Oxidized organic aerosol (OOA) is a major component of ambient particulate matter, substantially impacting climate, human health, and ecosystems. OOA is readily produced in the presence of sunlight, ...and requires days of photooxidation to reach the levels observed in the atmosphere. High concentrations of OOA are thus expected in the summer; however, our current mechanistic understanding fails to explain elevated OOA during wintertime periods of low photochemical activity that coincide with periods of intense biomass burning. As a result, atmospheric models underpredict OOA concentrations by a factor of 3 to 5. Here we show that fresh emissions from biomass burning exposed to NO₂ and O₃ (precursors to the NO₃ radical) rapidly form OOA in the laboratory over a few hours and without any sunlight. The extent of oxidation is sensitive to relative humidity. The resulting OOA chemical composition is consistent with the observed OOA in field studies in major urban areas. Additionally, this dark chemical processing leads to significant enhancements in secondary nitrate aerosol, of which 50 to 60% is estimated to be organic. Simulations that include this understanding of dark chemical processing show that over 70% of organic aerosol from biomass burning is substantially influenced by dark oxidation. This rapid and extensive dark oxidation elevates the importance of nocturnal chemistry and biomass burning as a global source of OOA.
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. In this study, we use the GEOS-Chem-TOMAS (GC-TOMAS) global aerosol
microphysics model to determine the impact on aerosol size distributions and
subsequent aerosol radiative effects from including MSA in the size-resolved
portion of the model. The effective equilibrium vapor pressure of MSA is
currently uncertain, and we use the Extended Aerosol Inorganics Model (E-AIM)
to build a parameterization for GC-TOMAS of MSA's effective volatility as a
function of temperature, relative humidity, and available gas-phase bases,
allowing MSA to condense as an ideally nonvolatile or semivolatile species or
too volatile to condense. We also present two limiting cases for MSA's
volatility, assuming that MSA is always ideally nonvolatile (irreversible
condensation) or that MSA is always ideally semivolatile (quasi-equilibrium
condensation but still irreversible condensation). We further present
simulations in which MSA participates in binary and ternary nucleation with
the same efficacy as sulfuric acid whenever MSA is treated as ideally
nonvolatile. When using the volatility parameterization described above (both
with and without nucleation), including MSA in the model changes the global
annual averages at 900 hPa of submicron aerosol mass by 1.2 %, N3
(number concentration of particles greater than 3 nm in diameter) by
−3.9 % (non-nucleating) or 112.5 % (nucleating), N80 by 0.8 %
(non-nucleating) or 2.1 % (nucleating), the cloud-albedo aerosol indirect
effect (AIE) by −8.6 mW m−2 (non-nucleating) or −26 mW m−2
(nucleating), and the direct radiative effect (DRE) by −15 mW m−2
(non-nucleating) or −14 mW m−2 (nucleating). The sulfate and
sulfuric acid from DMS oxidation produces 4–6 times more submicron mass than
MSA does, leading to an ∼10 times stronger cooling effect in the DRE.
But the changes in N80 are comparable between the contributions from MSA and
from DMS-derived sulfate/sulfuric acid, leading to comparable changes in the
cloud-albedo AIE. Model–measurement comparisons with the Heintzenberg et al. (2000) dataset
over the Southern Ocean indicate that the default model has a missing source
or sources of ultrafine particles: the cases in which MSA participates in
nucleation (thus increasing ultrafine number) most closely match the
Heintzenberg distributions, but we cannot conclude nucleation from MSA is the
correct reason for improvement. Model–measurement comparisons with
particle-phase MSA observed with a customized Aerodyne high-resolution
time-of-flight aerosol mass spectrometer (AMS) from the ATom campaign show
that cases with the MSA volatility parameterizations (both with and without
nucleation) tend to fit the measurements the best (as this is the first use
of MSA measurements from ATom, we provide a detailed description of these
measurements and their calibration). However, no one model sensitivity case
shows the best model–measurement agreement for both Heintzenberg and the
ATom campaigns. As there are uncertainties in both MSA's behavior (nucleation
and condensation) and the DMS emissions inventory, further studies on both
fronts are needed to better constrain MSA's past, current, and future impacts
upon the global aerosol size distribution and radiative forcing.
