A new technique has been developed to deconvolve and quantify the mass concentrations of hydrocarbon-like and oxygenated organic aerosols (HOA and OOA) using highly time-resolved organic mass spectra ...obtained with an Aerodyne Aerosol Mass Spectrometer (AMS). This technique involves a series of multivariate linear regressions that use mass-to-charge ratios (m/z's) 57 (mostly C4H9 +) and 44 (mostly CO2 +)the identified AMS mass spectral tracers for HOA and OOA, respectivelyas the initial principal components. Two algorithms have been developed: algorithm 1 is based solely on m/z 44 and m/z 57, and algorithm 2 is an iterative procedure expanded from algorithm 1. This technique was applied to the AMS organic aerosol data acquired at the EPA Pittsburgh Supersite during September 2002. The reconstructed organic concentrations (= HOA + OOA) agree well with the measured values (r 2 = 0.997, slope = 0.998), and the reconstructed organic data matrix (size = 3199 time steps × 300 m/z's) explains 99% of the variance in the measured time series. In addition, the extracted mass spectrum of HOA shows high similarity to those of diesel exhaust, lubricating oil, and freshly emitted traffic aerosols observed in urban areas, while the spectrum of OOA closely resembles those of aged organic aerosols sampled in rural areas and also shows similarity with the spectrum of fulvic acida humic-like substance that is ubiquitous in the environment and has previously been used as an analogue to represent polyacid components found in highly processed and oxidized atmospheric organic aerosols. There is evidence for the presence of a third component, although its contribution to the total organic signal appears to be small in this study. The most important result is that m/z 44 and m/z 57 are reliable AMS mass spectral “markers” that provide the “first guess” for algorithm 2 which allows the quantitative description of the organic aerosol concentration and mass spectra over a period of 16 days in a major urban area and allows the extraction of mass spectra of OOA and HOA that can be interpreted chemically. These findings indicate the potential of performing organic source apportionment on the basis of total particle mass, rather than on the basis of organic tracer compounds that contribute a small fraction of this mass.
Organic aerosol (OA) data acquired by the Aerosol Mass Spectrometer (AMS) in 37 field campaigns were deconvolved into hydrocarbon‐like OA (HOA) and several types of oxygenated OA (OOA) components. ...HOA has been linked to primary combustion emissions (mainly from fossil fuel) and other primary sources such as meat cooking. OOA is ubiquitous in various atmospheric environments, on average accounting for 64%, 83% and 95% of the total OA in urban, urban downwind, and rural/remote sites, respectively. A case study analysis of a rural site shows that the OOA concentration is much greater than the advected HOA, indicating that HOA oxidation is not an important source of OOA, and that OOA increases are mainly due to SOA. Most global models lack an explicit representation of SOA which may lead to significant biases in the magnitude, spatial and temporal distributions of OA, and in aerosol hygroscopic properties.
As has been the case in North America and western Europe,
the SO2 emissions have substantially reduced in the North China Plain (NCP) in
recent years. Differential rates of reduction in SO2 and NOx
...concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate
(pNO3-) over the NCP. In this
study, we observed a polluted episode with the particulate nitrate mass
fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during
wintertime in Beijing. Based on this typical pNO3--dominated haze
event, the linkage between aerosol water uptake and pNO3-
enhancement, further impacting on visibility degradation, has been
investigated based on field observations and theoretical calculations.
During haze development, as ambient relative humidity (RH) increased from
∼10 % to 70 %, the aerosol particle liquid water
increased from ∼1 µg m−3 at the beginning to
∼75 µg m−3 in the fully developed haze period. The
aerosol liquid water further increased the aerosol surface area and volume,
enhancing the condensational loss of N2O5 over particles. From the
beginning to the fully developed haze, the condensational loss of
N2O5 increased by a factor of 20 when only considering aerosol
surface area and volume of dry particles, while increasing by a factor of 25 when
considering extra surface area and volume due to water uptake. Furthermore,
aerosol liquid water favored the thermodynamic equilibrium of HNO3 in
the particle phase under the supersaturated HNO3 and NH3 in the
atmosphere. All the above results demonstrated that pNO3- is
enhanced by aerosol water uptake with elevated ambient RH during haze
development, in turn facilitating the aerosol take-up of water due to the
hygroscopicity of particulate nitrate salt. Such mutual promotion between
aerosol particle liquid water and particulate nitrate enhancement can
rapidly degrade air quality and halve visibility within 1 d. Reduction
of nitrogen-containing gaseous precursors, e.g., by control of traffic
emissions, is essential in mitigating severe haze events in the NCP.
