Coarse Aerosol Particles Research Articles

Analyzing Morphology and Composition of Coarse Aerosol Particles in Bangkok, Thailand: Implications to Sources and Impacts of Aged Aerosols on Ecosystem Health and Climate Dynamics

During dry seasons, elevated aerosol levels across Thailand pose nationwide problems. Understanding and addressing this issue is challenging due to the dynamic nature of aerosol modification and generation during transport. This study investigates the morphology and elemental compositions of coarse aerosol particles in Bangkok, Thailand, during the dry seasons of 2020/21. Through scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectrometry (EDS), the study reveals a complex mixture of anthropogenic, mineral, biogenic, and marine aerosols. Anthropogenic sources, including biomass burning, vehicular emissions, and industrial activities, contribute to carbonaceous particles like soot aggregates and tar balls. Mineral dust particles, predominantly calcium-rich and aluminosilicate, originate from various sources including construction, industry, and natural processes. Aging processes alter the composition and properties of both carbonaceous and mineral particles, influencing nutrient deposition, carbon sequestration, cloud condensation nuclei formation, and light scattering. These processes have multifaceted impacts on ecosystem health and climate dynamics, highlighting the need for further research and mitigation strategies to address the environmental consequences of aged aerosol particles in urban environments like Bangkok.

Observing super-coarse carbonaceous aerosol particles containing chloride in a tropical savanna climate at an agro-forest site in Thailand

Coarse aerosol particles containing chloride in tropical forests are significant for understanding biogeochemical cycles and atmospheric processes, with implications for environmental health and climate change mitigation. This study explored the sources of super-coarse carbonaceous aerosol particles containing chloride in a tropical savanna climate. Aerosol samples were collected from an agro-forest site in Thailand during the dry season and analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier-transform infrared (FTIR) spectroscopy. By examining the morphology and elemental compositions of individual aerosol particles, along with employing Positive Matrix Factorization (PMF) and backward trajectory analysis, potential sources were identified. The findings revealed two primary sources for the super-coarse aerosol particles: a mixture of biomass burning smoke and inorganic salts (likely from saline soil and sea salt), as well as halophilic fungal spores. FTIR analysis indicated the presence of compounds linked to biomass burning and clay minerals, influenced by prevailing northeast and southeast winds. Recommendations for future research include continued monitoring, correlation with meteorological parameters, and the application of transmission electron microscopy (TEM) for more detailed visualization and confirmation of aerosol sources.

The Cloud Indicator: A novel algorithm for automatic detection and classification of clouds using airborne in situ observations

Airborne in situ observations of aerosols and clouds provide relevant data sets crucial to improving the understanding of cloud microphysical processes and reducing uncertainties connected to future climate predictions. However, the selection and classification of cloud sequences in such data sets can be very time-consuming if done manually, and criteria used by the community are numerous and prone to misclassification, especially when coarse aerosol particles (> 1 μm diameter) are present.In this study, we present the Cloud Indicator, a novel algorithm that automatically detects and classifies measurement periods inside clouds. The Cloud Indicator was developed using data from three international airborne field campaigns, including ATom (Atmospheric Tomography; 2016–2018), A-LIFE (Absorbing aerosol layers in a changing climate: aging, lifetime and dynamics; 2017), and FIREX-AQ (Fire Influence on Regional to Global Environments Experiment and Air Quality; 2019). The algorithm utilizes size distribution measurements, combined with measurements of relative humidity and temperature, to automatically detect flight sequences in clouds and classify the cloud type. As an additional criterion for the Cloud Indicator, we established the cloud-aerosol volume factor fCA to ensure a precise and robust distinction between clouds and aerosol layers such as mineral dust or biomass burning thereby reducing misclassifications. The Cloud Indicator algorithm was developed with data from a second-generation Cloud, Aerosol, and Precipitation Spectrometer (CAPS) which was calibrated with a novel calibration procedure – introduced in this study - that results in a refractive index independent calibration and works also in the field. However, the Cloud Indicator algorithm is not restricted specifically to the CAPS and thus allows its application to a variety of in situ instruments that measure coarse aerosol and cloud particle size.Case studies from ATom and A-LIFE demonstrate the ability of the Cloud Indicator to precisely screen (airborne) in situ data sets for clouds. The algorithm separates the data set into cloud-free periods, Aerosol-Cloud Transition Regime (ACTR), liquid clouds, clouds in the Mixed-Phase Temperature Regime (MPTR), and cirrus clouds. The unique ability of the Cloud Indicator to successfully differentiate between layers of enhanced coarse-mode aerosol concentrations and clouds is demonstrated by measurements in a complex mixture of clouds embedded into a mineral dust layer during the A-LIFE. The empirically derived parameter thresholds of the Cloud Indicator are in good agreement with values in the literature.

