The formation and evolution of secondary organic aerosol
(SOA) were investigated at Yorkville, GA, in late summer (mid-August to mid-October 2016). The organic aerosol (OA) composition was
measured ...using two online mass spectrometry instruments, the
high-resolution time-of-flight aerosol mass spectrometer (AMS) and the
Filter Inlet for Gases and AEROsols coupled to a high-resolution
time-of-flight iodide-adduct chemical ionization mass spectrometer
(FIGAERO-CIMS). Through analysis of speciated organics data from
FIGAERO-CIMS and factorization analysis of data obtained from both
instruments, we observed notable SOA formation from isoprene and
monoterpenes during both day and night. Specifically, in addition to
isoprene epoxydiol (IEPOX) uptake, we identified isoprene SOA formation from non-IEPOX pathways
and isoprene organic nitrate formation via photooxidation in the presence of
NOx and nitrate radical oxidation. Monoterpenes were found to be the
most important SOA precursors at night. We observed significant
contributions from highly oxidized acid-like compounds to the aged OA factor
from FIGAERO-CIMS. Taken together, our results showed that FIGAERO-CIMS
measurements are highly complementary to the extensively used AMS
factorization analysis, and together they provide more comprehensive
insights into OA sources and composition.
We present results from a high-resolution chemical ionization time-of-flight mass spectrometer (HRToF-CIMS), operated with two different thermal desorption inlets, designed to characterize the gas ...and aerosol composition. Data from two field campaigns at forested sites are shown. Particle volatility distributions are estimated using three different methods: thermograms, elemental formulas, and measured partitioning. Thermogram-based results are consistent with those from an aerosol mass spectrometer (AMS) with a thermal denuder, implying that thermal desorption is reproducible across very different experimental setups. Estimated volatilities from the detected elemental formulas are much higher than from thermograms since many of the detected species are thermal decomposition products rather than actual SOA molecules. We show that up to 65% of citric acid decomposes substantially in the FIGAERO–CIMS, with ∼20% of its mass detected as gas-phase CO2, CO, and H2O. Once thermal decomposition effects on the detected formulas are taken into account, formula-derived volatilities can be reconciled with the thermogram method. The volatility distribution estimated from partitioning measurements is very narrow, likely due to signal-to-noise limits in the measurements. Our findings indicate that many commonly used thermal desorption methods might lead to inaccurate results when estimating volatilities from observed ion formulas found in SOA. The volatility distributions from the thermogram method are likely the closest to the real distributions.
Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254 nm produce OH radicals efficiently and are widely used in atmospheric chemistry and other fields. However, knowledge ...of detailed OFR chemistry is limited, allowing speculation in the literature about whether some non-OH reactants, including several not relevant for tropospheric chemistry, may play an important role in these OFRs. These non-OH reactants are UV radiation, O(1D), O(3P), and O3. In this study, we investigate the relative importance of other reactants to OH for the fate of reactant species in OFR under a wide range of conditions via box modeling. The relative importance of non-OH species is less sensitive to UV light intensity than to relative humidity (RH) and external OH reactivity (OHRext), as both non-OH reactants and OH scale roughly proportional to UV intensity. We show that for field studies in forested regions and also the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. We find that O(1D), O(3P), and O3 have relative contributions to VOC consumption that are similar or lower than in the troposphere. The impact of O atoms can be neglected under most conditions in both OFR and troposphere. Under pathological OFR conditions of low RH and/or high OHRext, the importance of non-OH reactants is enhanced because OH is suppressed. Some biogenics can have substantial destructions by O3, and photolysis at non-tropospheric wavelengths (185 and 254 nm) may also play a significant role in the degradation of some aromatics under pathological conditions. Working under low O2 with the OFR185 mode allows OH to completely dominate over O3 reactions even for the biogenic species most reactive with O3. Non-tropospheric VOC photolysis may have been a problem in some laboratory and source studies, but can be avoided or lessened in future studies by diluting source emissions and working at lower precursor concentrations in lab studies, and by humidification. SOA photolysis is shown to be insignificant for most functional groups, except for nitrates and especially aromatics, which may be photolyzed at high UV flux settings. Our work further establishes the OFR's usefulness as a tool to study atmospheric chemistry and enables better experiment design and interpretation, as well as improved future reactor design.
