A high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a ...suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (<20 ppm) and mass-resolving power (R > 5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity toward more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (>1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.
We present a comprehensive simulation of tropospheric chlorine
within the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmospheric
chemistry. The simulation includes explicit accounting of ...chloride
mobilization from sea salt aerosol by acid displacement of HCl and by other
heterogeneous processes. Additional small sources of tropospheric chlorine
(combustion, organochlorines, transport from stratosphere) are also included.
Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl,
HOCl, ClNO3, ClNO2, and minor species, is produced by the
HCl+OH reaction and by heterogeneous conversion of sea salt aerosol
chloride to BrCl, ClNO2, Cl2, and ICl. The model
successfully simulates the observed mixing ratios of HCl in marine air
(highest at northern midlatitudes) and the associated HNO3
decrease from acid displacement. It captures the high ClNO2 mixing
ratios observed in continental surface air at night and attributes the
chlorine to HCl volatilized from sea salt aerosol and transported inland
following uptake by fine aerosol. The model successfully simulates the
vertical profiles of HCl measured from aircraft, where enhancements in the
continental boundary layer can again be largely explained by transport inland
of the marine source. It does not reproduce the boundary layer Cl2
mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the
daytime, low at night); the model is too high at night, which could be due to
uncertainty in the rate of the ClNO2+Cl- reaction, but we have
no explanation for the high observed Cl2 in daytime. The global
mean tropospheric concentration of Cl atoms in the model is 620 cm−3
and contributes 1.0 % of the global oxidation of methane, 20 % of
ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry
increases global mean tropospheric BrO by 85 %, mainly through the
HOBr+Cl- reaction, and decreases global burdens of tropospheric
ozone by 7 % and OH by 3 % through the associated bromine radical
chemistry. ClNO2 chemistry drives increases in ozone of up to
8 ppb over polluted continents in winter.
The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of ...limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.
Sulfate (
SO
4
2
−
) and nitrate (
NO
3
−
) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade ...despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas–particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter
SO
4
2
−
and
NO
3
−
. GEOS-Chem reproduces observed
SO
4
2
−
−
NO
3
−
−
NH
4
+
particulate concentrations (2.45 μg sm-3) and composition (
SO
4
2
−
: 47%;
NO
3
−
: 32%;
NH
4
+
: 21%) during WINTER. Only 18% of SO₂ emissions were regionally oxidized to
SO
4
2
−
during WINTER, limited by low H₂O₂ and OH. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter SO₂ emissions from 2007 to 2015 and find that the H₂O₂ limitation on SO₂ oxidation weakened, which increased the fraction of SO₂ emissions oxidizing to
SO
4
2
−
. Simultaneously, NOx emissions decreased by 35%, but the modeled
NO
3
−
particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled
SO
4
2
−
and no change in
NO
3
−
, as observed. Wintertime
SO
4
2
−
and
NO
3
−
are expected to change slowly between 2015 and 2023, unless SO₂ and NOx emissions decrease faster in the future than in the recent past.
We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transport
model and apply it to investigate halogen radical cycling and ...implications for tropospheric oxidants. Improved representation of HOBr heterogeneous
chemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model to
include mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixing
ratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison to
surface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytime
measurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very large
missing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a global
mean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global mean
tropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane,
8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %,
NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozone
simulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.
Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. ...We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv–1 using a maximum enhancement method, with the median similar to previous observations of temperate forest fire plumes. Measured HONO to NO x enhancement ratios were generally factors of 2, or higher, at early plume ages than previous studies. Enhancement ratios scale with modified combustion efficiency and certain nitrogenous trace gases, which may be useful to estimate HONO release when HONO observations are lacking or plumes have photochemical exposures exceeding an hour as emitted HONO is rapidly photolyzed. We find that HONO photolysis is the dominant contributor to hydrogen oxide radicals (HO x = OH + HO2) in early stage (<3 h) wildfire plume evolution. These results highlight the role of HONO as a major component of reactive nitrogen emissions from wildfires and the main driver of initial photochemical oxidation.