Characteristics of aerosol at the research base “Ice Cape Baranova” in 2018–2023

Atmospheric aerosol plays an important role in the processes of radiative transfers and mass exchange by different substances in the “continent–atmosphere–ocean” system. In this paper we discuss the results of a five-year measurement cycle of the atmospheric aerosol characteristics at the polar station “Ice base Cape Baranov”, located on the Bolshevik Island (the Severnaya Zemlya Archipelago). The set of the characteristics analyzed includes: the aerosol optical depth (AOD) of the atmosphere; the ground concentration of aerosol particles in the radius range of 0.15–5microns; the content of the absorbing substance (soot) in the aerosol in the equivalent of elemental black carbon. The average values of the aerosol characteristics for the general measurement period (from April 2018 to May 2023) were: volumes of submicron and coarse aerosol particles 0.43 and 0.46 μm3/cm3, respectively; mass concentration of black carbon — 45.8 ng/m3; AOT of the atmosphere at a wavelength of 0.5 µm — 0.08; Angstrom selectivity exponent — 1.67. The average annual variation of aerosol and black carbon concentrations is characterized by a maximum in winter (January–March) and a minimum in summer (June–August). A comparison is made against the data from analogous measurements of aerosol characteristics at the polar station in Barentsburg (the Spitsbergen Archipelago) and against the data from model calculations, i. e., MERRA-2 reanalysis. A distinctive feature of the data in the Cape Baranov area is the low content of coarse aerosol — 1.7 less than in Barentsburg. There is agreement with the annual variation of black carbon concentrations at other polar stations, but the opposite nature of the seasonal variability of model (MERRA-2) concentrations: low values in winter and high values in summer. It is shown that the average spectral AODs of the atmosphere at the “Cape Baranov” are intermediate values between the data from polar stations in NyÅlesund and Barentsburg.

Measurement report: Stoichiometry of dissolved iron and aluminum as an indicator of the factors controlling the fractional solubility of aerosol iron – results of the annual observations of size-fractionated aerosol particles in Japan

