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Light scattering by kaolinite dust samples at 532 nm is studied using a newly developed laboratory apparatus. During the experiments, dust samples are suspended in water, aerosolized by a nebulizer, and then injected into the scattering zone, with or without going through a diffusion drier, to generate either dried dust particles or water droplets with dust inclusions. The light source is a dual wavelength (532 and 1064 nm) diode-pumped solid state laser. Light scattered by an ensemble of particles is collected by a charge-coupled device (CCD) camera, which is mounted on the rotating arm of a stepper motor. The stepper motor rotates the CCD to cover the scattering angle range from 3° to 177°. Polarized scattering light is measured for the horizontally and vertically polarized incident light. The apparatus is calibrated, using pure water droplets as the scattering media. The response function with respect to the scattering angle is obtained by comparing the measurements with Lorenz-Mie calculations and then used in the later data analysis. Measurements show that the backward scattering features of the water droplets are smoothened due to their dust inclusions. Numerical simulations and measurements are extensively compared and discussed. It is found that the Lorenz-Mie theory is inadequate to reproduce the scattering phase functions of either dust particles or water droplets with dust inclusions. A nonspherical aggregate model is applied to simulate the scattering phase functions. The simulation is able to reproduce the overall scattering features; however, substantial discrepancies still exist due to uncertainties in particle shape and refractive index. Copyright © 2018 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity for a series of alkylaminium carboxylate aerosols have been measured using a hygroscopicity tandem differential mobility analyzer coupled to a condensation particle counter and a CCN counter. The particles, consisting of the mixtures of monoacetic, propanoic, p-toluic, and ris-pinonic add) and dicarboxylic (oxalic, succinic, malic, adipic, and azelaic add) acid with alkylamine (mono-, di-, and trimethylamines), represent those commonly found under diverse environmental conditions. The hygroscopidty parameter (κ) of the alkylaminium carboxylate aerosols was derived from the HGF and CCN results and theoretically calculated. The HGF at 90% RH is in the range of 1.3 to 1.8 for alkylaminium monocarboxylates and 1.1 to 2.2 for alkylaminium dicarboxylates, dependent on the molecular functionality (i.e., the carboxylic or OH functional group in organic acids and methyl substitution in alkylamines). The κ value for all alkylaminium carboxylates is in the range of 0.06--1.37 derived from the HGF measurements at 90% RH, 0.05--0.49 derived from the CCN measurements, and 0.22--0.66 theoretically calculated. The measured hygroscopicity of the alkylaminium carboxylates increases with decreasing add to base ratio. The deliquescence point is apparent for several of the alkylaminium dicarboxylates but not for the alkylaminium monocarboxylates. Our results reveal that alkylaminium carboxylate aerosols exhibit distinct hygroscopic and deliquescent characteristics that are dependent on their molecular functionality, hence regulating their impacts on human health, air quality, and direct and indirect radiative forcing on climate. [ABSTRACT FROM AUTHOR]

Atmospheric aging of black carbon (BC) leads to changes in its physiochemical properties, exerting complex effects on environment and climate. In this study, we have conducted laboratory chamber experiments to investigate the effects of BC aging on its morphology, hygroscopicity and optical properties by exposing monodisperse fresh BC particles to ambient ubiquitous species of nitrogen dioxide (NO 2), sulfur dioxide (SO 2) and ammonia (NH 3) in absence of UV light. We show a rapid aging from highly fractal to compacted aggregates for the monodisperse BC particles with an initial diameter of 150 nm, with decline in the dynamic shape factor (χ) from about 1.8 to nearly 1. The effective density of the monodisperse BC particles increases from ∼0.54 to 1.50 g cm−3 accordingly. The aging process leads to that the light scattering, absorption, and single scattering albedo of the monodisperse BC particles are strongly enhanced by factors of 7.0, 1.8 and 3.0 respectively. By comparing with the BC aging from other mechanisms, we reveal a critical role of the composition of the coating materials on BC in determining its light absorption enhancement. Moreover, due to strong water uptake capacity of the aged BC particles, the light absorption enhancement (E abs) could be 40–60% higher at humid atmosphere compared with dry conditions. This BC aging process from NO 2 oxidation of SO 2 may occur commonly in polluted regions and thus considerably alter its effects on regional air quality and climate. [ABSTRACT FROM AUTHOR]

