The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing ...from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol–cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol–cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol–cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.
Food-cooking organic aerosols (COA) are one of the primary sources of submicron particulate matter in urban environments. However, there are still many questions surrounding source apportionment ...related to instrumentation as well as semivolatile partitioning because COA evolve rapidly in the ambient air, making source apportionment more complex. Online measurements of emissions from cooking different types of food were performed in a laboratory to characterize particles and gases. Aerosol mass spectrometer (AMS) measurements showed that the relative ionization efficiency for OA was higher (1.56–3.06) relative to a typical value of 1.4, concluding that AMS is over-estimating COA and suggesting that previous studies likely over-estimated COA concentrations. Food-cooking mass spectra were generated using AMS, and gas and particle food markers were identified with filter inlets for gases and aerosols–chemical ionization mass spectrometer (CIMS) measurements to be used in future food cooking-source apportionment studies. However, there is a considerable variability in both gas and particle markers, and dilution plays an important role in the particle mass budget, showing the importance of using these markers with caution during receptor modeling. These findings can be used to better understand the chemical composition of COA, and they provides useful information to be used in future source-apportionment studies.
Chemical transport models have historically struggled to
accurately simulate the magnitude and variability of observed organic
aerosol (OA), with previous studies demonstrating that models ...significantly
underestimate observed concentrations in the troposphere. In this study, we
explore two different model OA schemes within the standard GEOS-Chem
chemical transport model and evaluate the simulations against a suite of 15
globally distributed airborne campaigns from 2008 to 2017, primarily in the
spring and summer seasons. These include the ATom, KORUS-AQ, GoAmazon,
FRAPPE, SEAC4RS, SENEX, DC3, CalNex, OP3, EUCAARI, ARCTAS and ARCPAC
campaigns and provide broad coverage over a diverse set of
atmospheric composition regimes – anthropogenic, biogenic, pyrogenic and
remote. The schemes include significant differences in their treatment of
the primary and secondary components of OA – a “simple scheme” that models
primary OA (POA) as non-volatile and takes a fixed-yield approach to
secondary OA (SOA) formation and a “complex scheme” that simulates POA as
semi-volatile and uses a more sophisticated volatility basis set approach
for non-isoprene SOA, with an explicit aqueous uptake mechanism to model
isoprene SOA. Despite these substantial differences, both the simple and
complex schemes perform comparably across the aggregate dataset in their
ability to capture the observed variability (with an R2 of 0.41 and
0.44, respectively). The simple scheme displays greater skill in minimizing
the overall model bias (with a normalized mean bias of 0.04 compared to 0.30 for the
complex scheme). Across both schemes, the model skill in reproducing
observed OA is superior to previous model evaluations and approaches the
fidelity of the sulfate simulation within the GEOS-Chem model. However,
there are significant differences in model performance across different
chemical source regimes, classified here into seven categories.
Higher-resolution nested regional simulations indicate that model resolution
is an important factor in capturing variability in highly localized
campaigns, while also demonstrating the importance of well-constrained
emissions inventories and local meteorology, particularly over Asia. Our
analysis suggests that a semi-volatile treatment of POA is superior to a
non-volatile treatment. It is also likely that the complex scheme
parameterization overestimates biogenic SOA at the global scale. While this
study identifies factors within the SOA schemes that likely contribute to OA
model bias (such as a strong dependency of the bias in the complex scheme on
relative humidity and sulfate concentrations), comparisons with the skill of
the sulfate aerosol scheme in GEOS-Chem indicate the importance of other
drivers of bias, such as emissions, transport and deposition, that are
exogenous to the OA chemical scheme.
Particle resupply in the lower atmosphere Coe, Hugh
Science (American Association for the Advancement of Science),
07/2024, Letnik:
385, Številka:
6705
Journal Article
Recenzirano
New atmospheric particles form when the stratosphere intrudes into the troposphere
Aerosol particles smaller than 1 mm in diameter are ubiquitous in the troposphere, the lowest part of Earth’s ...atmosphere responsible for weather and a major part of the climate system. They act as condensation sites for water vapor from which cloud droplets grow and hence, are part of the hydrological cycle. Precipitation removes these particles on timescales of a few days to weeks. A major challenge is understanding the processes that supply new particles to the atmospheric system. Although several mechanisms have previously been described, they do not fully explain observations in the mid-troposphere of nanometer-sized particles, indicative of new particle formation ( 1 ). On page 210 of this issue, Zhang et al . ( 2 ) report that new particle events strongly correlate with intrusions of dry air from the stratosphere, when mixed with tropospheric air. These occurrences may underlie the resupply of new particles to these regions of the atmosphere.
Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur ...dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol-cloud interactions. Here we show that the massive 2014-2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets - consistent with expectations - but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around minus 0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.
Aerosol particles are of importance in the Earth's radiation budget since
they scatter and absorb sunlight. While extensive studies of aerosol optical
properties have been conducted at ground sites, ...vertical measurements and
characterization are very limited in megacities. In this work, we present
simultaneous real-time online measurements of aerosol optical properties at
ground level and at 260 m on a meteorological tower from 16 November to
13 December in 2016 in Beijing along with measurements of continuous vertical
profiles during two haze episodes. The average (±1σ) scattering
and absorption coefficients (bsca and babs; λ=630 nm) were 337.6 (±356.0) and 36.6 (±33.9) Mm−1 at
260 m, which were 26.5 % and 22.5 % lower than those at ground
level. Single scattering albedo (SSA), however, was comparable between the
two heights, with slightly higher values at ground level (0.89±0.04).
Although bsca and babs showed overall similar
temporal variations between ground level and 260 m, the ratios of 260 m to ground
varied substantially from less than 0.4 during the clean stages of haze
episodes to > 0.8 in the late afternoon. A more detailed
analysis indicates that vertical profiles of bsca,
babs, and SSA in the low atmosphere were closely related to the
changes in meteorological conditions and mixing layer height. The mass
absorption cross section (MAC) of equivalent black carbon (eBC, λ=630 nm) varied substantially from 9.5 to 13.2 m2 g−1
in winter in Beijing, and it was strongly associated with the mass ratio of
coating materials on refractory BC (rBC) to rBC (MR), and also the
oxidation degree of organics in rBC-containing particles. Our results show
that the increases in MAC of eBC in winter were mainly caused by
photochemically produced secondary materials. Light absorption of organic
carbon (brown carbon, BrC) was also important in winter, which on average
accounted for 46 (±8.5) % and 48 (±9.3) % of the total
absorption at 370 nm at ground level and 260 m, respectively. A linear
regression model combined with positive matrix factorization analysis was
used to show that coal combustion was the dominant source contribution of BrC
(48 %–55 %) followed by biomass burning (17 %) and
photochemically processed secondary organic aerosol (∼20 %) in
winter in Beijing.
Vegetation fires emit large quantities of aerosol into the
atmosphere, impacting regional air quality and climate. Previous work has
used comparisons of simulated and observed aerosol optical depth ...(AOD) in
regions heavily impacted by fires to suggest that emissions of aerosol particles
from fires may be underestimated by a factor of 2–5. Here we use surface,
aircraft and satellite observations made over the Amazon during September
2012, along with a global aerosol model to improve understanding of aerosol
emissions from vegetation fires. We apply three different satellite-derived
fire emission datasets (FINN, GFED, GFAS) in the model. Daily mean aerosol
emissions in these datasets vary by up to a factor of 3.7 over the Amazon
during this period, highlighting the considerable uncertainty in emissions.
We find variable agreement between the model and observed aerosol mass
concentrations. The model reproduces observed aerosol concentrations
over deforestation fires well in the western Amazon during dry season conditions
with FINN or GFED emissions and during dry–wet transition season conditions
with GFAS emissions. In contrast, the model underestimates aerosol
concentrations over savanna fires in the Cerrado environment east of the
Amazon Basin with all three fire emission datasets. The model generally
underestimates AOD compared to satellite and ground stations, even when the
model reproduces the observed vertical profile of aerosol mass
concentration. We suggest it is likely caused by uncertainties in the
calculation of AOD, which are as large as ∼90 %, with the
largest sensitivities due to uncertainties in water uptake and relative
humidity. Overall, we do not find evidence that particulate emissions from
fires are systematically underestimated in the Amazon region and we caution
against using comparison with AOD to constrain particulate emissions from
fires.
Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period. Here, we improve on previous ...simulations that overestimate boundary layer nucleation in the tropics and add changes to climate and land use to evaluate climate forcing. Our model includes both pure organic nucleation and heteromolecular nucleation of sulfuric acid and organics and reproduces the profile of aerosol number concentration measured in the Amazon. Organic nucleation decreases the sum of the total aerosol direct and indirect radiative forcing by 12.5%. The addition of climate and land use change decreases the direct radiative forcing (-0.38 W m
) by 6.3% and the indirect radiative forcing (-1.68 W m
) by 3.5% due to the size distribution and number concentration change of secondary organic aerosol and sulfate. Overall, the total radiative forcing associated with anthropogenic aerosols is decreased by 16%.
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