Abstract. The atmospheric deposition of iron (Fe) promotes primary production in the surface ocean, which results in the enhanced uptake of carbon dioxide into surface seawater. Given that microorganisms in seawater utilize dissolved Fe (d-Fe) as a nutrient, the bioavailability of Fe in aerosol particles depends on its solubility. However, the factors controlling fractional Fe solubility (Fesol %) in aerosol particles have not been fully understood. This study performed annual observations of the total and dissolved metal concentrations in size-fractionated (seven fractions) aerosol particles at Higashi-Hiroshima, Japan. The feasibility of the molar concentration ratio of d-Fe relative to dissolved Al ([d-Fe] / [d-Al]) as an indicator of sources of d-Fe in aerosol particles was investigated because this ratio is likely dependent on the emission sources of Fe (e.g., mineral dust, fly ash, and anthropogenic Fe oxides) and their dissolution processes (proton- and ligand-promoted dissolutions). Approximately 70 % of the total Fe in total suspended particulates (TSPs) was present in coarse aerosol particles, whereas about 70 % of d-Fe in TSPs was mainly found in fine aerosol particles. The average Fesol % in fine aerosol particles (11.4 ± 7.0 %) was higher than that of coarse aerosol particles (2.19 ± 2.27 %). In addition, the average ratio of [d-Fe] / [d-Al] in coarse aerosol particles (0.408 ± 0.168) was lower than that in fine aerosol particles (1.15 ± 0.80). The range of [d-Fe] / [d-Al] ratios in the coarse aerosol particles (0.121–0.927) was similar to that obtained by proton-promoted dissolution of mineral dust (0.1–1.0), which indicates that the d-Fe in coarse aerosol particles was derived from mineral dust. The [d-Fe] / [d-Al] ratios of fine aerosol particles ranged from 0.386 to 4.67, and [d-Fe] / [d-Al] ratios greater than 1.50 cannot be explained by proton- and ligand-promoted dissolutions (1.00 < [d-Fe] / [d-Al] < 1.50). The [d-Fe] / [d-Al] ratio correlated with the enrichment factor of Fe in fine aerosol particles (r: 0.505), which indicates that anthropogenic Fe with a high [d-Fe] / [d-Al] ratio was the source of d-Fe in fine aerosol particles. The high [d-Fe] / [d-Al] ratio was attributed to anthropogenic Fe oxides emitted from high-temperature combustions. Finally, the fraction of anthropogenic Fe oxides to d-Fe in TSPs was calculated based on the [d-Fe] / [d-Al] ratio of aerosols and their emission source samples. As a result, the fraction of anthropogenic Fe oxides to d-Fe in TSPs varied from 1.48 % to 80.7 %. A high fraction was observed in summer when air masses originated from industrial regions in Japan. By contrast, approximately 10 % of d-Fe in the TSPs collected in spring and during Asian dust events was derived from anthropogenic Fe oxides when air masses were frequently transported from East Asia to the Pacific Ocean. Thus, mineral dust was the dominant source of d-Fe in Asian outflow to the Pacific Ocean.

A blended TROPOMI+GOSAT satellite data product for atmospheric methane using machine learning to correct retrieval biases

Abstract. Satellite observations of dry-column methane mixing ratios (XCH4) from shortwave infrared (SWIR) solar backscatter radiation provide a powerful resource to quantify methane emissions in service of climate action. The TROPOspheric Monitoring Instrument (TROPOMI), launched in October 2017, provides global daily coverage at a 5.5 × 7 km2 (nadir) pixel resolution, but its methane retrievals can suffer from biases associated with SWIR surface albedo, scattering from aerosols and cirrus clouds, and across-track variability (striping). The Greenhouse gases Observing SATellite (GOSAT) instrument, launched in 2009, has better spectral characteristics and its methane retrieval is much less subject to biases, but its data density is 250 times sparser than TROPOMI. Here, we present a blended TROPOMI+GOSAT methane product obtained by training a machine learning (ML) model to predict the difference between TROPOMI and GOSAT co-located measurements, using only predictor variables included in the TROPOMI retrieval, and then applying the correction to the complete TROPOMI record from April 2018 to present. We find that the largest corrections are associated with coarse aerosol particles, high SWIR surface albedo, and across-track pixel index. Our blended product corrects a systematic difference between TROPOMI and GOSAT over water, and it features corrections exceeding 10 ppb over arid land, persistently cloudy regions, and high northern latitudes. It reduces the TROPOMI spatially variable bias over land (referenced to GOSAT data) from 14.3 to 10.4 ppb at a 0.25∘ × 0.3125∘ resolution. Validation with Total Carbon Column Observing Network (TCCON) ground-based column measurements shows reductions in variable bias compared with the original TROPOMI data from 4.7 to 4.4 ppb and in single-retrieval precision from 14.5 to 11.9 ppb. TCCON data are all in locations with a SWIR surface albedo below 0.4 (where TROPOMI biases tend to be relatively low), but they confirm the dependence of TROPOMI biases on SWIR surface albedo and coarse aerosol particles, as well as the reduction of these biases in the blended product. Fine-scale inspection of the Arabian Peninsula shows that a number of hotspots in the original TROPOMI data are removed as artifacts in the blended product. The blended product also corrects striping and aerosol/cloud biases in single-orbit TROPOMI data, enabling better detection and quantification of ultra-emitters. Residual coastal biases can be removed by applying additional filters. The ML method presented here can be applied more generally to validate and correct data from any new satellite instrument by reference to a more established instrument.