We describe the results from online measurements of nitrated phenols using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) with acetate as reagent ion in an oil and gas production ...region in January and February of 2014. Strong diurnal profiles were observed for nitrated phenols, with concentration maxima at night. Based on known markers (CH4, NOx, CO2), primary emissions of nitrated phenols were not important in this study. A box model was used to simulate secondary formation of phenol, nitrophenol (NP), and dinitrophenols (DNP). The box model results indicate that oxidation of aromatics in the gas phase can explain the observed concentrations of NP and DNP in this study. Photolysis was the most efficient loss pathway for NP in the gas phase. We show that aqueous-phase reactions and heterogeneous reactions were minor sources of nitrated phenols in our study. This study demonstrates that the emergence of new ToF-CIMS (including PTR-TOF) techniques allows for the measurement of intermediate oxygenates at low levels and these measurements improve our understanding on the evolution of primary VOCs in the atmosphere.
Mass spectrometry is an important analytical technique
within the field of atmospheric chemistry. Owing to advances in
instrumentation, particularly with regards to mass-resolving power and
...instrument response factors (sensitivities), hundreds of different
mass-to-charge (m/z) signals are routinely measured. This large number of
detected ions creates challenges for data visualization. Furthermore,
assignment of chemical formulas to these ions is time consuming and
increases in difficulty at the higher m/z ranges. Here, we describe generalized
Kendrick analysis (GKA) to facilitate the visualization and peak
identification processes for typical atmospheric organic (and to some extent
inorganic) compounds. GKA is closely related to resolution-enhanced Kendrick
mass defect analysis (REKMD), which introduces a tunable integer into the
Kendrick equation that effectively contracts or expands the mass scale. A
characteristic of all Kendrick analysis methods is that these changes
maintain the horizontal alignment of ion series related by integer multiples
of the chosen base unit. Compared to traditional Kendrick analysis, GKA and
REKMD use a tunable parameter (“scaling factor”) to alter the mass defect
spacing between different homologue ion series. As a result, the entire mass
defect range (−0.5 to 0.5) is more effectively used simplifying data
visualization and facilitating chemical formula assignment. We describe the
mechanism of this transformation and discuss base unit and scaling factor
selections appropriate for compounds typically found in atmospheric
measurements. We present an open-source graphical user interface (GUI) for
calculating and visualizing GKA results within the Igor Pro environment.
This paper examines the conditions under which a steady state analysis is valid for modeling NO3 and N2O5 chemistry in the atmosphere. The conclusions come from a simple box model analysis that ...considers a limited number of reactions between NO2, O3, NO3, N2O5 and the presumed sinks for the latter two. The applicability of the steady state depends on the strength of the sinks for NO3 and N2O5, the concentration of NO2, and the ambient temperature. Under clean conditions, weak sinks for NO3 prevent the system from passing through the induction period during the time between sunset and sunrise, thus keeping the system out of steady state. Under polluted (i.e., large NO2 concentrations) or cold conditions, the presence of an equilibrium between NO3 and N2O5 markedly slows the approach to steady state even though the two species are close to equilibrium. The time required to approach equilibrium between NO2, NO3, and N2O5 is not a good measure of the time required to achieve a steady state among these compounds. The paper considers the conditions for which steady state may be valid and outlines a method for identification of individual sinks for NO3 and N2O5 from observed concentration measurements for the steady state case.
