy covariance and static chamber techniques at a temperate forest in central Ontario, Canada

Jonathan M. Wang, Jennifer G. Murphy, Jeff A. Geddes, Carolyn L. Winsborough, Nate Basiliko, Sean C. Thomas
Biogeosciences, Volume 10, 4371-4382, 2013

Methane flux measurements were carried out at a temperate forest (Haliburton Forest and Wildlife Reserve) in central Ontario (45°17’11”N, 78°32’19”W) from June to October 2011. Continuous measurements were made by an off-axis integrated cavity output spectrometer that measures methane (CH4) at 10 Hz sampling rates. Fluxes were calculated from the gas measurements in conjunction with wind data collected by a 3-D sonic anemometer using the eddy covariance (EC) method. Observed methane fluxes showed net uptake of CH4 over the measurement period with an average uptake flux (±standard deviation of the mean) of −2.7 ± 0.13 nmol m−2 s−1. Methane fluxes showed a seasonal progression with average rates of uptake increasing from June through September and remaining high in October. This pattern was consistent with a decreasing trend in soil moisture content at the monthly timescale. On the diurnal timescale, there was evidence of increased uptake during the day, when the mid-canopy wind speed was at a maximum. These patterns suggest that substrate supply of CH4 to methanotrophs, and in certain cases hypoxic soil conditions supporting methanogenesis in low-slope areas, drives the observed variability in fluxes. A network of soil static chambers used at the tower site showed reasonable agreement with the seasonal trend and overall magnitude of the eddy covariance flux measurements. This suggests that soil-level microbial processes, and not abiological leaf-level CH4 production, drive overall CH4 dynamics in temperate forest ecosystems such as Haliburton Forest.