Climate and air quality impacts of boreal wildfires  —	new analytical approaches for the investigation of light absorption and atmospheric reactivity of wildfire particulate matter  
Creators: Ming Lyu
Publisher: name: University of Alberta Library
Year: 2022

<p>Wildfire smoke emissions contain substantial amounts of light-absorbing aerosols that can affect the radiation and cloud processes, resulting in climate impacts on regional and even global scales. The radiative impact of these light-absorbing aerosols is largely contributed by brown carbon (BrC), a type of organic aerosols with complex chemical composition. The light absorption of BrC has been found to have a strong dependence on the properties of biomass fuel as well as on combustion conditions; however, the fundamental factors underlying these correlations are not yet clear. In addition, the chemical species responsible for BrC’s light-absorbing properties and their evolution in the atmosphere are still not fully constrained. For these reasons, the total radiative impact of BrC is still uncertain. <br>
To address these knowledge gaps, I studied BrC produced from the systematic combustion of boreal peat collected in Alberta, Canada as a function of peat sampling depth and moisture content. Specifically, I investigated the correlation of fuel properties with the size-dependent light absorption of the resultant BrC, and the dynamic changes in these correlations and properties during simulated atmospheric photoaging. To further investigate the effect of fuel type, I also compared these results with those obtained for BrC from combustion of spruce foliage. This work is the first known attempt to systematically study the correlations between boreal peat properties and BrC light absorption.</p>

<p>The advances in our understanding of BrC properties described in this thesis were enabled by my development of a new analytical approach for characterization of BrC extracts using size exclusion chromatography (SEC) with photodiode array detection (PDA). I used this approach to classify BrC chromophores according to properties such as molecular size (represented here as “molecular weight”, MW) and polarity. I found that BrC chromophores in wildfire particulate matter extracts fell into two major fractions: first, a high-MW fraction with a featureless absorption spectrum that decreases from the UV to the visible wavelength region, which contributed most of the BrC absorption in the visible wavelength region; second, a low-MW fraction with structured absorption in the UV wavelength region. These two fractions also behaved differently upon photoaging: the low-MW BrC fraction was rapidly photobleached, whereas the high-MW BrC fraction underwent initial photo enhancement, followed by slow photobleaching. This suggests that the absorption associated with the high-MW fraction of BrC remains relatively stable in the atmosphere, and as a result, is responsible for the light absorption of BrC throughout most of its atmospheric lifetime, and therefore for its climate impact.</p>

<p>I also investigated the effect of fuel properties on the BrC absorption profiles described above, and found that the relative ratio of the high- and low-MW fractions of peat-BrC showed burn-to-burn variation. Specifically, the contribution from high-MW fractions increased with increasing moisture content and sampling depth. This observation is likely due to lower combustion efficiency as a result of increased water content and bulk density. These peat properties have little effect on the BrC aging profiles; however, a much stronger correlation between BrC properties and fuel types was observed. BrC from combustion of peat and spruce foliage are very different in both their absorption profile and aging behaviors, highlighting the importance of fuel dependence in estimating BrC radiative impacts.</p>

<p>PM in wildfire smoke plumes can also affect gas-phase chemical processes, because it provides a large surface area for partitioning between the gas and particle phases and the reactive uptake of gas-phase species. I examined the heterogeneous conversion of NO2 to HONO on the surface of BrC produced from peat combustion and wood pyrolysis, with a specific focus on the effect of fuel properties on the PM reactivity. For the first time, I observed a strong dark reaction with a 100% yield of HONO, which showed a significant dependence on the fuel type.<br>
This thesis provides new insights into the light-absorbing properties and chemical reactivity of wildfire BrC. In addition, it improves our understanding of the source dependence and aging mechanism of wildfire BrC, and ultimately assists with increasing our understanding of its climate effects.</p>