In-situ analysis of combustion aerosol using a supercontinuum lidar.

We report real-time monitoring of coarse aerosol particle distribution in a 9 m wide full-scale industrial boiler using a broadband supercontinuum lidar. The technique utilizes the light backscattered from the aerosol to map the extinction profile using the Klett inversion method, with measured extinction values of 0.04 - 0.2 m-1 across the furnace. The technique further exploits differential absorption of water molecules in the 1.25 - 1.5 µm region to map the water vapor concentration profile in the furnace up to a distance of 3.9 m with a spatial resolution of 30 cm. We also take advantage of the strong reflection from the boiler back-wall to simultaneously measure the average water vapor temperature and concentration in the boiler in good agreement with reference readings from the boiler. Our results open novel perspectives for versatile 3D profiling of flue gas parameters and other industrial process analysis.

Metals and organic species associated with fine and coarse aerosol particles in an electronic waste recycling plant

Metals and organic species associated with fine and coarse aerosol particles in an electronic waste recycling plant

Physicochemical Characterization of Aerosols in the Coastal Zone: Evidence of Persistent Carbon Soot in the Marine Atmospheric Boundary Layer (MABL) Background

Aerosol particles in coastal areas result from a complex mixing between sea-spray aerosols locally generated at the sea surface by breaking waves and a continental component issued from natural and/or anthropogenic sources. The aim of this paper is to study how the aerosols mix in the coastal marine atmosphere to evaluate the impact of the background pollution on the atmospheric aerosols. To this end, we have carried out a qualitative analysis of particulate matter sampled at two French coastal areas using a non-destructive methodology combining scanning electron microscopy (SEM)/X-ray fluorescence, transmission electron microscopy (TEM), X-ray diffraction, and Raman spectroscopy. Our analysis shows a dominant contribution of anthropogenic aerosols through strong levels of submicronic carbon soot and sulfate particles, even observed when the aerosol is sampled during pure maritime-air mass episodes. Our results also evidence the non-mixing between sea-spray, mainly composed of coarse aerosol particles, and this anthropogenic particulate matter of smaller sizes.

Iron (Fe) speciation in size-fractionated aerosol particles in the Pacific Ocean: The role of organic complexation of Fe with humic-like substances in controlling Fe solubility

