4 research outputs found
Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene
<div><p>The refractive index is a fundamental property controlling aerosol optical properties. Secondary organic aerosols have variable refractive indices, presumably reflecting variations in their chemical composition. Here, we investigate the real refractive indices (m<sub>r</sub>) and chemical composition of secondary organic aerosols (SOA) generated from the oxidation of α-pinene and limonene with ozone and NO<sub>x</sub>/sunlight at different HC/NO<sub>x</sub> ratios. Refractive indices were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532-nm light. Particle chemical composition was monitored with a high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS). For photochemically generated SOA, the values of refractive indices are consistent with prior results, and ranged from about 1.34 to 1.55 for limonene and from 1.44 to 1.47 for α-pinene, generally increasing as the particles grew. While AMS fragments are strongly correlated to the refractive index for each type of SOA, the relationships are in most cases quite different for different SOA types. Consistent with its wide range of refractive index, limonene SOA shows larger variations compared to α-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f<sub>43</sub> (<i>m/z</i> 43/organic), f<sub>C4H7</sub><sup>+</sup>, and others. Refractive indices for α-pinene ozonolysis SOA also fell in narrow ranges; 1.43–1.45 and 1.46–1.53 for particles generated at 19–22 and 23–29°C, respectively, with corresponding small changes of f<sub>43</sub> and H:C ratio and other parameters. Overall, H:C ratio, m/z 43 and 55 (C<sub>2</sub>H<sub>3</sub>O<sup>+</sup>, C<sub>4</sub>H<sub>7</sub><sup>+</sup>) were the best correlated with refractive index for all aerosol types investigated. The relationships between m<sub>r</sub> and most fragments support the notion that increasing condensation of less oxygenated semivolatile species (with a possible role for a concomitant decrease in low refractive index water) is responsible for the increasing m<sub>r</sub>s observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.</p>
</div
More unsaturated, cooking-type hydrocarbon-like organic aerosol particle emissions from renewable diesel compared to ultra low sulfur diesel in at-sea operations of a research vessel
<p>The aerosol particle emissions from R/V <i>Robert Gordon Sproul</i> were measured during two 5-day research cruises (29 September–3 October 2014; 4–7 and 26–28 September 2015) at four engine speeds (1600 rpm, 1300 rpm, 1000 rpm, and 700 rpm) to characterize the emissions under different engine conditions for ultra low sulfur diesel (ULSD) and hydrogenation derived renewable diesel (HDRD) fuels. Organic aerosol composition and mass distribution were measured on the aft deck of the vessel directly behind the exhaust stack to intercept the ship plume. The ship emissions for both fuels were composed of alkane-like compounds (H/C = 1.94 ± 0.003, O/C = 0.04 ± 0.001, C<i><sub>n</sub></i>H<sub>2</sub><i><sub>n</sub></i>) with mass spectral fragmentation patterns consistent with hydrocarbon-like organic aerosol (HOA). Single-particle mass spectra from emissions for both fuels showed two distinct HOA compositions, with one HOA type containing more saturated alkane fragments (C<i><sub>n</sub></i>H<sub>2</sub><i><sub>n</sub></i><sub>+1</sub>) and the other HOA type containing more monounsaturated fragments (C<i><sub>n</sub></i>H<sub>2</sub><i><sub>n</sub></i><sub>−1</sub>). The particles dominated by the C<i><sub>n</sub></i>H<sub>2</sub><i><sub>n</sub></i><sub>−1</sub> fragment series are similar to mass spectra previously associated with cooking emissions. More cooking-type organic particles were observed in the ship emissions for HDRD than for ULSD (45% and 38%, respectively). Changes in the plume aerosol composition due to photochemical aging in the atmosphere were also characterized. The higher fraction of alkene or aromatic (C<i><sub>n</sub></i>H<sub>2</sub><i><sub>n</sub></i><sub>−</sub><i><sub>m</sub></i><sub>,</sub> <i>m</i> ≥ 3) fragments in aged compared to fresh plume emissions suggest that some of the semivolatile alkane-like components partition back to the vapor phase as dilution increases, while alkene or aromatic hydrocarbons contribute more mass to the particle phase due to continuing photochemical oxidation and subsequent condensation from the vapor phase.</p> <p>Copyright © 2017 American Association for Aerosol Research</p