The Great Observatories All-sky LIRG Survey (GOALS) consists of a complete sample of 202 luminous infrared galaxies (LIRGs) selected from the IRAS Revised Bright Galaxy Sample (RBGS). The galaxies ...span the full range of interaction stages, from isolated galaxies to interacting pairs to late stage mergers. We present a comparison of the UV and infrared properties of 135 galaxies in GOALS observed by GALEX and Spitzer. For interacting galaxies with separations greater than the resolution of GALEX and Spitzer (~2''-6''), we assess the UV and IR properties of each galaxy individually. The contribution of the FUV to the measured star formation rate (SFR) ranges from 0.2% to 17.9%, with a median of 2.8% and a mean of 4.0% ± 0.4%. The specific star formation rate (SSFR) of the GOALS sample is extremely high, with a median value (3.9 × 10-10 yr-1) that is comparable to the highest SSFRs seen in the Spitzer Infrared Nearby Galaxies Survey sample. We examine the position of each galaxy on the IR excess-UV slope (IRX-beta) diagram as a function of galaxy properties, including IR luminosity and interaction stage. The LIRGs on average have greater IR excesses than would be expected based on their UV colors if they obeyed the same relations as starbursts with L IR < 1011 L sun or normal late-type galaxies. The ratio of L IR to the value one would estimate from the IRX-beta relation published for lower luminosity starburst galaxies ranges from 0.2 to 68, with a median value of 2.7. A minimum of 19% of the total IR luminosity in the RBGS is produced in LIRGs and ultraluminous infrared galaxies with red UV colors (beta>0). Among resolved interacting systems, 32% contain one galaxy which dominates the IR emission while the companion dominates the UV emission. Only 21% of the resolved systems contain a single galaxy which dominates both wavelengths.
The sensitivity of a chemical ionization mass spectrometer (ions formed per number density of analytes) is fundamentally limited by the collision frequency between reagent ions and analytes, known as ...the collision limit, the ion–molecule reaction time, and the transmission efficiency of product ions to the detector. We use the response of a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) to N2O5, known to react with iodide at the collision limit, to constrain the combined effects of ion–molecule reaction time, which is strongly influenced by mixing and ion losses in the ion–molecule reaction drift tube. A mass spectrometric voltage scanning procedure elucidates the relative binding energies of the ion adducts, which influence the transmission efficiency of molecular ions through the electric fields within the vacuum chamber. Together, this information provides a critical constraint on the sensitivity of a ToF-CIMS towards a wide suite of routinely detected multifunctional organic molecules for which no calibration standards exist. We describe the scanning procedure and collision limit determination, and we show results from the application of these constraints to the measurement of organic aerosol composition at two different field locations.
Iodide-based chemical ionization mass spectrometry (CIMS) has been used to detect and measure concentrations of several atmospherically relevant organic and inorganic compounds. The significant ...electronegativity of iodide and the strong acidity of hydroiodic acid makes electron transfer and proton abstraction essentially negligible, and the soft nature of the adduct formation ionization technique reduces the chances of sample fragmentation. In addition, iodide has a large negative mass defect, which, when combined with the high resolving power of a high resolution time-of-flight chemical ionization mass spectrometer (HR–ToF–CIMS), provides good selectivity. In this work, we use quantum chemical methods to calculate the binding energies, enthalpies and free energies for clusters of an iodide ion with a number of atmospherically relevant organic and inorganic compounds. Systematic configurational sampling of the free molecules and clusters was carried out at the B3LYP/6-31G* level, followed by subsequent calculations at the PBE/SDD and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP levels. The binding energies, enthalpies, and free energies thus obtained were then compared to the iodide-based University of Washington HR–ToF–CIMS (UW–CIMS) instrument sensitivities for these molecules. We observed a reasonably linear relationship between the cluster binding enthalpies and logarithmic instrument sensitivities already at the PBE/SDD level, which indicates that relatively simple quantum chemical methods can predict the sensitivity of an iodide-based CIMS instrument toward most molecules. However, higher level calculations were needed to treat some outlier molecules, most notably oxalic acid and methylerythritol. Our calculations also corroborated the recent experimental findings that the molecules that the UW–CIMS detects at maximum sensitivity usually have binding enthalpies to iodide which are higher than about 26 kcal/mol, depending slightly on the level of theory.
Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A ...large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas–particle equilibrium and (ii) have a short particle-phase lifetime (∼2–4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.
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