Abstract. Atmospheric deposition is one of the main sources of dissolved iron (Fe) in the ocean surfaces. Atmospheric processes are recognized as controlling fractional Fe solubility (Fesol%) in marine aerosol particles. However, the impact of these processes on Fesol% remains unclear. One of the reasons for this is the lack of field observations focusing on the relationship between Fesol% and Fe species in marine aerosol particles. In particular, the effects of organic ligands on Fesol% have not been thoroughly investigated in observational studies. In this study, Fe species in size-fractionated aerosol particles in the Pacific Ocean were determined using X-ray absorption fine structure (XAFS) spectroscopy. The internal mixing states of Fe and organic carbon were investigated using scanning transmission X-ray microscopy (STXM). The effects of atmospheric processes on Fesol% in marine aerosol particles were investigated based on the speciation results. Iron in size-fractionated aerosol particles was mainly derived from mineral dust, regardless of aerosol diameter, because the enrichment factor of Fe was almost 1 in both coarse (PM>1.3) and fine aerosol particles (PM1.3). Approximately 80 % of the total Fe (insoluble + labile Fe) was present in PM>1.3, whereas labile Fe was mainly present in PM1.3. The Fesol% in PM>1.3 was not significantly increased (2.56±2.53 %, 0.00 %–8.50 %, n=20) by the atmospheric processes because mineral dust was not acidified beyond the buffer capacity of calcite. In contrast, mineral dust in PM1.3 was acidified beyond the buffer capacity of calcite. As a result, Fesol% in PM1.3 (0.202 %–64.7 %, n=10) was an order of magnitude higher than that in PM>1.3. The PM1.3 contained ferric organic complexes with humic-like substances (Fe(III)-HULIS, but not Fe-oxalate complexes), and the abundance correlated with Fesol%. Iron(III)-HULIS was formed during transport in the Pacific Ocean because Fe(III)-HULIS was not found in aerosol particles in Beijing and Japan. The pH estimations of mineral dust in PM1.3 established that Fe was solubilized by proton-promoted dissolution under highly acidic conditions (pH < 3.0), whereas Fe(III)-HULIS was stabilized under moderately acidic conditions (pH 3.0–6.0). Since the observed labile Fe concentration could not be reproduced by proton-promoted dissolution under moderately acidic conditions, the pH of mineral dust increased after proton-promoted dissolution. The cloud process in the marine atmosphere increases the mineral dust pH because the dust particles are covered with organic carbon and Na. The precipitation of ferrihydrite was suppressed by Fe(III)-HULIS owing to its high water solubility. Thus, the organic complexation of Fe with HULIS plays a significant role in the stabilization of Fe that was initially solubilized by proton-promoted dissolution.

The effect of aerosol size on Fe solubility and deposition flux: A case study in the East China Sea

Aerosol sizes are highly associated with the solubilities and the deposition fluxes of aerosol Fe in the surface ocean since the sizes may reflect the sources and decide the deposition velocities. However, systematic studies for the association of the solubilities and fluxes have been limited. In this study, five size-fractions of dry aerosols were collected monthly for a year at two islets in the East China Sea, where large amounts of both fine anthropogenic and coarse lithogenic aerosols deposit. Both pure water and buffer leached methodologies were applied to determine the two operationally defined dissolvable Fe fractions, instantly dissolved Fe (DFe) and supposedly Fe-ligand complexed labile Fe (LFe), respectively. We found that the solubilities of DFe varied up to 4 orders of magnitude with the size spectrum and exhibited a highly linear correlation with non-sea-salt sulfur, indicating that the solubilities of DFe were closely associated with the acidity. Finer aerosols (PM 3) accounted for 90% of total DFe but coarser aerosols (>PM 3) contributed 66% of the difference between LFe and DFe (LFe-DFe). The increasing trend of the difference with increasing sizes indicates that the residence time of coarse aerosol particles and their interaction with Fe-ligands are critical factors deciding the total fluxes of LFe in the ocean. Considering the deposition velocities of each size of aerosols, the averaged fluxes of aerosol Fe of the fine and coarse aerosols were 1.8 and 5.9 nmol m−2 d−1 for DFe; and 2.8 and 62 nmol m−2 d−1 for LFe in the East China Sea, respectively. Attributed to the relatively low deposition velocities of fine aerosols, we found that either single or two averaged deposition velocities (fine/coarse) that were used in most of the previous studies would significantly overestimate dissolvable Fe fluxes in regions where the contribution of fine anthropogenic aerosols is dominant, such as the open ocean. In conclusion, this study demonstrates that aerosol sizes are essential and powerful parameters to accurately estimate the solubility and the fluxes of aerosol dissolvable Fe.

