Comparison study between nitrate and sulfate aerosols and their coating effect on the scattering properties of mineral aerosol

The existing numerical models generally employ sulfate instead of nitrate to simulate the scattering properties of aerosol, the corresponding radiative deviation needs to be evaluated urgently. Moreover, the sophisticated mixture of nitrate, sulfate, and mineral particles is formed through a series of chemical reactions, which makes it extremely challenging to understand the scattering properties of atmospheric aerosols. In this study, the core-shell ellipsoid model is used to flexibly characterize the morphology and mixed structures of nitrate, sulfate, and mineral in the fine mode radius range. The T-matrix method is used to compare the scattering properties of nitrate, sulfate, and mineral within different morphologies and mixing states at four selected wavelengths (0.44, 0.675, 0.87, and 1.02 μm). The results show that the difference of mean extinction efficiency factor (< Qe >) and mean single scattering albedo (< ω >) between nitrate and sulfate-containing particles is very small, mainly within 2%. However, their mean scattering phase function P11(θ) is quite different. The difference of forward scattering phase function P11(0) is up to 7%, while the difference of backward scattering phase function P11(π) can reach more than 25%. Overall, particle morphology and incident wavelength regulate the value of the optical parameters, whereas the coatings on the mineral play a more important role in drifting, but the differences between nitrate- and sulfate-containing particles are still very pronounced.

Apportioning PM1 in a contrasting receptor site in the Mediterranean region: Aerosol sources with an updated sulfur speciation

A multi-parametric experimental campaign was performed in Agri Valley (Basilicata, southern Italy) from July 2017 to January 2018. The investigated area, though basically rural and devoted to agricultural activities, hosts a huge on-shore oil reservoir, i.e. Centro Olio Val d'Agri (COVA), bringing substantial environmental modifications and impacts to the district landscape. Daily concentrations of PM₁ aerosol samples, Equivalent Black Carbon and number size distributions were evaluated. Chemical aerosol speciation based on elemental and ion analyses were carried out and source apportionment by Positive Matrix Factorization (PMF) was applied to reconstruct PM₁ source profile. The most significant emission sources found are torches from the oil treatment facility (37 % w/w), an unresolved factor constituted by soil resuspension, Saharan dust, and biomass burning (24 % w/w), ammonium sulphate (23 % w/w), emissions from the oil desulfurization (Claus process) (13 % w/w), and traffic + road dust (3 % w/w). SEM analysis on PM₁ single particles allowed to confirm the finding from PMF including the occurrence of elemental sulfur associated with the Claus process. The novelty of the present study consists in the identification of this latter fingerprint.

Strong Acids or Bases Displaced by Weak Acids or Bases in Aerosols: Reactions Driven by the Continuous Partitioning of Volatile Products into the Gas Phase