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.
Biomass burning is a significant global source of aerosol
number and mass. In fresh biomass burning plumes, aerosol coagulation
reduces aerosol number and increases the median size of aerosol size
...distributions, impacting aerosol radiative effects. Near-source biomass
burning aerosol coagulation occurs at spatial scales much smaller than the
grid boxes of global and many regional models. To date, these models have
ignored sub-grid coagulation and instantly mixed fresh biomass burning emissions into
coarse grid boxes. A previous study found that the rate of particle growth
by coagulation within an individual smoke plume can be approximated using
the aerosol mass emissions rate, initial size distribution median diameter
and modal width, plume mixing depth, and wind speed. In this paper, we use
this parameterization of sub-grid coagulation in the GEOS-Chem–TOMAS (TwO-Moment Aerosol Sectional) global
aerosol microphysics model to quantify the impacts on global aerosol size
distributions, the direct radiative effect, and the cloud-albedo aerosol
indirect effect. We find that inclusion of biomass burning sub-grid coagulation reduces the
biomass burning impact on the number concentration of particles larger than
80 nm (a proxy for CCN-sized particles) by 37 % globally. This cloud condensation nuclei
(CCN) reduction causes our estimated global biomass burning cloud-albedo aerosol
indirect effect to decrease from −76 to −43 mW m−2. Further, as
sub-grid coagulation moves mass to sizes with more efficient scattering,
including it increases our estimated biomass burning all-sky direct effect
from −224 to −231 mW m−2, with assumed external mixing of black carbon
and from −188 to −197 mW m−2 and with assumed internal mixing of black
carbon with core-shell morphology. However, due to differences in fire and
meteorological conditions across regions, the impact of sub-grid coagulation
is not globally uniform. We also test the sensitivity of the impact of
sub-grid coagulation to two different biomass burning emission inventories
to various assumptions about the fresh biomass burning aerosol size
distribution and to two different timescales of sub-grid coagulation. The
impacts of sub-grid coagulation are qualitatively the same regardless of
these assumptions.
Persons of color have been exposed to a disproportionate burden of air pollution across the United States for decades. Yet, the inequality in exposure to known toxic elements of air pollution is ...unclear. Here, we find that populations living in racially segregated communities are exposed to a form of fine particulate matter with over three times higher mass proportions of known toxic and carcinogenic metals. While concentrations of total fine particulate matter are two times higher in racially segregated communities, concentrations of metals from anthropogenic sources are nearly ten times higher. Populations living in racially segregated communities have been disproportionately exposed to these environmental stressors throughout the past decade. We find evidence, however, that these disproportionate exposures may be abated though targeted regulatory action. For example, recent regulations on marine fuel oil not only reduced vanadium concentrations in coastal cities, but also sharply lessened differences in vanadium exposure by segregation.
