Black carbon (BC) particles are responsible for substantial radiative heating of the atmosphere. However, the climate‐relevant properties of BC particles in the free troposphere (FT) are poorly ...constrained over longer time scales. Here, we report on in situ, intensive field campaigns deploying a single‐particle soot photometer during summer and winter seasons at the Jungfraujoch (3,580 m a.s.l., Switzerland), a site often located in the lower FT. The refractory BC (rBC) mass size distribution was very stable across the two seasons. BC was generally internally mixed (i.e., heavily coated with other aerosol material) in summer. However, against general expectations, BC was predominantly externally mixed in winter, with a high occurrence of negligibly‐to‐thinly coated BC. This strong seasonality in lower‐FT BC mixing state has not been previously observed and would substantially influence the lifetime, loading, and climate effects of free‐tropospheric BC. Future measurements should investigate the generality of these results.
Plain Language Summary
Black carbon (BC) aerosols (i.e., soot particles) play an important role in global climate change by causing atmospheric warming. To understand this role, we need to know what types of BC particles are found in the atmosphere; how big are they and how are they mixed with other aerosols? This type of information is lacking for BC particles found in the free troposphere (the atmospheric layer immediately above the mixed boundary layer of air at the Earth's surface). We performed measurements during winter and summer seasons at Jungfraujoch in the Swiss Alps (3,580 m a.s.l.) in order to fill this gap in knowledge. We discovered a large and unexpected seasonal contrast; in summer at this site, BC aerosols are heavily coated with other aerosol material, whereas in winter the BC is largely uncoated. This seasonal contrast has important implications for our understanding of how long BC particles stay in the atmosphere (coated particles are removed faster), and our estimates of BC climate impacts (coated particles absorb more sunlight).
Key Points
Laser‐induced‐incandescence measurements revealed no seasonal variability in black carbon (BC) mass size distributions at Jungfraujoch
BC was heavily coated in summer but largely uncoated in winter, a seasonality that has not been reported before
These measurements have important implications for the aging timescales and lifetime of BC in the free troposphere
Aerosol particles acting as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs) play a major role in the formation and glaciation of clouds. Thereby they exert a strong impact on the ...radiation budget of the Earth. Data on abundance and properties of both types of particles are sparse, especially for remote areas of the world, such as the Southern Ocean (SO). In this work, we present unique results from ship-borne aerosol-particle-related in situ measurements and filter sampling in the SO region, carried out during the Antarctic Circumnavigation Expedition (ACE) in the austral summer of 2016–2017. An overview of CCN and INP concentrations over the Southern Ocean is provided and, using additional quantities, insights regarding possible CCN and INP sources and origins are presented.
CCN number concentrations spanned 2 orders of magnitude, e.g. for a supersaturation of 0.3 % values ranged roughly from 3 to 590 cm−3.
CCN showed variable contributions of organic and inorganic material (inter-quartile range of hygroscopicity parameter κ from 0.2 to 0.9).
No distinct size dependence of κ was apparent, indicating homogeneous composition across sizes (critical dry diameter on average between 30 and 110 nm).
The contribution of sea spray aerosol (SSA) to the CCN number concentration was on average small.
Ambient INP number concentrations were measured in the temperature range from −5 to −27 ∘C using an immersion freezing method. Concentrations spanned up to 3 orders of magnitude, e.g. at −16 ∘C from 0.2 to 100 m−3.
Elevated values (above 10 m−3 at −16 ∘C) were measured when the research vessel was in the vicinity of land (excluding Antarctica), with lower and more constant concentrations when at sea. This, along with results of backward-trajectory analyses, hints towards terrestrial and/or coastal INP sources being dominant close to ice-free (non-Antarctic) land.
In pristine marine areas INPs may originate from both oceanic sources and/or long-range transport.
Sampled aerosol particles (PM10) were analysed for sodium and methanesulfonic acid (MSA). Resulting mass concentrations were used as tracers for primary marine and secondary aerosol particles, respectively.
Sodium, with an average mass concentration around 2.8 µg m−3, was found to dominate the sampled, identified particle mass.