Aerosols are ubiquitous in the atmosphere and profoundly affect climate systems and human health. To gain more insights on their broad impacts, we need to comprehensively understand the fundamental properties of atmospheric aerosols. Since aerosols are multiphase, a dispersion of condensed matter (solid particles or liquid droplets, hereafter particles) in gas, partitioning of volatile matter between the condensed and the gas phases is one defining characteristic of aerosols. For example, water content partitioning under different relative humidity conditions, known as aerosol hygroscopicity, has been extensively investigated in the past decades. Meanwhile, partitioning of volatile organic or inorganic components, which is referred to as aerosol volatility, remains understudied. Commonly, a bulk solution system is treated as a single phase, with volatility mainly determined by the nature of its components, and the composition partitioning between solution and gas phase is limited. Aerosols, however, comprise an extensive gas phase, and their volatility can also be induced by component reactions. These reactions occurring within aerosols are driven by the formation of volatile products and their continuous partitioning into the gas phase. As a consequence, the overall aerosol systems exhibit prominent volatility. Noteworthily, such volatility induced by reactions is a phenomenon exclusively observed in the multiphase aerosol systems, and it is trivial in bulk solutions due to the limited extent of liquid-gas partitioning. Take the chloride depletion in sea salt particles as an example. Recent findings have revealed that chloride depletion can be caused by reactions between NaCl and weak organic acids, which release HCl into the gas phase. Such a reaction can be described as a strong acid displaced by a weak acid, which is hardly observed in bulk phase. Generally, this unique partitioning behavior of aerosol systems and its potential to alter aerosol composition, size, reactivity, and other physicochemical properties merits more attention by atmospheric community.This Account focuses on the recent advancements in the research of component reactions that induce aerosol volatility. These reactions can be categorized into four types: chloride depletion, nitrate depletion, ammonium depletion, and salt hydrolysis. The depletion of chloride or nitrate can be regarded as a displacement reaction, in which a strong acid is displaced by a weak acid. Such a reaction releases highly volatile HCl or HNO₃ into the gas phase and leads to a loss of chloride or nitrate within the particles. Likewise, ammonium depletion is a displacement reaction in which a strong base is displaced by a weak base, resulting in release of ammonia and substantial changes in aerosol hygroscopicity. In addition, aerosol volatility can also be induced by salt hydrolysis in a specific case, which is sustained by the coexistence of proton acceptor and hydroxide ion acceptor within particles. Furthermore, we quantitatively discuss these displacement reactions from both thermodynamic and kinetic perspectives, by using the extended aerosol inorganic model (E-AIM) and Maxwell steady-state diffusive mass transfer equation, respectively. Given the ubiquity of component partitioning in aerosol systems, our discussion may provide a new perspective on the underlying mechanisms of aerosol aging and relevant climate effects.

First X-ray Spectroscopic Observations of Atmospheric Titanium Species: Size Dependence and the Emission Source

Titanium dioxide (TiO2) in mineral dust is considered as one of the driving forces of photocatalytic reaction at the aerosol surface in the atmosphere. As a precursor of mineral dust, soil contains ilmenite (FeTiO3) and titanite (CaSiTiO5), which have lower photochemical reactivities than TiO2. However, Ti species other than TiO2 in aerosol particles are not well recognized due to the lack of observation in ambient samples. In this study, Ti species in size-fractionated aerosol samples collected in the Noto Peninsula, Japan, were determined by macroscopic and semi-microscopic X-ray absorption fine structure spectroscopy. Regardless of aerosol particle size, Ti species were primarily composed of rutile, anatase, ilmenite, and titanite. Semi-microscopic Ti speciation showed that Ti-poor spots associated with mineral dust were composed of a mixture of rutile, anatase, ilmenite, and titanite, and Ti-rich spots were primarily composed of TiO2 (rutile or anatase) derived from authigenic minerals or anthropogenic materials. Thus, the Ti species in aerosol particles, especially mineral dust, were not composed solely of TiO2 polymorphs. Therefore, the photochemical reactivities of Ti in aerosol particles may be overestimated when laboratory experiments or model studies employ TiO2 as the representative Ti species.

Reducing Planetary Health Risks Through Short-Lived Climate Forcer Mitigation

Global air pollution and climate change are major threats to planetary health. These threats are strongly linked through the short-lived climate forcers (SLCFs); ozone (O3), aerosols, and methane (CH4). Understanding the impacts of ambitious SLCF mitigation in different source emission sectors on planetary health indicators can help prioritize international air pollution control strategies. A global Earth system model is applied to quantify the impacts of idealized 50% sustained reductions in year 2005 emissions in the eight largest global anthropogenic source sectors on the SLCFs and three indicators of planetary health: global mean surface air temperature change (∆GSAT), avoided PM2.5-related premature mortalities and gross primary productivity (GPP). The model represents fully coupled atmospheric chemistry, aerosols, land ecosystems and climate, and includes dynamic CH4. Avoided global warming is modest, with largest impacts from 50% cuts in domestic (-0.085 K), agriculture (-0.034 K), and waste/landfill (-0.033 K). The 50% cuts in energy, domestic, and agriculture sector emissions offer the largest opportunities to mitigate global PM2.5-related health risk at around 5%-7% each. Such small global impacts underline the challenges ahead in achieving the World Health Organization aspirational goal of a 2/3 reduction in the number of deaths from air pollution by 2030. Uncertainty due to natural climate variability in PM2.5 is an important underplayed dimension in global health risk assessment that can vastly exceed uncertainty due to the concentration-response functions at the large regional scale. Globally, cuts to agriculture and domestic sector emissions are the most attractive targets to achieve climate and health co-benefits through SLCF mitigation.