Secondary organic aerosol (SOA) has been shown to form in biomass-burning emissions in laboratory and field studies. However, there is significant variability among studies in mass enhancement, which ...could be due to differences in fuels, fire conditions, dilution, and/or limitations of laboratory experiments and observations. This study focuses on understanding processes affecting biomass-burning SOA formation in laboratory smog-chamber experiments and in ambient plumes. Vapor wall losses have been demonstrated to be an important factor that can suppress SOA formation in laboratory studies of traditional SOA precursors; however, impacts of vapor wall losses on biomass-burning SOA have not yet been investigated. We use an aerosol-microphysical model that includes representations of volatility and oxidation chemistry to estimate the influence of vapor wall loss on SOA formation observed in the FLAME III smog-chamber studies. Our simulations with base-case assumptions for chemistry and wall loss predict a mean OA mass enhancement (the ratio of final to initial OA mass, corrected for particle-phase wall losses) of 1.8 across all experiments when vapor wall losses are modeled, roughly matching the mean observed enhancement during FLAME III. The mean OA enhancement increases to over 3 when vapor wall losses are turned off, implying that vapor wall losses reduce the apparent SOA formation. We find that this decrease in the apparent SOA formation due to vapor wall losses is robust across the ranges of uncertainties in the key model assumptions for wall-loss and mass-transfer coefficients and chemical mechanisms.We then apply similar assumptions regarding SOA formation chemistry and physics to smoke emitted into the atmosphere. In ambient plumes, the plume dilution rate impacts the organic partitioning between the gas and particle phases, which may impact the potential for SOA to form as well as the rate of SOA formation. We add Gaussian dispersion to our aerosol-microphysical model to estimate how SOA formation may vary under different ambient-plume conditions (e.g., fire size, emission mass flux, atmospheric stability). Smoke from small fires, such as typical prescribed burns, dilutes rapidly, which drives evaporation of organic vapor from the particle phase, leading to more effective SOA formation. Emissions from large fires, such as intense wildfires, dilute slowly, suppressing OA evaporation and subsequent SOA formation in the near field. We also demonstrate that different approaches to the calculation of OA enhancement in ambient plumes can lead to different conclusions regarding SOA formation. OA mass enhancement ratios of around 1 calculated using an inert tracer, such as black carbon or CO, have traditionally been interpreted as exhibiting little or no SOA formation; however, we show that SOA formation may have greatly contributed to the mass in these plumes.In comparison of laboratory and plume results, the possible inconsistency of OA enhancement between them could be in part attributed to the effect of chamber walls and plume dilution. Our results highlight that laboratory and field experiments that focus on the fuel and fire conditions also need to consider the effects of plume dilution or vapor losses to walls.
Transport of anthropogenic aerosol into the Arctic in the spring
months has the potential to affect regional climate; however, modeling
estimates of the aerosol direct radiative effect (DRE) are ...sensitive to
uncertainties in the mixing state of black carbon (BC). A common approach in
previous modeling studies is to assume an entirely external mixture (all
primarily scattering species are in separate particles from BC) or internal
mixture (all primarily scattering species are mixed in the same particles as
BC). To provide constraints on the size-resolved mixing state of BC, we use
airborne single-particle soot photometer (SP2) and ultrahigh-sensitivity
aerosol spectrometer (UHSAS) measurements from the Alfred Wegener Institute
(AWI) Polar 6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate
coating thickness as a function of refractory BC (rBC) core diameter and
the fraction of particles containing rBC in the springtime Canadian high
Arctic. For rBC core diameters in the range of 140 to 220 nm, we find
average coating thicknesses of approximately 45 to 40 nm, respectively,
resulting in ratios of total particle diameter to rBC core diameters ranging
from 1.6 to 1.4. For total particle diameters ranging from 175 to 730 nm,
rBC-containing particle number fractions range from 16 % to 3 %,
respectively. We combine the observed mixing-state constraints with simulated
size-resolved aerosol mass and number distributions from GEOS-Chem–TOMAS to
estimate the DRE with observed bounds on mixing state as opposed to assuming
an entirely external or internal mixture. We find that the pan-Arctic average
springtime DRE ranges from −1.65 to −1.34 W m−2 when assuming
entirely externally or internally mixed BC. This range in DRE is reduced by
over a factor of 2 (−1.59 to −1.45 W m−2) when using the
observed mixing-state constraints. The difference in DRE between the two
observed mixing-state constraints is due to an underestimation of BC mass
fraction in the springtime Arctic in GEOS-Chem–TOMAS compared to Polar 6
observations. Measurements of mixing state provide important constraints for
model estimates of DRE.