Concentrations of NO3 and N2O5 were measured using an in situ instrument aboard the NOAA research vessel Ronald H. Brown in the marine boundary layer along the United States east coast as part of the ...New England Air Quality Study (NEAQS) in the summer of 2002. We analyze the results in terms of the loss partitioning and sink budgets for both of these compounds. Analysis of the data on nights with large N2O5 losses allowed for a determination of its heterogeneous uptake coefficient and gave γ(N2O5) = 0.03 ± 0.02. Reactions of NO3 with terrestrially emitted biogenic volatile organic compounds (isoprene and monoterpenes), advected into the marine boundary layer, and with DMS emitted from the ocean surface were also important. In general, loss of NO3 and N2O5 was rapid, and the partitioning between NO3 and N2O5 losses was roughly equal. Because rapid N2O5 loss consumes NOx at twice the rate of the reaction of NO2 with O3, whereas rapid NO3 loss leads to NOx removal at the same rate, the equal partitioning of losses indicates a nocturnal NOx loss rate of approximately 1.5 times the rate of NO2 + O3. Activation of halogens from the uptake of NO3 and N2O5 on sea salt was calculated to have produced substantial amounts of active Cl on some mornings through the nocturnal formation and sunrise photolysis of ClNO2 if the process proceeded at the rate determined by laboratory studies. However, there was no direct observational evidence to test the magnitude of the predicted source.
We report the first simultaneous in situ observation of a suite of compounds important in nocturnal nitrogen oxide chemistry. Measurements took place at a ground site near Boulder, Colorado, during ...the fall of 2001. Chemical measurements included NO3, N2O5, NO, NO2 and O3; meteorological data were also available. The concentrations of NO3 and N2O5 showed large dynamic ranges that were consistent with variations in NO2 and NO and with shifts in meteorological conditions at this site. The observed ratio of N2O5 to NO3 agreed with the ratio calculated from the measured NO2 concentration and the temperature‐dependent equilibrium constant. In addition, NO3 and N2O5 showed large short‐term variability that may indicate inhomogeneously mixed source and sink compounds and/or deposition at this ground‐based measurement site. Finally, N2O5 reached a peak concentration of nearly 3 ppbv under polluted conditions and accounted for an appreciable fraction of the total concentration of measured nitrogen oxide species.
Volatility and viscosity are important properties of organic aerosols (OA),
affecting aerosol processes such as formation, evolution, and partitioning of
OA. Volatility distributions of ambient OA ...particles have often been
measured, while viscosity measurements are scarce. We have previously
developed a method to estimate the glass transition temperature (Tg) of
an organic compound containing carbon, hydrogen, and oxygen. Based on
analysis of over 2400 organic compounds including oxygenated organic
compounds, as well as nitrogen- and sulfur-containing organic compounds, we
extend this method to include nitrogen- and sulfur-containing compounds
based on elemental composition. In addition, parameterizations are developed
to predict Tg as a function of volatility and the atomic
oxygen-to-carbon ratio based on a negative correlation between Tg and
volatility. This prediction method of Tg is applied to ambient
observations of volatility distributions at 11 field sites. The
predicted Tg values of OA under dry conditions vary mainly from 290 to 339 K
and the predicted viscosities are consistent with the results of ambient
particle-phase-state measurements in the southeastern US and the Amazonian
rain forest. Reducing the uncertainties in measured volatility distributions
would improve predictions of viscosity, especially at low relative humidity.
We also predict the Tg of OA components identified via positive matrix
factorization of aerosol mass spectrometer (AMS) data. The predicted viscosity of
oxidized OA is consistent with previously reported viscosity of secondary organic aerosols (SOA) derived
from α-pinene, toluene, isoprene epoxydiol (IEPOX), and diesel fuel.
Comparison of the predicted viscosity based on the observed volatility
distributions with the viscosity simulated by a chemical transport model
implies that missing low volatility compounds in a global model can lead to
underestimation of OA viscosity at some sites. The relation between
volatility and viscosity can be applied in the molecular corridor or
volatility basis set approaches to improve OA simulations in chemical
transport models by consideration of effects of particle viscosity in OA
formation and evolution.
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