Linear depolarization ratios of nitrate-coated mineral dust particles in haze episodes

This study reports an internal mixed particle model of dust and nitrate aerosols using the actual haze condition. We performed accurate calculations of linear depolarization ratios (LDR) of nitrate-coated mineral dust particles at three wavelengths (0.35, 0.53, and 1.06 µm) using the T-matrix method. The LDRs of the mono-disperse aerosol particles evolve differently as expressions in the Rayleigh and Mie domains. In the Rayleigh domain, the LDRs increase with the core-shell ratio and the aspect ratio and decrease when the wavelength increases. The forward and backward LDRs depend more on aspect ratio than on the core-shell ratio. In the Mie domain, the LDRs overall increase with the core-shell ratio and the aspect ratio, but there is no significant regular change. When the wavelength increases, the gradual change can be explained by the size parameter of the particles in the vicinity of the Rayleigh domain. For poly-disperse particles, the core-shell ratio mainly affects the position of the side-scattering peak, whereas aspect ratio affects the LDRs. The backscattering LDRs depend more on the variation of aspect ratio, and the core-shell ratio only affects LDRs in a small range. Furthermore, our results on the LDRs are highly promising for remote sensing of the non-spherical and inhomogeneous properties of fine aerosols compared with AERONET measurements. Our results provide a comprehensive understanding of the LDR evolution for coated non-spherical particles in a haze atmosphere. The LDRs can be used as an empirical reference for remote sensing to distinguish coated non-spherical particles.

Dust Heterogeneous Reactions during Long-Range Transport of a Severe Dust Storm in May 2017 over East Asia

Dust aerosol has important climate and environmental effects, which could be changed by internally mixing with anthropogenic aerosol as a result of heterogeneous reactions; however, the importance of these reactions is not fully understood yet. In this study, synergetic observations and an air quality model were used to analyze the transport of a severe dust storm and its impacts on nitrate and sulfate levels over East Asia between 3 and 11 May 2017. The model successfully reproduced the occurrence and transport of the dust storm compared to dust RGB imageries of the Himawari-8 satellite and dust extinction coefficients observed by LIDAR. The model also reasonably simulated the variations of observed nitrate and sulfate concentrations, and the results indicated that the dust heterogeneous reactions were dominant pathways for nitrate formation, but they had limited contribution for sulfate in both fine and coarse mode in Fukuoka, Japan. Dust nitrate formed rapidly after leaving China, and the highest period-averaged concentration of dust nitrate (>5 μg m−3) was shown over the Yellow Sea. Based on model results; we found that the mass ratio of dust nitrate to dust aerosol could reach 10% over the Pacific Ocean. Our results confirmed the importance of heterogeneous reactions on compositions of dust particles.

Influence of gaseous and particulate species on neutralization processes of polar aerosol and snow - A case study from Ny-Ålesund