MSA was highly variable over the SO, with mass concentrations up to 0.5 µg m−3 near the sea ice edge.
A correlation analysis yielded strong correlations between sodium mass concentration and particle number concentration in the coarse mode, unsurprisingly indicating a significant contribution of SSA to that mode.
CCN number concentration was highly correlated with the number concentration of Aitken and accumulation mode particles. This, together with a lack of correlation between sodium mass and Aitken and accumulation mode number concentrations, underlines the important contribution of non-SSA, probably secondarily formed particles, to the CCN population. INP number concentrations did not significantly correlate with any other measured aerosol physico-chemical parameter.
During summer, the Southern Ocean is largely unaffected by anthropogenic emissions, which makes this region an ideal place to investigate marine natural aerosol sources and processes. A better ...understanding of natural aerosol is key to constrain the preindustrial aerosol state and reduce the aerosol radiative forcing uncertainty in global climate models. We report the concentrations of gaseous sulfuric acid, iodic acid, and methanesulfonic acid (MSA) together with a characterization of new particle formation (NPF) events over a large stretch of the Southern Ocean. Measurements were conducted on board the Russian icebreaker Akademik Tryoshnikov from January to March 2017. Iodic acid is characterized by a particular diurnal cycle with reduced concentration around noon, suggesting a lower formation yield when solar irradiance is higher. Gaseous MSA does not have a diurnal cycle and measured concentrations in gas and condensed phase are compatible with this species being primarily produced via heterogeneous oxidation of dimethyl sulfide and subsequent partitioning into the gas phase. We also found that NPF in the boundary layer is mainly driven by sulfuric acid but it occurred very rarely over the vast geographical area probed and did not contribute to the cloud condensation nuclei budget in a directly observable manner. Despite the near absence of NPF events in the boundary layer, Aitken mode particles were frequently measured, supporting the hypothesis of a free tropospheric source. Iodic acid and MSA were not found to participate in nucleation, however, MSA may contribute to aerosol growth via heterogeneous formation in the aqueous phase.
Key Points
Production of methanesulfonic acid occurred predominantly in the condensed phase, followed by enhanced volatilization at lower relative humidity
The observed new particle formation events were probably driven by sulfuric acid and the environmental conditions (mainly temperature and condensation sink)
Boundary layer new particle formation over the Southern Ocean in summer occurred very sporadically and does not contribute to the cloud condensation nuclei budget
Aerosol measurements over the Southern Ocean are used to constrain
aerosol–cloud interaction radiative forcing (RFaci) uncertainty in a global climate model. Forcing uncertainty is quantified using 1 ...million climate model variants that sample the uncertainty in nearly 30 model parameters. Measurements of cloud condensation nuclei and other aerosol properties from an Antarctic circumnavigation expedition strongly constrain natural aerosol emissions: default sea spray emissions need to be increased by around a factor of 3 to be consistent with measurements. Forcing uncertainty is reduced by around 7 % using this set of several hundred measurements, which is comparable to the 8 % reduction achieved using a diverse and extensive set of over 9000 predominantly Northern Hemisphere measurements. When Southern Ocean and Northern Hemisphere measurements are combined, uncertainty in RFaci is reduced by 21 %, and the strongest 20 % of forcing values are ruled out as implausible. In this combined constraint, observationally plausible RFaci is around 0.17 W m−2 weaker (less negative) with 95 % credible values ranging from −2.51 to
−1.17 W m−2 (standard deviation of −2.18 to −1.46 W m−2). The Southern Ocean and Northern Hemisphere measurement datasets are complementary because they constrain different processes. These results
highlight the value of remote marine aerosol measurements.
