The underprediction of ambient secondary organic aerosol (SOA) levels by current atmospheric models in urban areas is well established, yet the cause of this underprediction remains elusive. ...Likewise, the relative contribution of emissions from gasoline- and diesel-fueled vehicles to the formation of SOA is generally unresolved. We investigate the source of these two discrepancies using data from the 2010 CalNex experiment carried out in the Los Angeles Basin (Ryerson et al., 2013). Specifically, we use gas-phase organic mass (GPOM) and CO emission factors in conjunction with measured enhancements in oxygenated organic aerosol (OOA) relative to CO to quantify the significant lack of closure between expected and observed organic aerosol concentrations attributable to fossil-fuel emissions. Two possible conclusions emerge from the analysis to yield consistency with the ambient data: (1) vehicular emissions are not a dominant source of anthropogenic fossil SOA in the Los Angeles Basin, or (2) the ambient SOA mass yields used to determine the SOA formation potential of vehicular emissions are substantially higher than those derived from laboratory chamber studies.
We reanalyze a data set of hydrocarbons in ambient air obtained by gas chromatography‐mass spectrometry at a surface site in Pasadena in the Los Angeles basin during the NOAA California Nexus study ...in 2010. The number of hydrocarbon compounds quantified from the chromatograms is expanded through the use of new peak‐fitting data analysis software. We also reexamine hydrocarbon removal processes. For alkanes, small alkenes, and aromatics, the removal is determined by the reaction with hydroxyl (OH) radicals. For several highly reactive alkenes, the nighttime removal by ozone and nitrate (NO3) radicals is also significant. We discuss how this nighttime removal affects the determination of emission ratios versus carbon monoxide (CO) and show that previous estimates based on nighttime correlations with CO were too low. We analyze model output from the Weather Research and Forecasting‐Chemistry model for hydrocarbons and radicals at the Pasadena location to evaluate our methods for determining emission ratios from the measurements. We find that our methods agree with the modeled emission ratios for the domain centered on Pasadena and that the modeled emission ratios vary by 23% across the wider South Coast basin. We compare the alkene emission ratios with published results from ambient measurements and from tunnel and dynamometer studies of motor vehicle emissions. We find that with few exceptions the composition of alkene emissions determined from the measurements in Pasadena closely resembles that of motor vehicle emissions.
Plain Language Summary
We report new measurements of hydrocarbons in ambient air in the Los Angeles basin. Chemical reactions between hydrocarbons and nitrogen oxides form ozone and fine particles, two important pollutants in Los Angeles smog. It is therefore important to understand hydrocarbon emission sources. In this work, we derive the composition of hydrocarbon emissions using ambient measurements at Pasadena in 2010. The study is complicated due to rapid chemical reactions that remove hydrocarbons in between the time of emission and measurement. After correcting for this chemistry, it is shown that the composition of reactive alkenes agrees closely with those emitted from motor vehicles.
Key Points
An expanded data set of hydrocarbons in ambient air in the Los Angeles basin is presented and analyzed
For reactive alkenes, removal by ozone and nitrate radicals at night is important in addition to their removal by hydroxyl radicals during the day, which complicates determining the composition of emissions
After correction for chemical removal, the composition of reactive alkene emissions is consistent with a source from motor vehicles
The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during ...the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6°S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.
