Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling and laboratory experiments demonstrate that coatings on BC can enhance BC’s light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ∼1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC’s warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models. Uncertainties in the absorptive properties of black and brown carbon particles limit our understanding of their warming potential. Following an extensive field campaign, Liuet al. report that the magnitude of warming is dependent on particle coatings, which vary due to source and photochemical aging.

/pdf/enhanced-light-absorption-by-mixed-source-black-and-brown-343ltkrsiy.pdf

Enhanced light absorption by mixed source black and brown carbon particles in UK winter

Predicting the formation of ice in the atmosphere presents one of the great challenges in physical sciences with important implications for the chemistry and composition of the Earth’s atmosphere, the hydrological cycle, and climate. Among atmospheric ice formation processes, heterogeneous ice nucleation proceeds on aerosol particles ranging from a few nanometers to micrometers in size, commonly referred to as ice nucleating particles (INPs). Research over the last two decades has demonstrated that organic matter (OM) is ubiquitous in the atmosphere, present as organic aerosol (OA) particles or as coatings on other particle types. The physicochemical properties of OM make predicting how OM can contribute to the INP population challenging. This review focuses on the role of OM in INPs, summarizing and highlighting recent advances in our understanding of the ice nucleation process gained from theoretical, laboratory, and field studies. Examination of ice residuals and INPs with analytical techniques demonst...

/pdf/the-role-of-organic-aerosol-in-atmospheric-ice-nucleation-a-4nzd9jtckc.pdf

The Role of Organic Aerosol in Atmospheric Ice Nucleation: A Review

Review on recent progress in observations, source identifications and countermeasures of PM2.5.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018RG000615

Aerosol Mixing State: Measurements, Modeling, and Impacts

. We present spectrally resolved optical and microphysical properties of
western Canadian wildfire smoke observed in a tropospheric layer from
5–6.5 km height and in a stratospheric layer from 15–16 km height during
a record-breaking smoke event on 22 August 2017. Three polarization/Raman
lidars were run at the European Aerosol Research Lidar Network (EARLINET)
station of Leipzig, Germany, after sunset on 22 August. For the first time,
the linear depolarization ratio and extinction-to-backscatter ratio (lidar
ratio) of aged smoke particles were measured at all three important lidar
wavelengths of 355, 532, and 1064 nm. Very different particle depolarization
ratios were found in the troposphere and in the stratosphere. The obviously
compact and spherical tropospheric smoke particles caused almost no
depolarization of backscattered laser radiation at all three wavelengths
(  %), whereas the dry irregularly shaped soot particles in the
stratosphere lead to high depolarization ratios of 22 % at 355 nm and
18 % at 532 nm and a comparably low value of 4 % at 1064 nm. The lidar
ratios were 40–45 sr (355 nm), 65–80 sr (532 nm), and 80–95 sr
(1064 nm) in both the tropospheric and stratospheric smoke layers
indicating similar scattering and absorption properties. The strong
wavelength dependence of the stratospheric depolarization ratio was probably
caused by the absence of a particle coarse mode (particle mode consisting of
particles with radius >500 nm ). The stratospheric smoke particles
formed a pronounced accumulation mode (in terms of particle volume or mass)
centered at a particle radius of 350–400 nm. The effective particle radius
was 0.32  µ m. The tropospheric smoke particles were much smaller
(effective radius of 0.17  µ m). Mass concentrations were of the
order of 5.5  µ g m −3 (tropospheric layer) and
40  µ g m −3 (stratospheric layer) in the night of
22 August 2017. The single scattering albedo of the stratospheric particles
was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm,
respectively.

/pdf/depolarization-and-lidar-ratios-at-355-532-and-1064-nm-and-3fmiz3blzy.pdf

Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke

The radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of <= 2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass-equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GL062404

Morphology and mixing state of aged soot particles at a remote marine free troposphere site: Implications for optical properties

3th Atmospheric Science Research (ASR), Science Team Meeting. Arlington, Virginia, March 12-16, 2012.

Aerosol measurements in the free troposphere at the North Atlantic Pico Mountain Observatory in the Azores

. Free tropospheric aerosol was sampled at the Pico Mountain Observatory located at 2225 m above mean sea level on Pico Island of the Azores archipelago in the North Atlantic. The observatory is located ~ 3900 km east and downwind of North America, which enables studies of free tropospheric air transported over long distances. Aerosol samples collected on filters from June to October 2012 were analyzed to characterize organic carbon, elemental carbon, and inorganic ions. The average ambient concentration of aerosol was 0.9 ± 0.7 μg m−3. On average, organic aerosol components represent the largest mass fraction of the total measured aerosol (60 ± 51%), followed by sulfate (23 ± 28%), nitrate (13 ± 10%), chloride (2 ± 3%), and elemental carbon (2 ± 2%). Water-soluble organic matter (WSOM) extracted from two aerosol samples (9/24 and 9/25) collected consecutively during a pollution event were analyzed using ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Approximately 4000 molecular formulas were assigned to each of the mass spectra in the range of m/z 100–1000. The majority of the assigned molecular formulas had unsaturated structures with CHO and CHNO elemental compositions. FLEXPART retroplume analyses showed the sampled air masses were very aged (average plume age > 12 days). These aged aerosol WSOM compounds had an average O/C ratio of ~ 0.45, which is relatively low compared to O/C ratios of other aged aerosol. The increase in aerosol loading during the measurement period of 9/24 was linked to biomass burning emissions from North America by FLEXPART retroplume analysis and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts. This was confirmed with biomass burning markers detected in the WSOM and with the morphology and mixing state of particles as determined by scanning electron microscopy. The presence of markers characteristic of aqueous-phase reactions of phenolic species suggests that the aerosol collected at the Pico Mountain Observatory had undergone cloud processing before reaching the site. Finally, the air masses of 9/25 were more aged and influenced by marine emissions, as indicated by the presence of organosulfates and other species characteristic of marine aerosol. The change in the air masses for the two samples was corroborated by the changes in ethane, propane, and ozone, morphology of particles, as well as by the FLEXPART retroplume simulations. This paper presents the first detailed molecular characterization of free tropospheric aged aerosol intercepted at a lower free troposphere remote location and provides evidence of low oxygenation after long-range transport. We hypothesize this is a result of the selective removal of highly aged and polar species during long-range transport, because the aerosol underwent a combination of atmospheric processes during transport facilitating aqueous-phase removal (e.g., clouds processing) and fragmentation (e.g., photolysis) of components.

/pdf/molecular-characterization-of-free-tropospheric-aerosol-4763n7fdhy.pdf

Molecular characterization of free tropospheric aerosol collected at the Pico Mountain Observatory: A case study with a long-range transported biomass burning plume

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Morphology and mixing state of aged soot particles at a remote marine free troposphere site: Implications for optical properties

Aerosol measurements in the free troposphere at the North Atlantic Pico Mountain Observatory in the Azores

Molecular characterization of free tropospheric aerosol collected at the Pico Mountain Observatory: A case study with a long-range transported biomass burning plume