We present the first data on the concentration of sea-salt aerosol throughout
most of the depth of the troposphere and over a wide range of latitudes,
which were obtained during the Atmospheric ...Tomography (ATom) mission.
Sea-salt concentrations in the upper troposphere are very small, usually less
than 10 ng per standard m3 (about 10 parts per trillion by mass) and
often less than 1 ng m−3. This puts stringent limits on the
contribution of sea-salt aerosol to halogen and nitric acid chemistry in the
upper troposphere. Within broad regions the concentration of sea-salt aerosol
is roughly proportional to water vapor, supporting a dominant role for wet
scavenging in removing sea-salt aerosol from the atmosphere. Concentrations
of sea-salt aerosol in the winter upper troposphere are not as low as in the
summer and the tropics. This is mostly a consequence of less wet scavenging
in the drier, colder winter atmosphere. There is also a source of sea-salt
aerosol over pack ice that is distinct from that over open water. With a
well-studied and widely distributed source, sea-salt aerosol provides an
excellent test of wet scavenging and vertical transport of aerosols in
chemical transport models.
Brown carbon (BrC) associated with aerosol particles in western United
States wildfires was measured between July and August 2019 aboard the NASA
DC-8 research aircraft during the Fire Influence on ...Regional to Global
Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods
are investigated, highly spectrally resolved light absorption in solvent
(water and methanol) extracts of particles collected on filters and in situ
bulk aerosol particle light absorption measured at three wavelengths (405,
532 and 664 nm) with a photoacoustic spectrometer (PAS). A light-absorption
closure analysis for wavelengths between 300 and 700 nm was performed. The
combined light absorption of particle pure black carbon material, including
enhancements due to internally mixed materials, plus soluble BrC and a
Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC,
was compared to absorption spectra from a power law fit to the three PAS
wavelengths. For the various parameters used, at a wavelength of roughly 400
nm they agreed, at lower wavelengths the individual component-predicted
particle light absorption significantly exceeded the PAS and at higher
wavelengths the PAS absorption was consistently higher but more variable.
Limitations with extrapolation of PAS data to wavelengths below 405 nm and
missing BrC species of low solubility that more strongly absorb at higher
wavelengths may account for the differences. Based on measurements closest
to fires, the emission ratio of PAS-measured BrC at 405 nm relative to
carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1; emission
ratios for soluble BrC are also provided. As the smoke moved away from the
burning regions, the evolution over time of BrC was observed to be highly
complex; BrC enhancement, depletion or constant levels with age were all
observed in the first 8 h after emission in different plumes. Within 8 h following emissions, 4-nitrocatechol, a well-characterized BrC
chromophore commonly found in smoke particles, was largely depleted relative
to the bulk BrC. In a descending plume where temperature increased by 15 K,
4-nitrocatechol dropped, possibly due to temperature-driven evaporation, but
bulk BrC remained largely unchanged. Evidence was found for reactions with
ozone, or related species, as a pathway for secondary formation of BrC under
both low and high oxides of nitrogen (NOx) conditions, while BrC was
also observed to be bleached in regions of higher ozone and low NOx,
consistent with complex behaviors of BrC observed in laboratory studies.
Although the evolution of smoke in the first hours following emission is
highly variable, a limited number of measurements of more aged smoke (15 to
30 h) indicate a net loss of BrC. It is yet to be determined how the
near-field BrC evolution in smoke affects the characteristics of smoke over
longer timescales and spatial scales, where its environmental impacts are likely
to be greater.
We conducted regional scale CO2 simulations using the Weather Research and Forecasting model (WRF) coupled with the Vegetation Photosynthesis and Respiration Model (VPRM). We contrasted simulated ...concentrations with column, ground and aircraft observations during the Korea-United States Air Quality (KORUS-AQ) 2016 field campaign. Overall, WRF-VPRM slightly underestimates CO2 concentrations at ground and column monitoring sites, but it significantly underestimates at an inland tower measurement site, especially within the stable (nocturnal) boundary layer in nighttime. The model successfully captures the airborne vertical profiles but showed a large offset within the planetary boundary layer (PBL) in the areas surrounding Seoul and around the Taeahn point source emissions in the west coastal area of the Korean Peninsula. A case study flight intended to capture Chinese influence observed no clear signals of long-range transport of CO2, due mainly to the much larger magnitude of background CO2 concentrations. The calculated Net Ecosystem Exchange (NEE) with flux measurements at a tower site in the South Korean Peninsula has also been evaluated comparing with CO2 flux measurements at a flux tower site, resulting in the underestimation by less than a factor of 1.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Halogens in the troposphere are increasingly recognized as playing an important role for atmospheric chemistry, and possibly climate. Bromine and iodine react catalytically to destroy ozone (O₃), ...oxidize mercury, and modify oxidative capacity that is relevant for the lifetime of greenhouse gases. Most of the tropospheric O₃ and methane (CH₄) loss occurs at tropical latitudes. Here we report simultaneous measurements of vertical profiles of bromine oxide (BrO) and iodine oxide (IO) in the tropical and subtropical free troposphere (10°N to 40°S), and show that these halogens are responsible for 34% of the column-integrated loss of tropospheric O₃. The observed BrO concentrations increase strongly with altitude (∼3.4 pptv at 13.5 km), and are 2–4 times higher than predicted in the tropical free troposphere. BrO resembles model predictions more closely in stratospheric air. The largest model low bias is observed in the lower tropical transition layer (TTL) over the tropical eastern Pacific Ocean, and may reflect a missing inorganic bromine source supplying an additional 2.5–6.4 pptv total inorganic bromine (Bry), or model overestimated Brywet scavenging. Our results highlight the importance of heterogeneous chemistry on ice clouds, and imply an additional Brysource from the debromination of sea salt residue in the lower TTL. The observed levels of bromine oxidize mercury up to 3.5 times faster than models predict, possibly increasing mercury deposition to the ocean. The halogen-catalyzed loss of tropospheric O₃ needs to be considered when estimating past and future ozone radiative effects.