A database consisting of approximately 4000 storm observations has been objectively analyzed to determine environmental characteristics that produce high radar reflectivities above the freezing ...level, large total lightning flash rates on the order of 10 flashes per minute, and anomalous vertical charge structures (most notably, dominant midlevel positive charge). The storm database is drawn from four regions of the United States featuring distinct environments, each with coinciding Lightning Mapping Array (LMA) network data. LMAs are able to infer total lightning flash rates using flash clustering algorithms, such as the one implemented in this study. Results show that anomalous charge structures inferred from LMA data, significant lightning flash rates, and increased radar reflectivities above the freezing level tend to be associated with environments that have high cloud base heights (approximately 3 km above ground level) and large atmospheric instability, quantified by normalized convective available potential energy (NCAPE) near 0.2 m s−2. Additionally, we infer that aerosols may affect storm intensity. Maximum flash rates were observed in storms with attributed aerosol concentrations near 1000 cm−3, while total flash rates decrease when aerosol concentrations exceed 1500 cm−3, consistent with previous studies. However, this effect is more pronounced in regions where the NCAPE and cloud base height are low. The dearth of storms with estimated aerosol concentrations less than 700 cm−3 (approximately 1% of total sample) does not provide a complete depiction of aerosol invigoration.
Key Points
Lightning flash rates computed from LMA data with novel clustering algorithm
Aerosol impacts on storm intensity dependent on thermodynamics
Unique charge structures observed for large NCAPE and high CBH
Primary emissions from wood and pellet stoves were aged in an atmospheric simulation chamber under daytime and nighttime conditions. The aerosol was analyzed with online aerosol mass spectrometry and ...offline Fourier transform infrared spectroscopy (FTIR). Measurements using the two techniques agreed reasonably well in terms of the organic aerosol (OA) mass concentration, OA:OC trends, and concentrations of biomass burning markers – lignin-like compounds and anhydrosugars. Based on aerosol mass spectrometry, around 15 % of the primary organic aerosol (POA) mass underwent some form of transformation during daytime oxidation conditions after 6–10 h of atmospheric exposure. A lesser extent of transformation was observed during the nighttime oxidation. The decay of certain semi-volatile (e.g., levoglucosan) and less volatile (e.g., lignin-like) POA components was substantial during aging, highlighting the role of heterogeneous reactions and gas–particle partitioning. Lignin-like compounds were observed to degrade under both daytime and nighttime conditions, whereas anhydrosugars degraded only under daytime conditions. Among the marker mass fragments of primary biomass burning OA (bbPOA), heavy ones (higher m/z) were relatively more stable during aging. The biomass burning secondary OA (bbSOA) became more oxidized with continued aging and resembled that of aged atmospheric organic aerosols. The bbSOA formed during daytime oxidation was dominated by acids. Organonitrates were an important product of nighttime reactions in both humid and dry conditions. Our results underline the importance of changes to both the primary and secondary biomass burning aerosols during their atmospheric aging. Heavier fragments from aerosol mass spectrometry seldom used in atmospheric chemistry can be used as more stable tracers of bbPOA and, in combination with the established levoglucosan marker, can provide an indication of the extent of bbPOA aging.
Cloud condensation nuclei (CCN) can affect cloud properties and therefore the Earth’s radiative balance. New particle formation (NPF) from condensable vapours in the free troposphere has been ...suggested to contribute to CCN, especially in remote, pristine atmospheric regions, but direct evidence is sparse, and the magnitude of this contribution is uncertain. Here we use in-situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observed intense NPF occurring at high altitude in tropical convective regions over both the Pacific and Atlantic Oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40% of the Earth’s surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where they can affect cloud properties. Our findings suggest that the production of CCN, as these new particles descend towards the surface, is currently not adequately captured in global models, because they tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth to CCN sizes. This has potential implications for cloud albedo and the global radiative balance.
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