New Strategies for Sampling, Analysing and Understanding Aerosols

Lead Research Organisation: University of Bristol
Department Name: Chemistry

Abstract

Aerosols are a collection of solid particles of liquid droplets dispersed in air and include smoke, fog, sea spray and pollution particles from vehicles. Particle sizes can range from the nanometre (a millionth of a millimetre) to the millimetre scale. Aerosols influence health, visibility, and climate and are finding increased technological application in the delivery of drugs to the lungs, the engineering of nanostructures through spray drying, and the delivery of fuels for combustion. This research will examine some of the fundamental challenges faced in understanding aerosols.A key to understanding the impact of aerosols on human health is to understand how they are transformed as they are inhaled and exhaled. In the humid environment found within the respiratory tract, water vapour can condense on inhaled particles leading to growth in size. This can influence their depth of penetration into the lungs, with smaller particles penetrating deeper. The targeted delivery of drugs to the lungs may be enhanced by altering their response to a humid environment. Further, the transmission of airborne viruses, such as the influenza virus, may be influenced by the humidity changes on exhalation and inhalation. Similarly, in atmospheric science it is crucial to understand how aerosol particles are influenced by humidity in order to predict the size of cloud droplets. Understanding the change in particle size with humidity is also critical for understanding the ability of particles to scatter and absorb sunlight and their impact on climate. Thus, key themes of this project will be to understand how particles change in size with humidity and their efficiency in scattering and absorbing light.It is also essential to know the chemical composition of particles to interpret their impact on health and the environment, particularly in polluted urban environments, and to understand how particles are chemically changed over time. Water is ubiquitous in our environment and largely regulates the composition of aerosol. However, many chemicals are not soluble in water. This is particularly true for many pollutants such as polycyclic aromatic hydrocarbons, which are considered to be carcinogenic and mutagenic, and for many of the agrochemicals that are used in crop production. Such chemicals may be very soluble and become concentrated in organic liquids. A key theme of this project will be to understand the properties of aerosol droplets that contain both water soluble and insoluble chemicals, and to understand how chemicals can dissolve into the organic phase within an aerosol. Indeed, many of the organic components may undergo chemical reactions in our environment and a further theme will be to explore the chemistry that can occur in aerosol and how it influences aerosol properties. From the delivery of drugs to the lungs, to the evaporation of fuel droplets in combustion, the spray drying of particles, and the growth of particles in the atmosphere, understanding the rate at which particles can change size and composition is also crucial. For example, the processes occurring during the evaporation of a volatile solvent in the use of metered does inhalers critically determine the delivery of active pharmaceuticals to the lungs. The solid structures formed when liquid droplets rapidly evaporate are controlled by how quickly the solvent evaporates in spray drying. Studies of these very rapid processes are a further theme.Finally, there is considerable interest in the analysis and chemical reactions of very small sample volumes, and this has led to the development of lab-on-a-chip technology. Commonly, solutions flow through very small fabricated channels to allow measurements on micro-litre samples. A final theme of this project will be to control reactions in aerosol droplets containing only picolitres of sample, a billionth of cubic centimetre, using laser beams to move particles around and control chemistry.

Publications

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Bones DL (2012) Comparing the mechanism of water condensation and evaporation in glassy aerosol. in Proceedings of the National Academy of Sciences of the United States of America

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Cai C (2015) Slow water transport in MgSO4 aerosol droplets at gel-forming relative humidities. in Physical chemistry chemical physics : PCCP

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Carruthers A (2010) Longitudinal optical trapping and sizing of aerosol droplets in Optics Express

 
Description Through the period of a 5 year fellowship, the following significant conclusions (summarising >50 publications) have been the following.

- We have extended the application of optical tweezers in the examination of aerosol processes, including the development of a commercial instrument that has now gone to market.

- We have resolved many of the fundamental questions as to the micorphysical processes that govern the evaporation and condensation of liquids, specifically through studying liquid droplets.

- We have provide unique data examining the heterogeneous chemical transformation of aerosol and how the chemical changes lead to changes in particle hygroscopicity, refractive index, composition and vapour pressure.

- We have demonstrated that individual particles can be captured in a Bessel light beam and probed by cavity ringdown spectroscopy.