Liquid clouds form by condensation of water vapour on aerosol particles in the atmosphere. Even black carbon (BC) particles, which are known to be slightly hygroscopic, have been shown to readily ...form cloud droplets once they have acquired water-soluble coatings by atmospheric aging processes. Accurately simulating the life cycle of BC in the atmosphere, which strongly depends on the wet removal following droplet activation, has recently been identified as a key element for accurate prediction of the climate forcing of BC. Here, to assess BC activation in detail, we performed in situ measurements during cloud events at the Jungfraujoch high-altitude station in Switzerland in summer 2010 and 2016. Cloud droplet residual and interstitial (unactivated) particles as well as the total aerosol were selectively sampled using different inlets, followed by their physical characterization using scanning mobility particle sizers (SMPSs), multi-angle absorption photometers (MAAPs) and a single-particle soot photometer (SP2). By calculating cloud droplet activated fractions with these measurements, we determined the roles of various parameters on the droplet activation of BC. The half-rise threshold diameter for droplet activation (Dhalfcloud), i.e. the size above which aerosol particles formed cloud droplets, was inferred from the aerosol size distributions measured behind the different inlets. The effective peak supersaturation (SSpeak) of a cloud was derived from Dhalfcloud by comparing it to the supersaturation dependence of the threshold diameter for cloud condensation nuclei (CCN) activation measured by a CCN counter (CCNC). In this way, we showed that the mass-based scavenged fraction of BC strongly correlates with that of the entire aerosol population because SSpeak modulates the critical size for activation of either particle type. A total of 50 % of the BC-containing particles with a BC mass equivalent core diameter of 90 nm was activated in clouds with SSpeak≈0.21 %, increasing up to ∼80 % activated fraction at SSpeak≈0.50 %. On a single-particle basis, BC activation at a certain SSpeak is controlled by the BC core size and internally mixed coating, which increases overall particle size and hygroscopicity. However, the resulting effect on the population averaged and on the size-integrated BC scavenged fraction by mass is small for two reasons: first, acquisition of coatings only matters for small cores in clouds with low SSpeak; and, second, variations in BC core size distribution and mean coating thickness are limited in the lower free troposphere in summer. Finally, we tested the ability of a simplified theoretical model, which combines the κ-Köhler theory with the Zdanovskii–Stokes–Robinson (ZSR) mixing rule under the assumptions of spherical core–shell particle geometry and surface tension of pure water, to predict the droplet activation behaviour of BC-containing particles in real clouds. Predictions of BC activation constrained with SSpeak and measured BC-containing particle size and mixing state were compared with direct cloud observations. These predictions achieved closure with the measurements for the particle size ranges accessible to our instrumentation, that is, BC core diameters and total particle diameters of approximately 50 and 180 nm, respectively. This clearly indicates that such simplified theoretical models provide a sufficient description of BC activation in clouds, as previously shown for activation occurring in fog at lower supersaturation and also shown in laboratory experiments under controlled conditions. This further justifies application of such simplified theoretical approaches in regional and global simulations of BC activation in clouds, which include aerosol modules that explicitly simulate BC-containing particle size and mixing state.
Properties of atmospheric black carbon (BC) particles were
characterized during a field experiment at a rural background site (Melpitz,
Germany) in February 2017. BC absorption at a wavelength of 870 ...nm was
measured by a photoacoustic extinctiometer, and BC physical properties (BC
mass concentration, core size distribution and coating thickness) were
measured by a single-particle soot photometer (SP2). Additionally, a
catalytic stripper was used to intermittently remove BC coatings by
alternating between ambient and thermo-denuded conditions. From these data
the mass absorption cross section of BC (MACBC) and its enhancement factor
(EMAC) were inferred for essentially water-free aerosol as present after
drying to low relative humidity (RH). Two methods were applied independently to investigate the
coating effect on EMAC: a correlation method (MACBC, ambient vs. BC coating
thickness) and a denuding method (MACBC, ambient vs. MACBC, denuded). Observed
EMAC values varied from 1.0 to 1.6 (lower limit from denuding method) or
∼1.2 to 1.9 (higher limit from correlation method), with the
mean coating volume fraction ranging from 54 % to 78 % in the dominating
mass equivalent BC core diameter range of 200–220 nm. MACBC and EMAC were
strongly correlated with coating thickness of BC. By contrast, other
potential drivers of EMAC variability, such as different BC sources (air mass
origin and absorption Ångström exponent), coating composition (ratio
of inorganics to organics) and BC core size distribution, had only minor effects. These results for ambient BC measured at Melpitz during winter show
that the lensing effect caused by coatings on BC is the main driver of the
variations in MACBC and EMAC, while changes in other BC particle properties
such as source, BC core size or coating composition play only minor roles at
this rural background site with a large fraction of aged particles. Indirect
evidence suggests that potential dampening of the lensing effect due to
unfavorable morphology was most likely small or even negligible.