The inter-conversion of nitrogen and sulfur species between the gas and particulate phases and their interaction with alkaline species influences the acidity of the aerosols and surface snow. To better understand these processes, a short field campaign was undertaken in Ny-Ålesund, Svalbard, during 13th April 2012 to 24th April 2012. Air measurements were carried out through a particulate sampler equipped with denuders and filter packs for simultaneous collection of trace gases (HNO₃, NO₂, SO₂ and reactive nitrogen compounds) and aerosols, with daily collection of snow samples. Ionic composition of the samples was analyzed using ion chromatography technique. The results suggested that nitrate-rich aerosols are formed when PAN (peroxy acetyl nitrate) disassociates to form NO₂ and HNO₃ which further hydrolyzes to form pNO₃^- (particulate nitrate). This resulted in a high contribution of pNO₃^- (62%) to the total nitrogen budget over the study area. The acidity of the aerosols and snow evaluated through cation/anion ratio (C/A) indicated alkaline conditions with C/A>2. The bicarbonates/carbonates of Mg²⁺ played an important role in neutralization processes of surface snow while the role of NH₃ was dominant in aerosol neutralization processes. Such neutralization processes can increase the aerosol hygroscopicity causing warming. Chloride depletion in the snow was significant as compared to the aerosols, indicating two important processes, scavenging of coarse sea salt by the snow and gaseous adsorption of SO₂ on the snow surface. However, a more systematic and long term study is required for a better understanding of the neutralization processes and chemical inter-conversions.

Characterisation of emission from open-field burning of crop residue during harvesting period in north-west India

Open-field crop residue burning is one of the important sources of atmospheric pollution in north-west India during the harvesting period. In this work, we studied NO₂ and SO₂ concentrations and physical and chemical properties of aerosols from open-field combustion of rice and wheat residue. NO₂ and SO₂ were analysed using UV-spectrophotometer and ion chromatography (IC) respectively. The aerosol particles were analysed by scan electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) for their physical dimension (size distribution) and elemental composition, and by IC for their ionic content. The measured concentrations of gases during burning showed rice straw burning spews more NO₂ and SO₂ than wheat straw burning. The calculated size of the particles ranged from 0.26 to 151.09 μm with high standard deviation. The median diameter of 1.64 μm (± 6.9) represented the central tendency of the particles emitted due to this combustion process. Comparative content analysis revealed that rice-borne particles are richer in Na, K, Al, Si and Zn, whereas, wheat-borne particles are more abundant in C, Mg, Fe, P and Cl. The results from IC and SEM-EDX evidenced the presence of fluoride, sulphate, carbonate, chloride, oxides and silicate compounds in particles. The emission of greenhouse gases (GHGs) and aerosols with this particle chemistry increases the atmospheric opacity through the absorption and scattering of incoming radiation at a significant amount in the UV-IR range causing high aerosol optical depth (AOD).

The formation and growth of calcium sulfate crystals through oxidation of SO2 by O3 on size-resolved calcium carbonate

Calcium sulfate is a major constituent of atmospheric sulfate, with a typical rod-like morphology ranging from several hundred nanometers to approximately two micrometers observed in field studies. However, the chemical formation mechanism is still not well known. In this study, the kinetics and mechanism for the formation and growth of rod-like calcium sulfate crystals through oxidation of SO2 by O3 on size-resolved CaCO3 at different relative humidity (RH) were investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and scanning electron microscopy (SEM). We found that the concentration and formation rate of sulfate decreased with the increasing diameter of CaCO3 particles, and thus smaller particles could enhance the formation of sulfate due to more reactive sites on smaller particles. The rod-like calcium sulfate crystals were formed only at RH above 60% and in the presence of reactant gases through the heterogeneous pathway. The liquid-like water layer formed by promotion of high RH in the presence of reactant gases could facilitate the formation and aggregation of calcium sulfate hydrates and thus promote the formation and growth of rod-like calcium sulfate crystals. This study provides a possible mechanism for the formation and growth of rod-like calcium sulfate crystals existing in the atmosphere.

