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

Particle hygroscopicity and its link to chemical composition in the urban atmosphere of Beijing, China, during summertime

Abstract: . Simultaneous measurements of particle number size distribution, particle hygroscopic properties, and size-resolved chemical composition were made during the summer of 2014 in Beijing, China. During the measurement period, the mean hygroscopicity parameters (κs) of 50, 100, 150, 200, and 250 nm particles were respectively 0.16 p 0.07, 0.19 p 0.06, 0.22 p 0.06, 0.26 p 0.07, and 0.28 p 0.10, showing an increasing trend with increasing particle size. Such size dependency of particle hygroscopicity was similar to that of the inorganic mass fraction in PM1. The hydrophilic mode (hygroscopic growth factor, HGF > 1.2) was more prominent in growth factor probability density distributions and its dominance of hydrophilic mode became more pronounced with increasing particle size. When PM2.5 mass concentration was greater than 50 μg m−3, the fractions of the hydrophilic mode for 150, 250, and 350 nm particles increased towards 1 as PM2.5 mass concentration increased. This indicates that aged particles dominated during severe pollution periods in the atmosphere of Beijing. Particle hygroscopic growth can be well predicted using high-time-resolution size-resolved chemical composition derived from aerosol mass spectrometer (AMS) measurements using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. The organic hygroscopicity parameter (κorg) showed a positive correlation with the oxygen to carbon ratio. During the new particle formation event associated with strongly active photochemistry, the hygroscopic growth factor or κ of newly formed particles is greater than for particles with the same sizes not during new particle formation (NPF) periods. A quick transformation from external mixture to internal mixture for pre-existing particles (for example, 250 nm particles) was observed. Such transformations may modify the state of the mixture of pre-existing particles and thus modify properties such as the light absorption coefficient and cloud condensation nuclei activation.

Effects of NO x and SO 2 on the secondary organic aerosol formation from photooxidation of α -pinene and limonene

Abstract: . Anthropogenic emissions such as NO x and SO 2 influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO x and SO 2 on the SOA formation from the photooxidation of α -pinene and limonene at ambient relevant NO x and SO 2 concentrations (NO x : 2 : < 0.05 to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO 2 induced nucleation and enhanced SOA mass formation. NO x strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO 2 which induced a high number concentration of particles after oxidation to H 2 SO 4 , the suppression of the mass yield of SOA by NO x was completely or partly compensated for. This indicates that the suppression of SOA yield by NO x was largely due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on and thus a significant influence of vapor wall loss on SOA mass yield. By compensating for the suppressing effect on nucleation of NO x , SO 2 also compensated for the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO x enhanced nitrate formation. The majority of the nitrate was organic nitrate (57–77 %), even in low-NO x conditions ( ∼ 1 ppb). Organic nitrate contributed 7–26 % of total organics assuming a molecular weight of 200 g mol −1 . SOA from α -pinene photooxidation at high NO x had a generally lower hydrogen to carbon ratio (H ∕ C), compared to low NO x . The NO x dependence of the chemical composition can be attributed to the NO x dependence of the branching ratio of the RO 2 loss reactions, leading to a lower fraction of organic hydroperoxides and higher fractions of organic nitrates at high NO x . While NO x suppressed new particle formation and SOA mass formation, SO 2 can compensate for such effects, and the combining effect of SO 2 and NO x may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.

Real-time measurements of ambient aerosols in a polluted Indian city: Sources, characteristics, and processing of organic aerosols during foggy and nonfoggy periods

Abstract: A detailed time-resolved chemical characterization of ambient nonrefractory submicron aerosols (NR-PM1) was conducted for the first time in India. The measurements were performed during the winter (November 2011 to January 2012) in a heavily polluted city of Kanpur, which is situated in the Indo-Gangetic Plain. Real-time measurements provided new insights into the sources and evolution of organic aerosols (OA) that could not be obtained using previously deployed filter-based measurements at this site. The average NR-PM1 loading was very high (>100 µg/m3) throughout the study, with OA contributing approximately 70% of the total aerosol mass. Source apportionment of the OA using positive matrix factorization revealed large contributions from fresh and aged biomass burning OA throughout the entire study period. A back trajectory analysis showed that the polluted air masses were affected by local sources and distant source regions where the burning of paddy residues occurs annually during winter. Several fog episodes were encountered during the study, and the OA composition varied between foggy and nonfoggy periods, with higher oxygen to carbon (O/C) ratios during the foggy periods. The evolution of OA and their elemental ratios (O:C and H:C) were investigated for the possible effects of fog processing.

Chemical characterization and toxicity of particulate matter emissions from roadside trash combustion in urban India

Abstract: Roadside trash burning is largely unexamined as a factor that influences air quality, radiative forcing, and human health even though it is ubiquitously practiced across many global regions, including throughout India. The objective of this research is to examine characteristics and redox activity of fine particulate matter (PM2.5) associated with roadside trash burning in Bangalore, India. Emissions from smoldering and flaming roadside trash piles (n = 24) were analyzed for organic and elemental carbon (OC/EC), brown carbon (BrC), and toxicity (i.e. redox activity, measured via the dithiothreitol “DTT” assay). A subset of samples (n = 8) were further assessed for toxicity by a cellular assay (macrophage assay) and also analyzed for trace organic compounds. Results show high variability of chemical composition and toxicity between trash-burning emissions, and characteristic differences from ambient samples. OC/EC ratios for trash-burning emissions range from 0.8 to 1500, while ambient OC/EC ratios were observed at 5.4 ± 1.8. Trace organic compound analyses indicate that emissions from trash-burning piles were frequently composed of aromatic di-acids (likely from burning plastics) and levoglucosan (an indicator of biomass burning), while the ambient sample showed high response from alkanes indicating notable representation from vehicular exhaust. Volume-normalized DTT results (i.e., redox activity normalized by the volume of air pulled through the filter during sampling) were, unsurprisingly, extremely elevated in all trash-burning samples. Interestingly, DTT results suggest that on a per-mass basis, fresh trash-burning emissions are an order of magnitude less redox-active than ambient air (13.4 ± 14.8 pmol/min/μgOC for trash burning; 107 ± 25 pmol/min/μgOC for ambient). However, overall results indicate that near trash-burning sources, exposure to redox-active PM can be extremely high.

Climate Forcing by Anthropogenic Aerosols

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.

The formation, properties and impact of secondary organic aerosol: current and emerging issues

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.

Evolution of Organic Aerosols in the Atmosphere

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.

Organic aerosol and global climate modelling: a review

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.

A single parameter representation of hygroscopic growth and cloud condensation nucleus activity

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.