- We explored the properties of ultraviscous and glassy aerosol, developing a new tool to measure the viscosity of particles over a range of 12 orders of magnitude and a method for examining the kinetics of water transport in viscous particles.
Exploitation Route Many of the techniques developed are now being applied to address problems in atmospheric aerosol science funded by grants from the NERC.
Sectors Environment,Healthcare,Other

 
Description In answer to "describe how the findings from the grant you are reporting on have been used and achieved impact beyond academia", I am not aware of any at present. The work contributed to our wider understanding of the processes and impacts of aerosols, and led to a significant number of publications.
First Year Of Impact 2009
Sector Environment,Healthcare,Other
 
Description A Prototype Aerosol Optical Tweezers Instrument
Amount £99,950 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 05/2013 
End 08/2014
 
Description New Frontiers in Aerosol Particle Measurements
Amount £333,338 (GBP)
Funding ID EP/L010569/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2014 
End 03/2017
 
Title A Complete Parameterization of the Relative Humidity and Wavelength Dependence of the Refractive Index of Hygroscopic Inorganic Aerosol Particles (Atmospheric Chemistry and Physics 2017) 
Description Calculations of aerosol radiative forcing require knowledge of wavelength-dependent aerosol optical properties, such as single scattering albedo. These aerosol optical properties can be calculated using Mie theory from knowledge of the key microphysical properties of particle size and refractive index, assuming that atmospheric particles are well-approximated to be spherical and homogeneous. We provide refractive index determinations for aqueous aerosol particles containing the key atmospherically relevant inorganic solutes of NaCl, NaNO3, (NH4)2SO4, NH4HSO4 and Na2SO4, reporting the refractive index variation with both wavelength (400 - 650 nm) and relative humidity (from 100% to the efflorescence value of the salt). The accurate and precise retrieval of refractive index is performed using single particle cavity ring-down spectroscopy. This approach involves probing a single aerosol particle confined in a Bessel laser beam optical trap through a combination of extinction measurements by cavity ring-down spectroscopy and elastic light scattering measurements. Further, we assess the accuracy of these refractive index measurements, comparing our data with previously reported data sets from different measurement techniques but at a single wavelength. Finally, we provide a Cauchy dispersion model that parameterizes refractive index measurements in terms of both wavelength and relative humidity. Our parameterizations should provide useful information to researchers requiring an accurate and comprehensive treatment of the wavelength and relative humidity dependence of refractive index for the inorganic component of atmospheric aerosol. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Assessing the Accuracy of Complex Refractive Index Retrievals from Single Aerosol Particle Cavity Ring-Down Spectroscopy 
Description Cavity ring-down spectroscopy (CRDS) of single, optically manipulated aerosol particles affords quantitative retrieval of refractive indices for particles of fixed or evolving composition with high precision. We quantify the accuracy with which refractive index determinations can be made by CRDS for single particles confined within the core of a Bessel laser beam and how that accuracy is degraded as the particle size is progressively reduced from the coarse mode (> 1 micrometre radius) to the accumulation mode (< 500 nm radius) regime. We apply generalised Lorenz-Mie theory to the intra-cavity standing wave to explore the effect of particle absorption on the distribution of extinction cross section determinations resulting from stochastic particle motion in the Bessel beam trap. The analysis provides an assessment of the accuracy with which the real, n, and imaginary, ?, components of the refractive index can be determined for a single aerosol particle. These data are published in M. I. Cotterell et al., Aerosol Science and Technology (2016) 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Commercial Aerosol Optical Tweezers Instrument 
Description Developed with Biral through the award of an EPSRC Impact Acceleration Award, the AOT-100 allows users to hold airborne particles for extended periods with ease. Holding particles in an optical trap enables many different parameters (droplet radius, refractive index, composition, phase, morphology etc.) to be studied continuously over times varying from milliseconds to hours. 
Type Of Technology Physical Model/Kit 
Year Produced 2014 
Impact Sale of first instrument to governmental laboratory in USA 
URL http://www.biral.com/particle-aerosol-analysers/aerosol-optical-tweezers
 
Description American Association of Aerosol Research Plenary Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited plenary talk
Year(s) Of Engagement Activity 2013
 
Description American Chemical Society Fall Conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited Talk
Year(s) Of Engagement Activity 2012
 
Description American Geophysical Union Conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited Talk
Year(s) Of Engagement Activity 2012
 
Description Berkeley Atmospheric Sciences Symposium 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk
Year(s) Of Engagement Activity 2013
 
Description Seminars at ETH-Zurich (Switzerland), University of Kyoto (Japan), and York (UK) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talks
Year(s) Of Engagement Activity 2013
 
Description University of California, Irvine 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk at University
Year(s) Of Engagement Activity 2013
 
Description University of California, San Diego 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk at University
Year(s) Of Engagement Activity 2013
 
Description Workshop on Physical Chemistry of Aerosol 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited Talk
Year(s) Of Engagement Activity 2014
 
Description Workshop on Physical Chemistry of Aerosol 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk
Year(s) Of Engagement Activity 2012