Characterizing the Aging of Biomass Burning Organic Aerosol by Use of Mixing Ratios: A Meta-analysis of Four Regions
Characteristic organic aerosol (OA) emission ratios (ERs)
and normalized
excess mixing ratios (NEMRs) for biomass burning (BB) events have
been calculated from ambient measurements recorded during four field
campaigns. Normalized OA mass concentrations measured using Aerodyne
Research Inc. quadrupole aerosol mass spectrometers (Q-AMS) reveal
a systematic variation in average values between different geographical
regions. For each region, a consistent, characteristic ratio is seemingly
established when measurements are collated from plumes of all ages
and origins. However, there is evidence of strong regional and local-scale
variability between separate measurement periods throughout the tropical,
subtropical, and boreal environments studied. ERs close to source
typically exceed NEMRs in the far-field, despite apparent compositional
change and increasing oxidation with age. The absence of any significant
downwind mass enhancement suggests no regional net source of secondary
organic aerosol (SOA) from atmospheric aging of BB sources, in contrast
with the substantial levels of net SOA formation associated with urban
sources. A consistent trend of moderately reduced ΔOA/ΔCO ratios
with aging indicates a small net loss of OA, likely as a result of
the evaporation of organic material from initial fire emissions. Variability
in ERs close to source is shown to substantially exceed the magnitude
of any changes between fresh and aged OA, emphasizing the importance
of fuel and combustion conditions in determining OA loadings from
biomass burning
Comparison of Gasoline Direct-Injection (GDI) and Port Fuel Injection (PFI) Vehicle Emissions: Emission Certification Standards, Cold-Start, Secondary Organic Aerosol Formation Potential, and Potential Climate Impacts
Recent
increases in the Corporate Average Fuel Economy standards have led
to widespread adoption of vehicles equipped with gasoline direct-injection
(GDI) engines. Changes in engine technologies can alter emissions.
To quantify these effects, we measured gas- and particle-phase emissions
from 82 light-duty gasoline vehicles recruited from the California
in-use fleet tested on a chassis dynamometer using the cold-start
unified cycle. The fleet included 15 GDI vehicles, including 8 GDIs
certified to the most-stringent emissions standard, superultra-low-emission
vehicles (SULEV). We quantified the effects of engine technology,
emission certification standards, and cold-start on emissions. For
vehicles certified to the same emissions standard, there is no statistical
difference of regulated gas-phase pollutant emissions between PFIs
and GDIs. However, GDIs had, on average, a factor of 2 higher particulate
matter (PM) mass emissions than PFIs due to higher elemental carbon
(EC) emissions. SULEV certified GDIs have a factor of 2 lower PM mass
emissions than GDIs certified as ultralow-emission vehicles (3.0 ±
1.1 versus 6.3 ± 1.1 mg/mi), suggesting improvements in engine
design and calibration. Comprehensive organic speciation revealed
no statistically significant differences in the composition of the
volatile organic compounds emissions between PFI and GDIs, including
benzene, toluene, ethylbenzene, and xylenes (BTEX). Therefore, the
secondary organic aerosol and ozone formation potential of the exhaust
does not depend on engine technology. Cold-start contributes a larger
fraction of the total unified cycle emissions for vehicles meeting
more-stringent emission standards. Organic gas emissions were the
most sensitive to cold-start compared to the other pollutants tested
here. There were no statistically significant differences in the effects
of cold-start on GDIs and PFIs. For our test fleet, the measured 14.5%
decrease in CO<sub>2</sub> emissions from GDIs was much greater than
the potential climate forcing associated with higher black carbon
emissions. Thus, switching from PFI to GDI vehicles will likely lead
to a reduction in net global warming