CCN closure study: Effects of aerosol chemical composition and mixing state

TL;DR: In this article, the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols were investigated during the winter season in Kanpur.

Abstract: This study presents a detailed cloud condensation nuclei (CCN) closure study that investigates the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols. Measurements of the chemical composition, aerosol size distribution, total number concentration, and CCN concentration at supersaturation (SS = 0.2–1.0%) were performed during the winter season in Kanpur, India. Among the two cases considered here, better closure results are obtained for case 1 (low total aerosol loading, 49.54 ± 26.42 μg m−3, and high O:C ratio, 0.61 ± 0.07) compared to case 2 (high total aerosol loading, 101.05 ± 18.73 μg m−3, and low O:C ratio, 0.42 ± 0.06), with a maximum reduction of 3–81% in CCN overprediction for all depleted SS values (0.18–0.60%). Including the assumption that less volatile oxidized organic aerosols represent the soluble organic fraction reduced the overprediction to at most 40% and 129% in the internal and external mixing scenarios, respectively. At higher depleted SS values (0.34–0.60%), size-resolved chemical composition with an internal mixing state performed well in CCN closure among all organic solubility scenarios. However, at a lower depleted SS value (0.18%), closure is found to be more sensitive to both the chemical composition and mixing state of aerosols. At higher SS values, information on the solubility of organics and size-resolved chemical composition is required for accurate CCN predictions, whereas at lower SS values, information on the mixing state in addition to the solubility of organics and size-resolved chemical composition is required. Overall, κtotal values are observed to be independent of the O:C ratio [κtotal = (0.36 ± 0.01) × O:C − (0.03 ± 0.01)] in the range of 0.2

TL;DR: In this article, the mixing state index (χ) was used to quantify the degree of external and internal mixing of aerosol populations, and combined with particle-resolved model simulations to quantify error in predicted cloud condensation nuclei (CCN) predictions when mixing state information is neglected.

Abstract: . It is commonly assumed that models are more prone to errors in predicted cloud condensation nuclei (CCN) concentrations when the aerosol populations are externally mixed. In this work we investigate this assumption by using the mixing state index (χ) proposed by Riemer and West (2013) to quantify the degree of external and internal mixing of aerosol populations. We combine this metric with particle-resolved model simulations to quantify error in CCN predictions when mixing state information is neglected, exploring a range of scenarios that cover different conditions of aerosol aging. We show that mixing state information does indeed become unimportant for more internally mixed populations, more precisely for populations with χ larger than 75 %. For more externally mixed populations (χ below 20 %) the relationship of χ and the error in CCN predictions is not unique and ranges from lower than −40 % to about 150 %, depending on the underlying aerosol population and the environmental supersaturation. We explain the reasons for this behavior with detailed process analyses.

TL;DR: While particle collection for Toxicological purposes by the ESP is somewhat inferior to a direct aerosol-into-liquid collection, the ESP performs equally well, if not better, than conventional filter samplers and can be utilized as a simple and adequately efficient PM collector for toxicological studies.

TL;DR: In this article, the authors presented the first time-resolved estimates of hygroscopicity parameter and cloud condensation nuclei (CCN), spanning for more than a year, derived from chemical composition and size distribution data.

Abstract: . Delhi is a megacity subject to high local anthropogenic emissions and long-range transport of pollutants. This work presents for the first time time-resolved estimates of hygroscopicity parameter ( κ ) and cloud condensation nuclei (CCN), spanning for more than a year, derived from chemical composition and size distribution data. As a part of the Delhi Aerosol Supersite (DAS) campaign, the characterization of aerosol composition and size distribution was conducted from January 2017 to March 2018. Air masses originating from the Arabian Sea (AS), Bay of Bengal (BB), and southern Asia (SA) exhibited distinct characteristics of time-resolved sub-micron non-refractory PM 1 (NRPM 1 ) species, size distributions, and CCN number concentrations. The SA air mass had the highest NRPM 1 loading with high chloride and organics, followed by the BB air mass, which was more contaminated than AS, with a higher organic fraction and nitrate. The primary sources were identified as biomass-burning, thermal power plant emissions, industrial emissions, and vehicular emissions. The average hygroscopicity parameter ( κ ), calculated by the mixing rule, was approximately 0.3 (varying between 0.13 and 0.77) for all the air masses ( 0.32±0.06 for AS, 0.31±0.06 for BB, and 0.32±0.10 for SA). The diurnal variations in κ were impacted by the chemical properties and thus source activities. The total, Aitken, and accumulation mode number concentrations were higher for SA, followed by BB and AS. The mean values of estimated CCN number concentration ( NCCN ; 3669–28926 cm −3 ) and the activated fraction ( af ; 0.19–0.87), for supersaturations varying from 0.1 % to 0.8 %, also showed the same trend, implying that these were highest in SA, followed by those in BB and then those in AS. The size turned out to be more important than chemical composition directly, and the NCCN was governed by either the Aitken or accumulation modes, depending upon the supersaturation (SS) and critical diameter ( Dc ). af was governed mainly by the geometric mean diameter (GMD), and such a high af ( 0.71±0.14 for the most dominant sub-branch of the SA air mass – R1 – at 0.4 % SS) has not been seen anywhere in the world for a continental site. The high af was a consequence of very low Dc (25–130 nm, for SS ranging from 0.1 % to 0.8 %) observed for Delhi. Indirectly, the chemical properties also impacted CCN and af by impacting the diurnal patterns of Aitken and accumulation modes, κ and Dc . The high-hygroscopic nature of aerosols, high NCCN , and high af can severely impact the precipitation patterns of the Indian monsoon in Delhi, impact the radiation budget, and have indirect effects and need to be investigated to quantify this impact.

