Impacts of Photoinitiated Chemical Processing on Climate Relevant Aerosol Properties

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's 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.

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 aerosol 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.