Integrative analysis of desert dust size and abundance suggests less dust climate cooling

Desert dust aerosols affect Earth's global energy balance through interactions with radiation^1,2, clouds^3,4, and ecosystems⁵. But the magnitudes of these effects are so uncertain that it remains unclear whether atmospheric dust has a net warming or cooling effect on global climate^1,4,6. Consequently, it is still uncertain whether large changes in atmospheric dust loading over the past century have slowed or accelerated anthropogenic climate change^4,7-9, and the climate impact of possible future alterations in dust loading is similarly disputed^9,10. Here we use an integrative analysis of dust aerosol sizes and abundance to constrain the climatic impact of dust through direct interactions with radiation. Using a combination of observational, experimental, and model data, we find that atmospheric dust is substantially coarser than represented in current climate models. Since coarse dust warms global climate, the dust direct radiative effect (DRE) is likely less cooling than the ~0.4 W/m² estimated by models in a current ensemble^2,11-13. We constrain the dust DRE to - 0.20 (-0.48 to +0.20) W/m², which suggests that the dust DRE produces only about half the cooling that current models estimate, and raises the possibility that dust DRE is actually net warming the planet.

Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide

Heterogeneous reaction of SO2 on mineral dust seems to be an important sink for SO2. However, kinetic data about this reaction on authentic mineral dust are scarce and are mainly limited to low relative humidity (RH) conditions. In addition, little is known about the role of hydrogen peroxide (H2O2) in this reaction. Here, we investigated the uptake kinetics of SO2 on three authentic mineral dusts (i.e., Asian mineral dust (AMD), Tengger desert dust (TDD), and Arizona test dust (ATD)) in the absence and presence of H2O2 at different RHs using a filter-based flow reactor, and applied a parameter (effectiveness factor) to the estimation of the effective surface area of particles for the calculation of the corrected uptake coefficient (γc). We found that with increasing RH, the γc decreases on AMD particles, but increases on ATD and TDD particles. This discrepancy is probably due to the different mineralogy compositions and aging extents of these dust samples. Furthermore, the presence of H2O2 can promote the uptake of SO2 on mineral dust at different RHs. The probable explanations are that H2O2 rapidly reacts with SO2 on mineral dust in the presence of adsorbed water, and OH radicals, which can be produced from the heterogeneous decomposition of H2O2 on the mineral dust, immediately react with adsorbed SO2 as well. Our results suggest that the removal of SO2 via the heterogeneous reaction on mineral dust is an important sink for SO2 and has the potential to alter the physicochemical properties (e.g., ice nucleation ability) of mineral dust particles in the atmosphere.

Volcanic Ash versus Mineral Dust: Atmospheric Processing and Environmental and Climate Impacts

This review paper contrasts volcanic ash and mineral dust regarding their chemical and physical properties, sources, atmospheric load, deposition processes, atmospheric processing, and environmental and climate effects. Although there are substantial differences in the history of mineral dust and volcanic ash particles before they are released into the atmosphere, a number of similarities exist in atmospheric processing at ambient temperatures and environmental and climate impacts. By providing an overview on the differences and similarities between volcanic ash and mineral dust processes and effects, this review paper aims to appeal for future joint research strategies to extend our current knowledge through close cooperation between mineral dust and volcanic ash researchers.

Comparison of experimental and modeled absorption enhancement by black carbon (BC) cored polydisperse aerosols under hygroscopic conditions

The quantification of the radiative impacts of light absorbing ambient black carbon (BC) particles strongly depends on accurate measurements of BC mass concentration and absorption coefficient (β(abs)). In this study, an experiment has been conducted to quantify the influence of hygroscopic growth of ambient particles on light absorption. Using the hygroscopic growth factor (i.e., Zdanovskii-Stokes-Robinson (ZSR) approach), a model has been developed to predict the chemical composition of particles based on measurements, and the absorption and scattering coefficients are derived using a core-shell assumption with light extinction estimates based on Mie theory. The estimated optical properties agree within 7% for absorption coefficient and 30% for scattering coefficient with that of measured values. The enhancement of absorption is found to vary according to the thickness of the shell and BC mass, with a maximum of 2.3 for a shell thickness of 18 nm for the particles. The findings of this study underline the importance of considering aerosol-mixing states while calculating their radiative forcing.