A particle‐into‐liquid sampler (PILS) was coupled to a total organic carbon (TOC) analyzer for 3 s integrated measurements of water‐soluble organic carbon (WSOC) in PM1 ambient particles. The ...components of the instrument are described in detail. The PILS‐TOC was deployed on the NOAA WP‐3D aircraft during the NEAQS/ITCT 2004 program to investigate WSOC sources over the northeastern United States and Canada. Two main sources were identified: biomass burning emissions from fires in Alaska and northwestern Canada and emissions emanating from urban centers. Biomass burning WSOC was correlated with carbon monoxide (CO) and acetonitrile (r2 > 0.88). These plumes were intercepted in layers at altitudes between 3 and 4 km and contained the highest fine particle volume and WSOC concentrations of the mission. Apart from the biomass burning influence, the lowest WSOC concentrations were recorded in rural air masses that included regions of significant biogenic emissions. Highest concentrations were at low altitudes in distinct plumes from urban centers. WSOC and CO were highly correlated (r2 > 0.78) in these urban plumes. The ratio of the enhancement in WSOC relative to CO enhancement was found to be low (∼3 μg C/m3/ppmv) in plumes that had been in transit for a short time, and increased with plume age, but appeared to level off at ∼32 ± 4 μg C/m3/ppmv after ∼1 day of transport from the sources. The results suggest that the production of WSOC in fine particles depends on compounds coemitted with CO and that this process is rapid with a time constant of ∼1 day.
The use of ethanol as a transportation fuel in the U.S. increased significantly from 2000–2009, and in 2010 nearly all gasoline contained 10% ethanol. In accordance with this increased use, ...atmospheric measurements of volatile organic compounds in Los Angeles in 2010 were significantly enriched in ethanol compared to measurements in urban outflow in the Northeast U.S. in 2002 and 2004. Mixing ratios of acetaldehyde, an atmospheric oxidation product of ethanol, decreased between 2002 and 2010 in Los Angeles. Previous work has suggested that large‐scale use of ethanol may have detrimental effects on air quality. While we see no evidence for this in the U.S., our study indicates that ethanol has become a ubiquitous compound in urban air and that better measurements are required to monitor its increase and effects.
Key Points
The use of fuel ethanol has increased by a factor of ten in the last decade
Measurements show that ethanol in urban air has increased strongly
Acetaldehyde, a hazardous air pollutant produced from ethanol, has decreased
A negative-ion proton-transfer chemical ionization mass spectrometric technique (NI-PT-CIMS), using acetate as the reagent ion, was applied to the measurement of volatile inorganic acids of ...atmospheric interest: hydrochloric (HCl), nitrous (HONO), nitric (HNO3), and isocyanic (HNCO) acids. Gas phase calibrations through the sampling inlet showed the method to be intrinsically sensitive (6–16 cts/pptv), but prone to inlet effects for HNO3 and HCl. The ion chemistry was found to be insensitive to water vapor concentrations, in agreement with previous studies of carboxylic acids. The inlet equilibration times for HNCO and HONO were 2 to 4 s, allowing for measurement in biomass burning studies. Several potential interferences in HONO measurements were examined: decomposition of HNO3·NO3− clusters within the CIMS, and NO2-water production on inlet surfaces, and were quite minor (≤1%, 3.3%, respectively). The detection limits of the method were limited by the instrument backgrounds in the ion source and flow tube, and were estimated to range between 16 and 50 pptv (parts per trillion by volume) for a 1 min average. The comparison of HONO measured by CIMS and by in situ FTIR showed good correlation and agreement to within 17%. The method provided rapid and accurate measurements of HNCO and HONO in controlled biomass burning studies, in which both acids were seen to be important products.
Recent work has quantified the delay times in measurements of volatile
organic compounds (VOCs) caused by the partitioning between the gas phase
and the surfaces of the inlet tubing and instrument ...itself. In this study we
quantify wall partitioning effects on time responses and transmission of
multifunctional, semivolatile, and intermediate-volatility organic compounds
(S/IVOCs) with saturation concentrations (C∗) between 100 and 104 µg m−3. The instrument delays of several chemical ionization
mass spectrometer (CIMS) instruments increase with decreasing C∗, ranging from
seconds to tens of minutes, except for the NO3- CIMS where it is
always on the order of seconds. Six different tubing materials were tested.
Teflon, including PFA, FEP, and conductive PFA, performs better than metals
and Nafion in terms of both delay time and transmission efficiency.
