Impacts of Photoinitiated Chemical Processing on Climate Relevant Aerosol Properties

Lead Research Organisation: University of Bristol
Department Name: Chemistry


Sunlight and atmospheric aerosols are ever-present in our environment. Aerosols (airborne particles or droplets) can impact air quality and climate. Air pollution costs the UK £15 billion/year in damage to human health. The World Health Organization estimates nearly 12% of all deaths worldwide are due to indoor and outdoor air pollution. Aerosols are the largest uncertainty in our understanding of climate. Aerosols interact with sunlight by scattering or absorbing it, which can reduce visibility (e.g. smog) or create a beautiful sunset. However, we have not studied how sunlight can initiate chemistry in aerosols and change the properties of the particles. Recent experiments indicate that light-initiated chemistry may be a common occurrence. However, the impacts of light-initiated chemistry on a range of aerosol properties relevant to climate and air quality are not understood. These reactions may change the identities of the molecules that make up an aerosol particle and may alter fundamental properties including its size and ability to scatter or absorb light. Moreover, this chemistry may produce molecules that partition into the gas phase and can then undergo further chemistry to form new particles.

This work will investigate the role of light-initiated chemistry on a range of aerosol properties that are relevant to climate (how much they scatter or absorb sunlight) and to air quality (composition and size). This work will be accomplished using a novel combination of single particle measurements and a photochemical aerosol reactor. In single particle studies, an aerosol particle is captured using an optical trap, irradiated with light of a chosen wavelength, and monitored for changes to the particle's properties. Specifically, the impacts on the size, refractive index, hygroscopic properties, and phase of the particle will be determined. Refractive index determines the ability of the particle to scatter and absorb light. Hygroscopicity determines how a particle' size changes with relative humidity, which also ultimately impacts on refractive index. Phase describes whether the particle is a solid, liquid, or between the two, which can affect how the particle responds to its environment. Additionally, the yield of specific light-induced reactions will be determined at different wavelengths and particle sizes. Further experiments will examine how the surface composition of the particle may impact the chemistry. The photochemical aerosol reactor experiments will allow testing of simulations that scale the single particle measurements to ensemble measurements as well as allow precise elucidation of the molecular pathways operative in the experiments.

The results of these experiments will provide a systematic understanding of how light can interact with aerosols to induce chemistry and how that chemistry ultimately impacts climate and air quality relevant particle properties. This systematic understanding will enable predictions of the significance of light induced chemistry on climate and air quality. We then plan to assess some of these impacts by incorporating the newly resolved chemistry into an aerosol chemistry model.

Planned Impact

Atmospheric aerosols impact air quality, human health, and global climate. Indoor and outdoor air pollution contributes to nearly 12% of all deaths worldwide, and 92% of the world's population lives in areas where air quality exceeds WHO limits (World Health Organization). In the UK, air pollution costs £15 billion/year in damage to human health (NERC Strategy Statement). Aerosols are thought to cool climate but represent one of the largest sources of uncertainty in understanding climate change and future climate. Impacts on health and climate are determined largely by the composition, size distribution, and concentration of the aerosol. In order to understand and predict these aerosol properties, it is key to understand how atmospheric aerosol is processed. The particular type of processing this project will investigate is how solar radiation may induce chemical processing that ultimately impacts the size, refractive index, and hygroscopicity of the aerosol. The effects of photoinitiated chemistry on these properties has not been studied in detail previously, yet may be very important due to the ubiquity of both atmospheric aerosol and solar radiation. Mounting evidence implicates photoinitiated chemistry in changes to climate and air quality relevant aerosol physicochemical properties, changes to reactive gas phase molecule concentrations, and increases in aerosol number concentrations. An improved understanding of photoinitiated chemistry will benefit the atmospheric science and modelling community as well as the general public because it will enable better predictions of aerosol climate effects. Reductions in uncertainties in future climate will have well established social and economic benefits.

One pathway for delivering impact will occur through incorporation of the experimental results into aerosol chemistry models in order to assess the magnitude of atmospheric importance. Although some preliminary studies have suggested its importance, photoinitiated aersol chemistry is so poorly understood that it currently cannot be included in climate models. To rapidly move the laboratory work into models, Dr. Bzdek will collaborate with Project Partner Hartmut Herrmann to modify an existing and widely used aerosol chemistry model to include the experimental results. The modified model will more accurately describe atmospheric aerosol processing and will be coupled to phase transfer and gas phase models. The magnitude of impact on both aerosol and gas phase product concentrations will be assessed, thereby constraining the impacts on atmospheric chemistry and composition. Future beneficiaries will include climate modellers, as photoinitiated chemistry may represent a key aerosol process that has been previously neglected. The wider science community will also benefit from this work through overlapping research areas, including microdroplet reactivity and synthesis.

Another beneficiary of the proposed experiments includes the commercial sector. WP3 will interface a commercially available optical tweezers instrument (manufactured by Biral) with mass spectrometry in order to determine the molecular composition of the trapped droplet. Successful completion of WP3 will greatly expand the versatility of the commercial instrument and may result in scientific and economic benefits to Biral as well as further cooperation between academia and industry.