A hygroscopicity tandem differential mobility analyzer (HTDMA) was used to measure the water uptake (hygroscopicity) of secondary organic aerosol (SOA) formed during the chemical and photochemical ...oxidation of several organic precursors in a smog chamber. Electron ionization mass spectra of the non-refractory submicron aerosol were simultaneously determined with an aerosol mass spectrometer (AMS), and correlations between the two different signals were investigated. SOA hygroscopicity was found to strongly correlate with the relative abundance of the ion signal m/z 44 expressed as a fraction of total organic signal (f44). m/z 44 is due mostly to the ion fragment CO2+ for all types of SOA systems studied, and has been previously shown to strongly correlate with organic O/C for ambient and chamber OA. The analysis was also performed on ambient OA from two field experiments at the remote site Jungfraujoch, and the megacity Mexico City, where similar results were found. A simple empirical linear relation between the hygroscopicity of OA at subsaturated RH, as given by the hygroscopic growth factor (GF) or "ϰorg" parameter, and f44 was determined and is given by ϰorg = 2.2 × f44 − 0.13. This approximation can be further verified and refined as the database for AMS and HTDMA measurements is constantly being expanded around the world. The use of this approximation could introduce an important simplification in the parameterization of hygroscopicity of OA in atmospheric models, since f44 is correlated with the photochemical age of an air mass.
A comprehensive chamber investigation of photochemical
secondary organic aerosol (SOA) formation and transformation in mixtures of
anthropogenic (o-cresol) and biogenic (α-pinene and isoprene)
...volatile organic compound (VOC) precursors in the presence of NOx and
inorganic seed particles was conducted. To enable direct comparison across
systems, the initial concentration (hence reactivity) of the systems towards
the dominant OH oxidant was adjusted. Comparing experiments conducted in
single-precursor systems at various initial reactivity levels (referenced to
a nominal base case VOC concentration, e.g. halving the initial
concentration for a 1/2 initial reactivity experiment) as well as their
binary and ternary mixtures, we show that the molecular interactions from
the mixing of the precursors can be investigated and discuss challenges in
their interpretation. The observed average SOA particle mass yields (the
organic particle mass produced for a mass of VOC consumed) in descending
order were found for the following systems: α-pinene (32 ± 7 %), α-pinene–o-cresol (28 ± 9 %), α-pinene at 1/2
initial reactivity (21 ± 5 %), α-pinene–isoprene (16 ± 1 %), α-pinene at 1/3 initial reactivity (15 ± 4 %), o-cresol
(13 ± 3 %), α-pinene–o-cresol–isoprene (11 ± 4 %),
o-cresol at 1/2 initial reactivity (11 ± 3 %), o-cresol–isoprene (6 ± 2 %), and isoprene (0 ± 0 %). We find a
clear suppression of the SOA mass yield from α-pinene when it is
mixed with isoprene, whilst no suppression or enhancement of SOA particle
yield from o-cresol was found when it was similarly mixed with isoprene. The
α-pinene–o-cresol system yield appeared to be increased compared to
that calculated based on the additivity, whilst in the α-pinene–o-cresol–isoprene system the measured and predicted yields were comparable.
However, in mixtures in which more than one precursor contributes to the SOA
particle mass it is unclear whether changes in the SOA formation potential
are attributable to physical or chemical interactions, since the reference
basis for the comparison is complex. Online and offline chemical composition
as well as SOA particle volatility, water uptake, and “phase” behaviour
measurements that were used to interpret the SOA formation and behaviour are
introduced and detailed elsewhere.
Two Aerodyne Aerosol Mass Spectrometers (AMS) were deployed at three sites representing urban, semi-rural and rural areas during the Pacific 2001 experiment in the Lower Fraser Valley (LFV), British ...Columbia, Canada in August 2001. The AMS provides on-line quantitative measurements of the size and chemical composition of the non-refractory fraction of submicron aerosol particles. A significant accumulation mode with a peak around 400–500nm was observed at all sites that was principally composed of sulphate, organics, ammonium and some nitrate. Another significant mode with a peak below 200nm was also observed at the urban site and when urban plumes affected the other sites. This paper focuses on the variability of the organic particulate composition and size distribution as a function of location and photochemical activity with a particular emphasis on the urban and rural areas. The small organic mode at the urban site was well correlated with gas phase CO, 1,3-butadiene, benzene and toluene with Pearson's r values of 0.76, 0.71, 0.79 and 0.69, respectively, suggesting that combustion-related emissions are likely to be the main source of the small organic mode at this site. The mass spectra of the urban organic particulate are similar to those of internal combustion engine lubricating oil, and of diesel exhaust aerosol particles, implying that they were composed of a mixture of n-alkanes, branched alkanes, cycloalkanes, and aromatics. In contrast, organic particulate at the rural site was dominated by shorter chain oxidized organic compounds. Correlations between the two organic modes and gas phase compounds at the rural site indicated that a significant part of the small mode originated from combustion sources, while the large accumulation organic mode appeared to be the result of photochemical processing. Processing of organic particulate during a relatively high O3 episode at the rural site appeared to increase the modal diameter of the accumulation mode from about 400 to 600nm and almost doubled its mass loading.