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.
Wildfire smoke influences on air quality and atmospheric chemistry have been underscored by the increasing fire prevalence in recent years, and yet, the connection between fire, smoke emissions, and ...the subsequent transformation of this smoke in the atmosphere remains poorly constrained. Toward improving these linkages, we present a new method for coupling high time‐resolution satellite observations of fire radiative power with in situ observations of smoke aerosols and trace gases. We apply this technique to 13 fire plumes comprehensively characterized during the recent FIREX‐AQ mission and show that changes in fire radiative power directly translate into changes in conserved smoke tracers (CO2, CO, and black carbon aerosol) observed in the downwind smoke plume. The correlation is particularly strong for CO2 (mean r > 0.9). This method is important for untangling the competing effects of changing fire behavior versus the influence of dilution and atmospheric processing on the downwind evolution of measured smoke properties.
Key Points
Geostationary satellite observations of fire radiative power are highly correlated with in situ airborne measurements of primary‐emission smoke tracers
High‐resolution satellite observations are needed to disentangle how fire activity and plume dilution impact the downwind evolution of smoke
Diurnal fire activity for wildfires observed during FIREX‐AQ is best parameterized using a bimodal Gaussian distribution to inform models
Formaldehyde (HCHO) has been measured from space for more
than 2 decades. Owing to its short atmospheric lifetime, satellite HCHO
data are used widely as a proxy of volatile organic compounds (VOCs; ...please
refer to Appendix A for abbreviations and acronyms), providing constraints
on underlying emissions and chemistry. However, satellite HCHO products from
different satellite sensors using different algorithms have received little
validation so far. The accuracy and consistency of HCHO retrievals remain
largely unclear. Here we develop a validation platform for satellite HCHO
retrievals using in situ observations from 12 aircraft campaigns with a chemical
transport model (GEOS-Chem) as the intercomparison method. Application to
the NASA operational OMI HCHO product indicates negative biases (−44.5 %
to −21.7 %) under high-HCHO conditions, while it indicates high biases (+66.1 % to
+112.1 %) under low-HCHO conditions. Under both conditions, HCHO a priori
vertical profiles are likely not the main driver of the biases. By providing
quick assessment of systematic biases in satellite products over large
domains, the platform facilitates, in an iterative process, optimization of
retrieval settings and the minimization of retrieval biases. It is also
complementary to localized validation efforts based on ground observations
and aircraft spirals.
The exchange of carbon between the Earth's atmosphere and biosphere influences the atmospheric abundances of carbon dioxide (CO2) and methane (CH4). Airborne eddy covariance (EC) can quantify ...surface-atmosphere exchange from landscape-to-regional scales, offering a unique perspective on carbon cycle dynamics. We use extensive airborne measurements to quantify fluxes of sensible heat, latent heat, CO2, and CH4 across multiple ecosystems in the Mid-Atlantic region during September 2016 and May 2017. In conjunction with footprint analysis and land cover information, we use the airborne dataset to explore the effects of landscape heterogeneity on measured fluxes. Our results demonstrate large variability in CO2 uptake over mixed agricultural and forested sites, with fluxes ranging from −3.4 0.7 to −11.5 1.6 mol m−2 s−1 for croplands and −9.1 1.5 to −22.7 3.2 mol m−2 s−1 for forests. We also report substantial CH4 emissions of 32.3 17.0 to 76.1 29.4 nmol m−2 s−1 from a brackish herbaceous wetland and 58.4 12.0 to 181.2 36.8 nmol m−2 s−1 from a freshwater forested wetland. Comparison of ecosystem-specific aircraft observations with measurements from EC flux towers along the flight path demonstrate that towers capture ∼30%-75% of the regional variability in ecosystem fluxes. Diel patterns measured at the tower sites suggest that peak, midday flux measurements from aircraft accurately predict net daily CO2 exchange. We discuss next steps in applying airborne observations to evaluate bottom-up flux models and improve understanding of the biophysical processes that drive carbon exchange from landscape-to-regional scales.