Dr. Bzdek will directly benefit from the proposed work through his establishment in UK academia in an underdeveloped area of atmospheric science. Dr. Bzdek will engage with the GW4+ doctoral training partnership to recruit Ph.D. students and will also mentor final year project students.


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Description We have developed a new approach to measure the surface tension of aerosol droplets, which have opened up new research questions relevant to the formation of cloud droplets.

We also provided the first direct evidence that the surface tensions of aerosol droplets can differ significantly from the solutions that produced them. This observation is significant because surface tension is a key parameter that governs whether atmospheric particles grow into cloud droplets and affect climate. We therefore show that a key assumption about the surface tensions of aerosols is unlikely to be reasonable.

Lastly, we have collected preliminary data where we measure a change in droplet size due to photochemistry in an individual aerosol droplet.
Exploitation Route These outcomes are significant to aerosol modellers as we provide key data they can test their models against. In addition, our results are an important first step in accomplishing our goal of resolving the role of sunlight in atmospheric aerosol chemistry.
Sectors Environment,Manufacturing, including Industrial Biotechology

Description Development of Predictive Frameworks for Indoor Air Quality
Amount £3,350 (GBP)
Organisation GW4 
Sector Academic/University
Country United Kingdom
Start 09/2018 
End 03/2019
Description Dynamic Surface Properties of Atmospheric Aerosol and Resulting Climate Impacts
Amount £100,000 (GBP)
Funding ID 2274588 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 09/2019 
End 09/2023
Description Measurements of Indoor Air Quality in a Victorian Home
Amount £3,040 (GBP)
Organisation Dyson 
Sector Private
Country United Kingdom
Start 04/2019 
End 04/2020
Description NERC GW4+ DTP Studentship: Impacts of Photoinitiated Chemical Processing on Climate Relevant Aerosol Properties
Amount £100,000 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 10/2018 
End 03/2022
Description Supporting Early Career Researchers at the University of Bristol
Amount £424,999 (GBP)
Funding ID EP/S018050/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2018 
End 05/2020
Title Data from Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age 
Description This data set provides the data underlying the figures in our publication "Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age", Journal of Physical Chemistry A, 2019. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Title Data from The Surface Tension of Surfactant-Containing, Finite Volume Droplets 
Description These are data supporting a paper entitled The Surface Tension of Surfactant-Containing, Finite Volume Droplets, currently in press at PNAS. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact No impact yet. 
Description Modelling of Surfactant Partitioning in Picoliter Aerosol Droplets 
Organisation University of Oulu
Country Finland 
Sector Academic/University 
PI Contribution We devised an approach to measure the surface tensions of picoliter droplets containing surfactants in order to study the partitioning behaviour for the surfactants. These experiments are highly relevant to describing cloud droplet nucleation in the atmosphere, as surfactants are important components of aerosol composition.
Collaborator Contribution Our partners have developed a state-of-the-art model to describe the partitioning of surfactants to the surface of small droplets. The validation of this model against our experimental data has allowed the collaborators to predict the effect of surfactants on cloud droplet formation through additional simulations.
Impact One publication in press at PNAS .
Start Year 2016
Description Modelling the Surface Tensions of Aerosols 
Organisation University of Minnesota
Department Department of Mechanical Engineering
Country United States 
Sector Academic/University 
PI Contribution We have provided experimental data that our collaborators are using to develop models that predict the surface tension of multicomponent systems containing both non-surfactants and surfactants.
Collaborator Contribution Our collaborators have modelled our experimental data of the surface tension of multicomponent chemical systems.
Impact Multidisciplinary: chemistry and engineering. Output: One publication was accepted into the Journal of Physical Chemistry A (2019). A second publication is in preparation for submission to a scholarly journal.
Start Year 2017
Description GW4 Sponsored Workshop on Indoor Air Quality 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact I organised a GW4-sponsored workshop on Indoor Air Quality, assembling researchers across Bath, Bristol, Cardiff, and Exeter along with participants from government agencies (Public Health England) and industry. Participants discussed many challenges in indoor air quality, spanning engineering and scientific challenges to social challenges. The outcomes included a ranked list of key problems to address potentially through future grant proposals as well as the development of a diverse collaborative network to find multidisciplinary solutions to complex problems
Year(s) Of Engagement Activity 2019
Description Participation in the GW4 Crucible 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Study participants or study members
Results and Impact Participated in the 2018 NERC GW4 Crucible (theme: Resilience, Environment, and Sustainability), a series of workshops designed for young faculty to foster interdisciplinary and inter-institutional collaborations as well as engage with government, media, and the public. The goal is to develop collaborations across multiple sectors.
Year(s) Of Engagement Activity 2018
Description Royal Society Pairing Scheme One-Day Workshop at Houses of Parliament 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Participated in the Royal Society Pairing Scheme one-day workshop at the Houses of Parliament and the Royal Society, which included workshops on how science has impacted policy and a reception with Ministers, MPs, Peers, and Civil Servants at the Houses of Parliament
Year(s) Of Engagement Activity 2018
Description School Visit (Ramapo High School) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact Gave a presentation to senior science students (~25 students) about environmental science and careers in research. The teacher was impressed with the outcome and has invited me to do this as a yearly presentation to his students.
Year(s) Of Engagement Activity 2019