Analogous to instrument responses, tubing delays increase as C∗ decreases,
from less than a minute to >100 min. The delays caused by Teflon
tubing vs. C∗ can be modeled using the simple chromatography model of Pagonis
et al. (2017). The model can be used to estimate the equivalent absorbing
mass concentration (Cw) of each material, and to estimate delays under
different flow rates and tubing dimensions. We also include time delay
measurements from a series of small polar organic and inorganic analytes in
PFA tubing measured by CIMS. Small polar molecules behave differently than
larger organic ones, with their delays being predicted by their Henry's law
constants instead of their C∗, suggesting the dominance of partitioning to
small amounts of water on sampling surfaces as a result of their polarity
and acidity properties. PFA tubing has the best performance for gas-only
sampling, while conductive PFA appears very promising for sampling S/IVOCs
and particles simultaneously. The observed delays and low transmission both
affect the quality of gas quantification, especially when no direct
calibration is available. Improvements in sampling and instrument response
are needed for fast atmospheric measurements of a wide range of S/IVOCs
(e.g., by aircraft or for eddy covariance). These methods and results are
also useful for more general characterization of surface–gas interactions.
Losses of gas-phase compounds or delays on their transfer through tubing are
important for atmospheric measurements and also provide a method to
characterize and quantify gas–surface interactions. ...Here we expand recent
results by comparing different types of Teflon and other polymer tubing, as
well as glass, uncoated and coated stainless steel and aluminum, and other
tubing materials by measuring the response to step increases and decreases
in organic compound concentrations. All polymeric tubings showed absorptive
partitioning behavior with no dependence on humidity or concentration, with
PFA Teflon tubing performing best in our tests. Glass and uncoated and
coated metal tubing showed very different phenomenology due to adsorptive
partitioning to a finite number of surface sites. Strong dependencies on
compound concentration, mixture composition, functional groups, humidity,
and memory effects were observed for glass and uncoated and coated metals,
which (except for Silonite-coated stainless steel) also always caused longer
delays than Teflon for the compounds and concentrations tested. Delays for
glass and uncoated and coated metal tubing were exacerbated at low relative
humidity but reduced for RH >20 %. We find that conductive PFA
and Silonite tubing perform best among the materials tested for gas-plus-particle sampling lines, combining reduced gas-phase delays with good
particle transmission.
Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US ...and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO
measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.
An extensive set of volatile organic compounds (VOCs) and particulate organic matter (POM) was measured in polluted air during the New England Air Quality Study in 2002. Using VOC ratios, the ...photochemical age of the sampled air masses was estimated. This approach was validated (1) by comparing the observed rates at which VOCs were removed from the atmosphere with the rates expected from OH oxidation, (2) by comparing the VOC emission ratios inferred from the data with the average composition of urban air, and (3) by the ability to describe the increase of an alkyl nitrate with time in terms of the chemical kinetics. A large part of the variability observed for oxygenated VOCs (OVOCs) and POM could be explained by a description that includes the removal of the primary anthropogenic emissions, the formation and removal of secondary anthropogenic species, and a biogenic contribution parameterized by the emissions of isoprene. The OVOC sources determined from the data are compared with the available literature, and a satisfactory agreement is obtained. The observed sub‐μm POM was highly correlated with secondary anthropogenic gas‐phase species, strongly suggesting that the POM was from secondary anthropogenic sources. The results are used to describe the speciation and total mass of gas‐ and particle‐phase organic carbon as a function of the photochemical age of an urban air mass. Shortly after emission the organic carbon mass is dominated by primary VOCs, while after two days the dominant contribution is from OVOCs and sub‐μm POM. The total measured organic carbon mass decreased by about 40% over the course of two days. The increase in sub‐μm POM could not be explained by the removal of aromatic precursors alone, suggesting that other species must have contributed and/or that the mechanism for POM formation is more efficient than previously assumed.
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