Atmospheric marine aerosol particles impact Earth's albedo and climate. These particles can be primary or secondary and come from a variety of sources, including sea salt, dissolved organic matter, ...volatile organic compounds, and sulfur-containing compounds. Dimethylsulfide (DMS) marine emissions contribute greatly to the global biogenic sulfur budget, and its oxidation products can contribute to aerosol mass, specifically as sulfuric acid and methanesulfonic acid (MSA). Further, sulfuric acid is a known nucleating compound, and MSA may be able to participate in nucleation when bases are available. As DMS emissions, and thus MSA and sulfuric acid from DMS oxidation, may have changed since pre-industrial times and may change in a warming climate, it is important to characterize and constrain the climate impacts of both species. Currently, global models that simulate aerosol size distributions include contributions of sulfate and sulfuric acid from DMS oxidation, but to our knowledge, global models typically neglect the impact of MSA on size distributions.
Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM1 pH as a ...function of geographical location and altitude, focusing on the northeastern U.S., based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity campaign (1 February to 15 March 2015). Particle pH and water were predicted with the ISORROPIA‐II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic‐inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 µg m−3). Including refractory ions in the pH calculations did not improve model predictions, suggesting they were externally mixed with PM1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were found to be minimal. Overall, particle pH for altitudes up to 5000 m ranged between −0.51 and 1.9 (10th and 90th percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern U.S. and eastern Mediterranean. This expansive aircraft data set is used to investigate causes in variability in pH and pH‐dependent aerosol components, such as PM1 nitrate, over a wide range of temperatures (−21 to 19°C), RH (20 to 95%), inorganic gas, and particle concentrations and also provides further evidence that particles with low pH are ubiquitous.
Key Points
Highly acidic aerosols (pH = 0.77 ± 0.96) for altitudes up to 5000 m in the northeastern U.S. in wintertime
Thermodynamically predicted HNO3−NO3− partitioning by ISORROPIA‐II agrees with observation above 40% RH
Particle pH should be explicitly determined to accurately assess properties impacted by aerosol acidity, such as HNO3−NO3− partitioning
Organic aerosol (OA) is an important fraction of submicron aerosols. However,
it is challenging to predict and attribute the specific organic compounds and
sources that lead to observed OA loadings, ...largely due to contributions from
secondary production. This is especially true for megacities surrounded by
numerous regional sources that create an OA background. Here, we utilize
in situ gas and aerosol observations collected on board the NASA DC-8 during
the NASA–NIER KORUS-AQ (Korea–United States Air Quality) campaign to
investigate the sources and hydrocarbon precursors that led to the secondary
OA (SOA) production observed over Seoul. First, we investigate the
contribution of transported OA to total loadings observed over Seoul by
using observations over the Yellow Sea coupled to FLEXPART Lagrangian
simulations. During KORUS-AQ, the average OA loading advected into Seoul was
∼1–3 µg sm−3. Second, taking this background into
account, the dilution-corrected SOA concentration observed over Seoul was
∼140 µgsm-3ppmv-1 at 0.5 equivalent photochemical
days. This value is at the high end of what has been observed in other
megacities around the world (20–70 µgsm-3ppmv-1 at 0.5
equivalent days). For the average OA concentration observed over Seoul
(13 µg sm−3), it is clear that production of SOA from locally
emitted precursors is the major source in the region. The importance
of local SOA production was supported by the following observations.
(1) FLEXPART source contribution calculations indicate any
hydrocarbons with a lifetime of less than 1 day, which are shown to dominate the
observed SOA production, mainly originate from South Korea. (2) SOA
correlated strongly with other secondary photochemical species, including
short-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxy
nitrates, dihydroxytoluene, and nitrate aerosol). (3) Results from
an airborne oxidation flow reactor (OFR), flown for the first time, show a
factor of 4.5 increase in potential SOA concentrations over Seoul versus over
the Yellow Sea, a region where background air masses that are advected into
Seoul can be measured. (4) Box model simulations reproduce SOA
observed over Seoul within 11 % on average and suggest that short-lived
hydrocarbons (i.e., xylenes, trimethylbenzenes, and semi-volatile and intermediate-volatility compounds) were the main SOA precursors over Seoul. Toluene
alone contributes 9 % of the modeled SOA over Seoul. Finally, along with
these results, we use the metric ΔOA/ΔCO2 to
examine the amount of OA produced per fuel consumed in a megacity, which
shows less variability across the world than ΔOA∕ΔCO.