Chemical composition and radiative forcing of atmospheric aerosols over the high-altitude Western Himalayas of India

Aerosol behavior over the Himalayas plays an important role in the regional climate of South Asia. Previous studies at high-altitude observatories have provided evidence of the impact of long-range transport of pollutants from the Indo-Gangetic Plain (IGP). However, little information exists for the valley areas in the high Himalayas where significant local anthropogenic emissions can act as additional sources of short-living climate forcers and pollutants. The valley areas host most economic activities based on agriculture, forestry, and pilgrimage during every summer season. We report here first measurements at a valley site at ~2600 m a.s.l. on the trek to the Gangotri glacier (Gaumukh), in the Western Himalayas, where local infrastructures for atmospheric measurements are absent. The study comprised short-term measurement of aerosols, chemical characterization, and estimation of aerosol radiative forcing (ARF) during the winter and summer periods (2015-2016). The particulate matter mass concentrations were observed to be higher than the permissible limit during the summer campaigns. We obtained clear evidence of the impact of local anthropogenic sources: particulate nitrate is associated with coarse aerosol particles, the black carbon (BC) mass fraction appears undiluted with respect to measurements performed in the lower Himalayas, and in winter, both BC and sulfate concentrations in the valley site are well above the background levels reported from literature studies for mountain peaks. Finally, high concentrations of trace metals such as copper point to anthropogenic activities, including combustion and agriculture. While most studies in the Himalayas have addressed pollution in the high Himalayas in terms of transport from IGP, our study provides clear evidence that local sources cannot be overlooked over the high-altitude Himalayas. The estimated direct clear-sky ARF was estimated to be in the range of -0.1 to +1.6 W m-2, with significant heating in the atmosphere over the high-altitude Himalayan study site. These results indicate the need to establish systematic aerosol monitoring activities in the high Himalayan valleys.

The role of coarse aerosol particles as a sink of HNO<sub>3</sub> in wintertime pollution events in the Salt Lake Valley

Abstract. Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH3, HNO3, HCl, and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl−, and SO42- were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3-) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3- was nearly 3 times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3- and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse-mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO3- : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution.

Statistical aerosol properties associated with fire events from 2002 to 2019 and a case analysis in 2019 over Australia

Abstract. Wildfires are an important contributor to atmospheric aerosols in Australia and could significantly affect the regional and even global climate. This study investigates the impact of fire events on aerosol properties along with the long-range transport of biomass-burning aerosol over Australia using multi-year measurements from Aerosol Robotic Network (AERONET) at 10 sites over Australia, a satellite dataset derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), reanalysis data from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), and back-trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The fire count, fire radiative power (FRP), and aerosol optical depth (AOD) showed distinct and consistent interannual variations, with high values during September–February (biomass-burning period, BB period) and low values during March–August (non-biomass-burning period, non-BB period) every year. Strong correlation (0.62) was found between FRP and AOD over Australia. Furthermore, the correlation coefficient between AOD and fire count was much higher (0.63–0.85) during October–January than other months (−0.08 to 0.47). Characteristics of Australian aerosols showed pronounced differences between the BB period and non-BB period. AOD values significantly increased and fine-mode aerosol dominated during the BB period, especially in northern and southeastern Australia. Carbonaceous aerosol was the main contributor to total aerosols during the BB period, especially in September–December when carbonaceous aerosol contributed the most (30.08 %–42.91 %). Aerosol size distributions showed a bimodal character, with both fine and coarse aerosol particles generally increasing during the BB period. The megafires during the BB period of 2019/2020 further demonstrated the significant impact of wildfires on aerosol properties, such as the extreme increase in AOD for most of southeastern Australia, the dominance of fine particle aerosols, and the significant increase in carbonaceous and dust aerosols in southeastern and central Australia, respectively. Moreover, smoke was found to be the dominant aerosol type detected at heights from 2.5 to 12 km in southeastern Australia in December 2019 and at heights from roughly 6.2 to 12 km in January 2020. In contrast, dust was detected more frequently at heights from 2 to 5 km in November 2019 and January and February 2020. A case study emphasized that the transport of biomass-burning aerosols from wildfire plumes in eastern and southern Australia significantly impacted the aerosol loading, aerosol particle size, and aerosol type of central Australia.

Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

Abstract. A small fraction of freezing cloud droplets probably initiates much of the precipitation above continents. Only a minute fraction of aerosol particles, so-called ice-nucleating particles (INPs), can trigger initial ice formation at −15 ∘C, at which cloud-top temperatures are frequently associated with snowfall. At a mountaintop site in the Swiss Alps, we found that concentrations of INPs active at −15 ∘C can be parameterised by different functions of coarse (> 2 µm) aerosol particle concentrations, depending on whether an air mass is (a) precipitating, (b) non-precipitating, or (c) carrying a substantial fraction of dust particles while non-precipitating. Consequently, we suggest that a parameterisation at moderate supercooling should consider coarse particles in combination with air mass differentiation.

Relationships between Immersion Freezing and Crystal Habit for Arctic Mixed-Phase Clouds—A Numerical Study

AbstractThe number concentration of ice particles in Arctic mixed-phase clouds is a major controlling factor of cloud lifetime. The relationships between ice nucleation mode and ice crystal habit development are not yet constrained by observations. This study uses a habit-predicting microphysical scheme within a 3D large-eddy simulation model to evaluate the relationship between immersion freezing and ice habit in a simulated Arctic mixed-phase cloud case. Three immersion freezing parameterizations are considered: a volume-dependent freezing scheme (VF), a parameterization limited to activated droplets (C-AC), and a parameterization limited to coarse aerosol particles (C-CM). Both C-AC and C-CM are based on classical nucleation theory. The freezing rate with VF is found to be greater in downdraft regions than in updraft regions due to the downdraft having a higher number concentration of large droplets. The C-AC cases show active freezing of small droplets near cloud top, whereas in the C-CM cases, mainly the 8–32-μm-sized droplets freeze in updraft regions near the cloud base. Because the initial crystal size is assumed to affect the axis ratio of hexagonal plates, the VF cases produce crystals with larger axis ratios, resulting in smaller mode radii than the C-AC cases. In all cases, irregular polycrystals dominate near cloud top and a band-like structure develops within the cloud, which qualitatively agrees with previous observations. In the VF and C-CM cases, unactivated large droplets arising from coarse-mode aerosol particles contributed significantly to the freezing rate, producing an important influence on crystal habit.

Characterization of carbonaceous aerosols over the Northern Adriatic Sea in the JERICO-NEXT project framework

The coastal aerosol represents a complex mixing between sea spray aerosols locally generated at the sea surface and a component generated by natural and/or anthropogenic sources on the nearby land. The aim of this paper is to study the atmospheric concentrations of carbonaceous compounds as measured in the coastal zone of the Northern Adriatic Sea between April 2017 and April 2018. The results show a quite large contribution of carbonaceous compounds, even when the sampled air mass is predominantly maritime. Although it is generally assumed that 70% and 90% of bulk organic carbon (OC) and elemental carbon (EC) concentrations, respectively, are found in the fine mode, we observed higher OC and EC concentrations in the coarse fraction (up to about 37% and 25%, respectively). This confirms the occurrence of mixing between anthropogenic matter and sea spray, mainly composed of coarse aerosol particles, during the atmospheric transport over the Mediterranean basin.

Variations in Aerosol Microstructure under the Influence of Smokes from Forest Fires according to Inversion of Spectral Extinction Characteristics in the Near-Surface Layer and in Vertical Atmospheric Column

We studied the specific features of aerosol microstructure in the near-surface layer and in the vertical column of the atmosphere after smoke from fires enters the atmosphere. The aerosol microstructure parameters were determined by solving the inverse problem for spectral measurements of the aerosol extinction coefficient by the path method and aerosol optical depth by the sun photometry method in a clean atmosphere and during its pollution by smoke near Tomsk. The geometric cross section, volume concentration, and mean radius of submicron and coarse aerosol particles were calculated. The coarse fraction is typically predominant in the total volume of aerosol under the background conditions. The submicron fraction of aerosol particles becomes dominant after air is polluted by smoke. As a result, the particle radius averaged over the total ensemble decreases in the smoke-polluted atmosphere. Also, the character of regression interrelations between the microstructure parameters of different aerosol fractions and the spectral characteristics of aerosol extinction significantly change after the atmosphere is polluted by smoke.