TL;DR: New insights are provided into the combined effects of OA loading and fog aqueous processing on the evolution of ambient organic aerosols (OA) for the first time and Fog episodes (n=17) studied here seem to be reducing the magnitude of the negative impact of Oa loading on O/C ratio.

TL;DR: Insight is provided into the seasonal variations of OA sources and evolution in a very polluted and complex environment with significant changes in types and relative contribution from oxidized OAs (OOA).

TL;DR: The aerosol forcing has likely offset global greenhouse warming to a substantial degree, however, differences in geographical and seasonal distributions of these forcings preclude any simple compensation.

Abstract: Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be –1 to –2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.

TL;DR: In this article, an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and analytical techniques used to determine the chemical composition of SOA is presented.

Abstract: Secondary organic aerosol (SOA) accounts for a significant fraction of ambient tropospheric aerosol and a detailed knowledge of the formation, properties and transformation of SOA is therefore required to evaluate its impact on atmospheric processes, climate and human health. The chemical and physical processes associated with SOA formation are complex and varied, and, despite considerable progress in recent years, a quantitative and predictive understanding of SOA formation does not exist and therefore represents a major research challenge in atmospheric science. This review begins with an update on the current state of knowledge on the global SOA budget and is followed by an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and the analytical techniques used to determine the chemical composition of SOA. A survey of recent laboratory, field and modeling studies is also presented. The following topical and emerging issues are highlighted and discussed in detail: molecular characterization of biogenic SOA constituents, condensed phase reactions and oligomerization, the interaction of atmospheric organic components with sulfuric acid, the chemical and photochemical processing of organics in the atmospheric aqueous phase, aerosol formation from real plant emissions, interaction of atmospheric organic components with water, thermodynamics and mixtures in atmospheric models. Finally, the major challenges ahead in laboratory, field and modeling studies of SOA are discussed and recommendations for future research directions are proposed.

TL;DR: A unifying model framework describing the atmospheric evolution of OA that is constrained by high–time-resolution measurements of its composition, volatility, and oxidation state is presented, which can serve as a basis for improving parameterizations in regional and global models.

Abstract: Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high-time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

TL;DR: In this article, the authors reviewed existing knowledge with regard to organic aerosol (OA) of importance for global climate modelling and defined critical gaps needed to reduce the involved uncertainties, and synthesized the information to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosols.

Abstract: The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

TL;DR: In this paper, a method to describe the relationship between particle dry diameter and cloud condensation activity using a single hygroscopicity parameter is presented. But this method is limited to single and multi-component particles with varying amounts of inorganic, organic and surface active compounds.

Abstract: We present a method to describe the relationship between particle dry diameter and cloud condensation nu- clei (CCN) activity using a single hygroscopicity parameter . Values of the hygroscopicity parameter are between 0.5 and 1.4 for highly-CCN-active salts such as sodium chlo- ride, between 0.01 and 0.5 for slightly to very hygroscopic organic species, and 0 for nonhygroscopic components. Ob- servations indicate that atmospheric particulate matter is typ- ically characterized by 0.1<< 0.9. If compositional data are available and if the hygroscopicity parameter of each com- ponent is known, a multicomponent hygroscopicity parame- ter can be computed by weighting component hygroscopic- ity parameters by their volume fractions in the mixture. In the absence of information on chemical composition, exper- imental data for complex, multicomponent particles can be fitted to obtain the hygroscopicity parameter. The hygroscop- icity parameter can thus also be used to conveniently model the CCN activity of atmospheric particles, including those containing insoluble components. We confirm the applica- bility of the hygroscopicity parameter and its mixing rule by applying it to published hygroscopic diameter growth fac- tor and CCN-activation data for single- and multi-component particles containing varying amounts of inorganic, organic and surface active compounds. We suggest that may be fit to CCN data assuming s/a=0.072 J m 2 and present a table of derived for this value and T=298.15 K. The predicted hygroscopicities for mixtures that contain the surfactant ful- vic acid agree within uncertainties with the measured values. It thus appears that this approach is adequate for predict- ing CCN activity of mixed particles containing surface ac- tive materials, but the generality of this assumption requires further verification.