We analyse aerosol particle composition measurements from five research missions between 2014 and 2018 to assess the meridional extent of particles
containing meteoric material in the upper ...troposphere and lower stratosphere (UTLS). Measurements from the Jungfraujoch mountaintop site and a
low-altitude aircraft mission show that meteoric material is also present within middle- and lower-tropospheric aerosol but within only a very small
proportion of particles. For both the UTLS campaigns and the lower- and mid-troposphere observations, the measurements were conducted with single-particle laser ablation mass spectrometers with bipolar-ion detection, which enabled us to measure the chemical composition of particles in a diameter
range of approximately 150 nm to 3 µm. The five UTLS aircraft missions cover a latitude range from 15 to 68∘ N,
altitudes up to 21 km, and a potential temperature range from 280 to 480 K. In total, 338 363 single particles were analysed, of
which 147 338 were measured in the stratosphere. Of these total particles, 50 688 were characterized by high abundances of magnesium and iron,
together with sulfuric ions, the vast majority (48 610) in the stratosphere, and are interpreted as meteoric material immersed or dissolved within
sulfuric acid. It must be noted that the relative abundance of such meteoric particles may be overestimated by about 10 % to 30 % due to the
presence of pure sulfuric acid particles in the stratosphere which are not detected by the instruments used here. Below the tropopause, the observed
fraction of the meteoric particle type decreased sharply with 0.2 %–1 % abundance at Jungfraujoch, and smaller abundances
(0.025 %–0.05 %) were observed during the lower-altitude Canadian Arctic aircraft measurements. The size distribution of the meteoric sulfuric
particles measured in the UTLS campaigns is consistent with earlier aircraft-based mass-spectrometric measurements, with only 5 %–10 %
fractions in the smallest particles detected (200–300 nm diameter) but with substantial (> 40 %) abundance fractions for particles
from 300–350 up to 900 nm in diameter, suggesting sedimentation is the primary loss mechanism. In the tropical lower stratosphere, only a
small fraction (< 10 %) of the analysed particles contained meteoric material. In contrast, in the extratropics the observed fraction of
meteoric particles reached 20 %–40 % directly above the tropopause. At potential temperature levels of more than 40 K above the
thermal tropopause, particles containing meteoric material were observed in much higher relative abundances than near the tropopause, and, at these
altitudes, they occurred at a similar abundance fraction across all latitudes and seasons measured. Above 440 K, the observed fraction of meteoric
particles is above 60 % at latitudes between 20 and 42∘ N. Meteoric smoke particles are transported from the mesosphere into the
stratosphere within the winter polar vortex and are subsequently distributed towards low latitudes by isentropic mixing, typically below a potential temperature of 440 K. By contrast, the findings from the UTLS measurements show that meteoric material is found in stratospheric
aerosol particles at all latitudes and seasons, which suggests that either isentropic mixing is effective also above 440 K or that meteoric
fragments may be the source of a substantial proportion of the observed meteoric material.
Chloride (Cl−) displacement from sea salt particles is an extensively studied phenomenon with implications for human health, visibility, and the global radiation budget. Past works have investigated ...Cl− depletion over the northwest Atlantic (NWA); however, an updated, multi-seasonal, and geographically expanded account of sea salt reactivity over the region is needed. This study uses chemically resolved mass concentrations and meteorological data from the airborne Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) to quantify seasonal, spatial, and meteorological trends in Cl− depletion and to explore the importance of quantifying (1) non-sea salt sources of Na+ and (2) mass concentrations of lost Cl− (instead of relative amounts displaced). Lost Cl− mass concentrations are lowest in December–February and March, moderate around Bermuda in June, and highest in May (median losses of 0.04, 0.04, 0.66, and 1.76 µg m−3, respectively), with losses in May that are high enough to potentially accelerate tropospheric oxidation rates. Inorganic acidic species can account for all Cl− depletion in December–February, March, and June near Bermuda but none of the lost Cl− in May, suggesting that organic acids may be of importance for Cl− displacement in certain months. Contributions of dust to Na+ are not important seasonally but may cause relevant overestimates of lost Cl− in smoke and dust plumes. Higher percentages of Cl− depletion often do not correspond to larger mass concentrations of lost Cl−, so it is highly recommended to quantify the latter to place depletion reactions in context with their role in atmospheric oxidation and radiative forcing.
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