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.
The spatial distribution and properties of submicron organic aerosol (OA)
are among the key sources of uncertainty in our understanding of aerosol
effects on climate. Uncertainties are particularly ...large over remote regions
of the free troposphere and Southern Ocean, where very few data have been
available and where OA predictions from AeroCom Phase II global models span 2 to 3 orders of magnitude, greatly exceeding the model spread over
source regions. The (nearly) pole-to-pole vertical distribution of
non-refractory aerosols was measured with an aerosol mass spectrometer
onboard the NASA DC-8 aircraft as part of the Atmospheric Tomography (ATom)
mission during the Northern Hemisphere summer (August 2016) and winter
(February 2017). This study presents the first extensive characterization of
OA mass concentrations and their level of oxidation in the remote
atmosphere. OA and sulfate are the major contributors by mass to submicron
aerosols in the remote troposphere, together with sea salt in the marine
boundary layer. Sulfate was dominant in the lower stratosphere. OA
concentrations have a strong seasonal and zonal variability, with the
highest levels measured in the lower troposphere in the summer and over the
regions influenced by biomass burning from Africa (up to 10 µg sm−3). Lower concentrations (∼0.1–0.3 µg sm−3)
are observed in the northern middle and high latitudes and very low
concentrations (<0.1 µg sm−3) in the southern middle and
high latitudes. The ATom dataset is used to evaluate predictions of eight
current global chemistry models that implement a variety of commonly used
representations of OA sources and chemistry, as well as of the AeroCom-II
ensemble. The current model ensemble captures the average vertical and
spatial distribution of measured OA concentrations, and the spread of the
individual models remains within a factor of 5. These results are
significantly improved over the AeroCom-II model ensemble, which shows large
overestimations over these regions. However, some of the improved agreement
with observations occurs for the wrong reasons, as models have the tendency
to greatly overestimate the primary OA fraction and underestimate the
secondary fraction. Measured OA in the remote free troposphere is highly
oxygenated, with organic aerosol to organic carbon (OA ∕ OC) ratios of
∼2.2–2.8, and is 30 %–60 % more oxygenated than in current
models, which can lead to significant errors in OA concentrations. The
model–measurement comparisons presented here support the concept of a more
dynamic OA system as proposed by Hodzic et al. (2016), with enhanced removal
of primary OA and a stronger production of secondary OA in global models
needed to provide better agreement with observations.
From 2016 to 2018 a DC-8 aircraft operated by the US National
Aeronautics and Space Administration (NASA) made four series of flights,
profiling the atmosphere from 180 m to ∼12 km above sea level ...(km a.s.l.)
from the Arctic to the Antarctic over both the Pacific and Atlantic oceans.
This program, the Atmospheric Tomography Mission (ATom), sought to sample
the troposphere in a representative manner, making measurements of
atmospheric composition in each season. This paper describes the
aerosol microphysical measurements and derived quantities obtained during
this mission. Dry size distributions from 2.7 nm to 4.8 µm in
diameter were measured in situ at 1 Hz using a battery of instruments: 10
condensation particle counters with different nucleation diameters, two
ultra-high-sensitivity aerosol size spectrometers (UHSASs), one of which
measured particles surviving heating to 300 ∘C, and a laser
aerosol spectrometer (LAS). The dry aerosol measurements were complemented
by size distribution measurements from 0.5 to 930 µm diameter at
near-ambient conditions using a cloud, aerosol, and precipitation
spectrometer (CAPS) mounted under the wing of the DC-8. Dry aerosol number,
surface area, and volume, and optical scattering and asymmetry parameters at
several wavelengths from the near-UV to the near-IR ranges were calculated from the
measured dry size distributions (2.7 nm to 4.8 µm). Dry aerosol mass
was estimated by combining the size distribution data with particle density
estimated from independent measurements of aerosol composition with a
high-resolution aerosol mass spectrometer and a single-particle soot
photometer. We describe the instrumentation and fully document the aircraft
inlet and flow distribution system, the derivation of uncertainties, and the
calculation of data products from combined size distributions. Comparisons
between the instruments and direct measurements of some aerosol properties
confirm that in-flight performance was consistent with calibrations and
within stated uncertainties for the two deployments analyzed. The unique
ATom dataset contains accurate, precise, high-resolution in situ
measurements of dry aerosol size distributions, and integral parameters, and
estimates and measurements of optical properties, for particles < 4.8 µm in diameter that can be used to evaluate aerosol abundance and
processes in global models.
Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary ...aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH₂SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.
Aerosol intercomparisons are inherently complex as they convolve
instrument-dependent detection efficiencies vs. size (which often change
with pressure, temperature, or humidity) and variations in ...the sampled
aerosol population, in addition to differences in chemical detection
principles (e.g., inorganic-only nitrate vs. inorganic plus
organic nitrate for two instruments). The NASA Atmospheric Tomography
Mission (ATom) spanned four separate aircraft deployments which sampled the remote marine troposphere from 86∘ S to 82∘ N over
different seasons with a wide range of aerosol concentrations and
compositions. Aerosols were quantified with a set of carefully characterized and calibrated instruments, some based on particle sizing and some on composition measurements. This study aims to provide a critical evaluation of inlet transmissions impacting aerosol intercomparisons, and of aerosol quantification during ATom, with a focus on the aerosol mass spectrometer (AMS). The volume determined from physical sizing instruments (aerosol microphysical properties, AMP, 2.7 nm to 4.8 µm optical diameter) is compared in detail with that derived from the chemical measurements of the AMS and the single particle soot photometer (SP2). Special attention was paid to characterize the upper end of the AMS size-dependent transmission with in-field calibrations, which we show to be critical for accurate comparisons across instruments with inevitably different size cuts. Observed differences between campaigns emphasize the importance of characterizing AMS transmission for each instrument and field study for meaningful interpretation of instrument comparisons. Good agreement (regression slope =0.949 and 1.083 for ATom-1 and ATom-2, respectively; SD =0.003) was found between the composition-based volume (including AMS-quantified sea salt) and that derived from AMP after applying the AMS inlet transmission. The AMS captured, on average, 95±15 % of the standard PM1 volume (referred to as the URG Corp. standard cut 1 µm cyclone operated at its nominal efficiency). These results support the absence of significant unknown biases and the appropriateness of the accuracy estimates for AMS total mass and volume for the mostly aged air masses encountered in ATom. The particle size ranges (and their altitude dependence) that are sampled by the AMS and complementary composition instruments (such as soluble acidic gases and aerosol, SAGA, and particle analysis by laser mass spectrometry, PALMS) are investigated to inform their use in future studies.
Organosulfur compounds are important components of secondary organic aerosols (SOA). While the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS) has been extensively used in ...aerosol studies, the response of the AMS to organosulfur compounds is not well-understood. Here, we investigated the fragmentation patterns of organosulfurs and inorganic sulfates in the AMS, developed a method to deconvolve total sulfate into components of inorganic and organic origins, and applied this method in both laboratory and field measurements. Apportionment results from laboratory isoprene photooxidation experiment showed that with inorganic sulfate seed, sulfate functionality of organic origins can contribute ∼7% of SOA mass at peak growth. Results from measurements in the Southeastern U.S. showed that 4% of measured sulfate is from organosulfur compounds. Methanesulfonic acid was estimated for measurements in the coastal and remote marine boundary layer. We explored the application of this method to unit mass-resolution data, where it performed less well due to interferences. Our apportionment results demonstrate that organosulfur compounds could be a non-negligible source of sulfate fragments in AMS laboratory and field data sets. A reevaluation of previous AMS measurements over the full range of atmospheric conditions using this method could provide a global estimate/constraint on the contribution of organosulfur compounds.
Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenic emissions of organic compounds, constitutes a substantial fraction of the mass of submicron aerosol in populated areas around ...the world and contributes to poor air quality and premature mortality. However, the precursor sources of ASOA are poorly understood, and there are large uncertainties in the health benefits that might accrue from reducing anthropogenic organic emissions. We show that the production of ASOA in 11 urban areas on three continents is strongly correlated with the reactivity of specific anthropogenic volatile organic compounds. The differences in ASOA production across different cities can be explained by differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds, indicating the importance of controlling these ASOA precursors. With an improved model representation of ASOA driven by the observations, we attribute 340 000 PM2.5-related premature deaths per year to ASOA, which is over an order of magnitude higher than prior studies. A sensitivity case with a more recently proposed model for attributing mortality to PM2.5 (the Global Exposure Mortality Model) results in up to 900 000 deaths. A limitation of this study is the extrapolation from cities with detailed studies and regions where detailed emission inventories are available to other regions where uncertainties in emissions are larger. In addition to further development of institutional air quality management infrastructure, comprehensive air quality campaigns in the countries in South and Central America, Africa, South Asia, and the Middle East are needed for further progress in this area.
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