Lidar observations of Saharan dust loads above Sofia, Bulgaria: dust layers extending throughout the troposphere (a case study)

We report the lidar observations of a prolonged unusual event of atmospheric aerosol load extending up to about 15 km above the city of Sofia, Bulgaria. Based on the results obtained and air-transport modeling data, we show that the observed diffused aerosol layers consisted predominantly of Saharan mineral dust partly mixed with other aerosol types. We present retrieved time-averaged vertical profiles of the aerosol backscatter coefficient (at 532 and 1064 nm) in order to display the aerosol density height distribution, whereas the aerosol layering temporal dynamics is illustrated by color-coded maps of range-corrected lidar signals. We also calculated the profiles of the backscatter-related Angstrom exponent (BAE), which showed the domination of moderately coarse aerosol particle modes. Further, frequency count distributions of the BAE occurrences were obtained and analyzed statistically. Particular attention was paid to the revealed BAE distribution skewness, in view of better identifying and characterizing the aerosol size and composition changes. The results obtained enabled us to argue that the atmosphere above Sofia in the period considered had been strongly affected by intrusion of Saharan dust. Desert aerosols had perturbed or covered over the top of the atmospheric boundary layer and filled up the free troposphere, considerably influencing the local meteorological conditions. We also ascertained that aged volcanic aerosols had been likely present and detected at tropopause altitudes.

Evaluation and comparison of MODIS Collection 6.1 aerosol optical depth against AERONET over regions in China with multifarious underlying surfaces

In this study, we evaluated the performance of the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6.1 (C6.1) aerosol optical depth (AOD) products and compared them with the Collection 6 (C6) products over regions in China with multifarious underlying surfaces during 2001–2016. The AOD retrievals were validated against 20 AERONET (V3) sites, and the results show that the correlation coefficient (R) for dark target (DT) retrievals in C6.1 is 0.946, while the fraction within the expected error (EE) can be considered relatively low at only 54.03%. Deep blue (DB) retrievals in C6.1 have a slightly lower R value (0.931), but the other criteria are superior to DT. Comparing the results over urban and vegetation areas in C6.1, the overall quality of the DB retrievals is better than the DT retrievals in urban areas. The performance of DT is significantly superior to DB in the low elevation vegetation (LEV) areas. For the high elevation vegetation (HEV) areas, DB performs integrally better than DT. In the spatial distribution aspect in C6.1, most of the DB AOD values are less than those of DT, and the relationship between DT and DB varies with the land cover types and surface reflectance. For the AOD coverage in C6.1, DT retrievals with high coverage mainly distribute in east-central China. However, the effects of high surface reflectance lead to low AOD coverage in the southwest. In contrast, the AOD coverage of DB tends to be high in areas where the main land cover type is arid or semiarid and tends to be low in areas affected by snow. In terms of the comparison between C6.1 and C6, the overestimation of DT over urban areas in C6 is effectively mitigated in C6.1. However, a nearly systematic decline in DT is discovered in C6.1 as well. With respect to DB, consistent AOD coverage distribution is observed, with only subtle distinction. The AOD coverage of DB in C6.1 appears higher than that in C6 in the middle, south, and northeast of China. The quality of the DB retrievals in C6.1 increases slightly compared to C6, and the major improvements are observed for the coarse aerosol particles and for high elevation areas at surface reflectance from 0.11 to 0.2, respectively.