In this study, we investigate the occurrence of primary biological aerosol particles (PBAP) over all sectors of the Southern Ocean (SO) based on a 90‐day data set collected during the Antarctic ...Circumnavigation Expedition (ACE) in austral summer 2016–2017. Super‐micrometer PBAP (1–16 µm diameter) were measured by a wide band integrated bioaerosol sensor (WIBS‐4). Low (3σ) and high (9σ) fluorescence thresholds are used to obtain statistics on fluorescent and hyper‐fluorescent PBAP, respectively. Our focus is on data obtained over the pristine ocean, that is, more than 200 km away from land. The results indicate that (hyper‐)fluorescent PBAP are correlated to atmospheric variables associated with sea spray aerosol (SSA) particles (wind speed, total super‐micrometer aerosol number concentration, chloride and sodium concentrations). This suggests that a main source of PBAP over the SO is SSA. The median percentage contribution of fluorescent and hyper‐fluorescent PBAP to super‐micrometer SSA was 1.6% and 0.13%, respectively. We demonstrate that the fraction of (hyper‐)fluorescent PBAP to total super‐micrometer particles positively correlates with concentrations of bacteria and several taxa of pythoplankton measured in seawater, indicating that marine biota concentrations modulate the PBAP source flux. We investigate the fluorescent properties of (hyper‐)fluorescent PBAP for several events that occurred near land masses. We find that the fluorescence signal characteristics of particles near land is much more variable than over the pristine ocean. We conclude that the source and concentration of fluorescent PBAP over the open ocean is similar across all sampled sectors of the SO.
Key Points
Fluorescent primary bioaerosol particles (PBAP) were measured over all sectors of the Southern Ocean
Moderate to good correlations were observed between PBAP and sea spray aerosol (SSA) proxies
PBAP fractions in SSA were positively correlated to concentrations of certain marine biota
Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by ...the UV–visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE365) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m2 g–1, which was significantly higher than that for aged OA (0.24 ± 0.18 m2 g–1). Secondary OA (SOA) dominated aged OA and the decreased MAE365 after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE365 of SOA (0.14 ± 0.08 m2 g–1) was much lower than that of POA. A comparison of MAE365 of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.
We investigate the possibility that the refractory, infrared-light-absorbing carbon particulate material known as “tarballs” or tar brown carbon (tar brC) generates a unique signal in the scattering ...and incandescent detectors of a single particle soot photometer (SP2).
As recent studies have defined tar brC in different ways, we begin by reviewing the literature and proposing a material-based definition of tar.
We then show that tar brC results in unique SP2 signals due to a combination of complete or partial evaporation, with no or very little incandescence.
Only a subset of tar brC particles exhibited detectable incandescence (70 % by number); for these particles the ratio of incandescence to light scattering was much lower than that of soot black carbon (BC).
At the time of incandescence the ratio of light scattering to incandescence from these particles was up to 2-fold greater than from soot (BC).
In our sample, where the mass of tar was 3-fold greater than the mass of soot, this led to a bias of <5 % in SP2-measured soot mass, which is negligible relative to calibration uncertainties.
The enhanced light scattering of tar is interpreted as being caused by tar being more amorphous and less graphitic than soot BC.
The fraction of the tar particle which does incandesce was likely formed by thermal annealing during laser heating. These results indicate that laser-induced incandescence, as implemented in the SP2, is the only BC measurement technique which can quantify soot BC concentrations separately from tar while also potentially providing real-time evidence for the presence of tar.
In contrast, BC measurement techniques based on thermal–optical (EC: elemental carbon) and absorption (eBC: equivalent BC) measurements cannot provide such distinctions.
The optical properties of our tar particles indicate a material similarity to the tar particles previously reported in the literature.
However, more- and less-graphitized tar samples have also been reported, which may show stronger and weaker SP2 responses, respectively.
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