Diesel exhaust emissions were introduced into an atmospheric simulation
chamber and measured using thermal desorption (TD) comprehensive
two-dimensional gas chromatography coupled to a flame ...ionisation detector
(GC × GC-FID). An extensive set of measurements were performed to
investigate the effect of different engine conditions (i.e. load, speed,
“driving scenarios”) and emission control devices (with or without diesel
oxidative catalyst, DOC) on the composition and abundance of unregulated
exhaust gas emissions from a light-duty diesel engine, fuelled with ultra-low
sulfur diesel (ULSD). A range of exhaust dilution ratios were investigated
(range = 1 : 60 to 1 : 1158), simulating the chemical and physical
transformations of the exhaust gas from near to downwind of an emission
source. In total, 16 individual and 8 groups of compounds (aliphatics and
single-ring aromatics) were measured in the exhaust gas ranging from volatile
to intermediate volatility (VOC-IVOC), providing both detailed chemical
speciation and groupings of compounds based on their structure and
functionality. Measured VOC-IVOC emission rates displayed excellent
reproducibility from replicate experiments using similar exhaust dilution
ratios. However, at the extremes of the investigated exhaust dilution ratios
(comparison of 1 : 60 and 1 : 1158), measured VOC-IVOC emission rates
displayed some disagreement owing to poor reproducibility and highlighted the
importance of replicate sample measurements. The investigated DOC was found
to remove 43±10 % (arithmetic mean ± experimental
uncertainty) of the total speciated VOC-IVOC (∑SpVOC-IVOC) emissions.
The compound class-dependant removal efficiencies for the investigated DOC
were 39±12 % and 83±3 % for the aliphatics and
single-ring aromatics, respectively. The DOC aliphatic removal efficiency
generally decreased with increasing carbon chain length. The ∑SpVOC-IVOC emission rates varied significantly with different engine
conditions, ranging from 70 to 9268 mg kg−1 (milligrams of mass
emitted per kilogram of fuel burnt). ∑SpVOC-IVOC emission rates
generally decreased with increasing engine load and temperature, and to a
lesser degree, engine speed. The exhaust gas composition changed considerably
as a result of two influencing factors: engine combustion and DOC hydrocarbon
(HC) removal efficiency. Increased engine combustion efficiency resulted in a
greater percentage contribution of the C7 to C12 n-alkanes to the
∑SpVOC-IVOC emission rate. Conversely, increased DOC HC removal
efficiency resulted in a greater percentage contribution of the C7 to
C12 branched aliphatics to the ∑SpVOC-IVOC emission rate. At low
engine temperatures (<150 ∘C, below the working temperature of
the DOC), the contribution of n-alkanes in the exhaust gas increased with
increasing combustion efficiency and may be important in urban environments,
as n-alkanes are more efficient at producing secondary organic aerosol
(SOA) than their branched counterparts. At very high engine temperatures
(maximum applied engine speed and load, engine
temperature = 700 ∘C), the n-alkane contribution increased by
a factor of 1.6 times greater than that observed in the cold-start experiment
(most similar to unburnt fuel) and may suggest liquid-fuel-based estimates of
SOA yields may be inconsistent with exhaust SOA yields, particularly at high
engine speeds and loads (i.e. high engine temperatures). Emission rates were
found to be 65 times greater from a cold-start experiment than at maximum
applied engine speed and load. To our knowledge, this is the first study
which uses an atmospheric simulation chamber to separate the effects of the
DOC and combustion efficiency on the exhaust gas composition.
Wood‐burning for domestic heating purposes is becoming more important owing to the increasing use of wood as a renewable fuel. Particle emissions from residential wood combustion contribute ...substantially to particulate matter during winter. An Aerodyne quadrupole aerosol mass spectrometer was used to study the variability of the mass spectra of organic aerosol particles emitted from the burning of different wood types as a function of burning conditions and burning technologies. Previously found wood‐burning mass fragment markers in ambient air and for levoglucosan such as m/z 60, 73, and 29 were confirmed as a feature of wood‐burning aerosol. They were enhanced during the flaming phase and reduced in the smoldering phase when burning was conducted in a small wood stove. The mass spectra during the smoldering phase were dominated by oxygenated species and exhibited a strong resemblance to the mass spectrum of fulvic acid which is used as a model compound for highly oxidized aerosol. A strong resemblance between the mass spectra of fulvic acid and organic particles emitted during wood‐burning in an automatic furnace was found. In general, we found larger differences in the mass spectra between flaming and smoldering phases of one wood type than between different wood types within the same phase. Furthermore it was observed that during one experiment where white fir bark was burned the contribution of polycyclic aromatic hydrocarbons to the total organic matter was very high (∼30%) compared to other wood‐burning experiments (0.4–2.2%).
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