A first-of-a-kind, extended-term cloud aircraft campaign was conducted to obtain an in situ statistical characterization of continental boundary layer clouds needed to investigate cloud processes and refine retrieval algorithms. Coordinated by the Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF), the Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign operated over the ARM Southern Great Plains (SGP) site from 22 January to 30 June 2009, collecting 260 h of data during 59 research flights. A comprehensive payload aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft measured cloud microphysics, solar and thermal radiation, physical aerosol properties, and atmospheric state parameters. Proximity to the SGP's extensive complement of surface measurements provides ancillary data that support modeling studies and facilitates evaluation of a variety of surface retrieval algorithms. The five-month duration enabled sampling a range of conditions associated with the seasonal transition from winter to summer. Although about two thirds of the flights during which clouds were sampled occurred in May and June, boundary layer cloud fields were sampled under a variety of environmental and aerosol conditions, with about 77% of the cloud flights occurring in cumulus and stratocumulus. Preliminary analyses illustrate use of these data to analyze aerosol-cloud relationships, characterize the horizontal variability of cloud radiative impacts, and evaluate surface-based retrievals. We discuss how an extended-term campaign requires a simplified operating paradigm that is different from that used for typical, short-term, intensive aircraft field programs. [ABSTRACT FROM AUTHOR]

The article discusses the coupling of the land ecosystem and the atmospheric hydrologic cycle through biogenic aerosol. Biogenic and other aerosols, which comprise organic material can be effective cloud condensation nuclei which influence microphysical and optical properties of clouds. The importance of aerosols coupling the biogeochemical and atmospheric hydrological cycles need to be assessed in the context of energy and water exchanges. The organic compounds in the aerosols influence the radiative properties of the aerosol.

The compositions of six aggregated aerosol samples from the Storm Peak Laboratory site have been comprehensively analysed (Hallar et al., 2013), focusing particularly on the large water-extractable organic fraction which consists of both high molecular weight organic compounds and a range of acids and sugar-alcohols. The contribution of the soluble organic fraction of atmospheric aerosols to their hygroscopicity is hard to quantify, largely because of the lack of a detailed knowledge of both composition and the thermodynamic properties of the functionally complex compounds and structures the fraction contains. In this work we: (i) develop a means of predicting the relative solubility of the compounds in the water-extractable organic material from the Storm Peak site, based upon what is known about their chemical composition; (ii) derive the probable soluble organic fraction from comparisons of model predictions with the measured hygroscopicity; (iii) test a model of the water uptake of the total aerosol (inorganic plus total water-extractable organic compounds). Using a novel UNIFAC-based method, different assignments of functional groups to the high molecular weight water soluble organic compounds (WSOC) were explored, together with their effects on calculated hygroscopic growth factors, constrained by the known molecular formulae and the double bond equivalents associated with each molecule. The possible group compositions were compared with the results of ultrahigh resolution mass spectrometry measurements of the organic material, which suggest large numbers of alcohol (–OH) and acid (–COOH) groups. A hygroscopicity index (HI) was developed. The measured hygroscopic growth is found to be consistent with a dissolution of the WSOC material that varies approximately linearly with RH , such that the dissolved fraction is about 0.45–0.85 at 90% relative humidity when ordering by HI , depending on the assumptions made. This relationship, if it also applies to other types of organic aerosol material, provides a simple approach to calculating both water uptake and CCN activity (and the κ parameter for hygroscopic growth). The hygroscopicity of the total aerosol was modelled using a modified Zdanovskii-Stokes-Robinson approach as the sum of that of the three analysed fractions: inorganic ions (predicted), individual organic acids and "sugar alcohols" (predicted), and the high molecular weight WSOC fraction (measured). The calculated growth factors broadly agree with the measurements, and validate the approach taken. The insights into the dissolution of the organic material seem likely to apply to other largely biogenic secondary organic aerosols from similar remote locations. • A hygroscopicity index is developed for secondary organic aerosol material. • The estimated molecular group compositions confirm large numbers of –OH and –COOH. • Dissolution of water-extractable organic material varies linearly with RH. • Results suggest simple methods of modelling water uptake of atmospheric organics. • Measured and predicted growth factors of the total soluble aerosol agree well. [ABSTRACT FROM AUTHOR]

Traffic emissions are a dominant source of secondary organic aerosol (SOA) in urban environments. Though tailpipe exhaust has drawn extensive attention, the impact of non-tailpipe emissions on atmospheric SOA has not been well studied. Here, a closure study was performed combining urban tunnel experiments and dynamometer tests using an oxidation flow reactor in situ photo-oxidation. Results show a significant gap between field and laboratory research; the average SOA formation potential from real-world fleet is 639 ± 156 mg kg fuel -1 , higher than the reconstructed result (188 mg kg fuel -1 ) based on dynamometer tests coupled with fleet composition inside the tunnel. Considering the minimal variation of SOA/CO in emission standards, we also reconstruct CO and find the critical role of high-emitting events in the real-world SOA burden. Different profiles of organic gases are detected inside the tunnel than tailpipe exhaust, such as more abundant C 6 -C 9 aromatics, C 11 -C 16 species, and benzothiazoles, denoting contributions from non-tailpipe emissions to SOA formation. Using these surrogate chemical compounds, we roughly estimate that high-emitting, evaporative emission, and asphalt-related and tire sublimation share 14, 20, and 10% of the SOA budget, respectively, partially explaining the gap between field and laboratory research. These experimental results highlight the importance of non-tailpipe emissions to atmospheric SOA.