Anhydrosugars as tracers of fire air quality effects, carbon cycling and paleoclimate
Suciu, Loredana G
Masiello, Caroline A; Griffin, Robert J
Doctor of Philosophy
Wild and prescribed fires are important sources of a broad suite of organic compounds collectively termed pyrogenic carbon (PyC). Most PyC compounds have additional sources beyond fire, adding uncertainty to their use as tracers. However, members of the anhydrosugar family of isomeric compounds - levoglucosan, galactosan and mannosan – are generated exclusively by the pyrolysis and combustion of cellulose and hemicellulose. Although anhydrosugars are some of the only unique organic markers for fire, their use as tracers in atmospheric, marine, and terrestrial systems is challenging because there is no clear theoretical framework to deal with their reactivity and phase partitioning. The atmospheric science community has made the first approximation that they are unreactive on timescales of interest. This assumption is problematic, because there is ample evidence of anhydrosugar reactivity on short timescales. On the other hand, the terrestrial and marine science communities have not yet seen wide use of anhydrosugars as tracers because our understanding of their biogeochemistry and transport through the Earth system is poorly constrained. Chapter 2 of this thesis reviews evidence for anhydrosugar production, degradation and detection in various environments and use this information to develop a framework for uses of anhydrosugars in research on PyC and organic matter in the Earth system. Anhydrosugars are chemically reactive in all phases (gaseous, aqueous and particulate), molecularly diffusive in semisolid matter, semivolatile, water-soluble, and biodegradable. Their chemical composition also suggests that they sorb to soil mineral surfaces. Together, these characteristics mean that anhydrosugars are not conservative tracers. While these traits have historically been perceived as drawbacks, here I argue that they present opportunities for new research avenues, including tracking organic matter transport and degradation in multiple environments. Chapter 3 of this thesis provides insights on the model development and simulations of the atmospheric degradation of the most abundant anhydrosugar emitted from biomass burning - levoglucosan (LEV) - and its effects on the formation of secondary organic aerosols (SOA) and other gases, using a zero-dimensional (0-D) modeling framework. Existing chemical mechanisms (homogeneous gas-phase chemistry and heterogeneous chemistry) were updated to include the chemical degradation of LEV and its intermediary degradation products in both phases (gas and aerosol). In addition, the gas-particle partitioning mechanism was added to the model to account for the effect of evaporation and condensation on the concentrations of LEV and its degradation products. Comparison of simulation results with measurements from various chamber experiments show that the degradation time scale of LEV varied by phase, 1.5-3.5 days (gas-phase) and 8-21 hours (aerosol-phase); these relatively short time scales suggest that most of the initial LEV concentration can be lost chemically or deposited locally before being transported regionally. Estimated secondary organic aerosol SOA yields (5-32%) reveal that conversion of LEV to secondary products is significant and occurs rapidly in the studied scenarios. The chemical degradation of LEV has effects on other gases, such as increasing the concentrations of radicals and total reactive nitrogen. Decreases of nitrogen oxides (NOx) appear to drive a more rapid increase in ozone (O3) compared to volatile organic compounds (VOC) levels. Future model evaluations and subsequent implementation of the 0-D multiphase LEV chemistry (extended to include its isomers) in CTMs will allow to model both regional transport and deposition of anhydrosugars and thus better assess their atmospheric implications and use as tracers. Another application of anhydrosugars would be to trace regional air transported to highly polluted urban areas, such as the Houston area. Vegetation fires occurring outside this region contribute emissions of O3 precursors, such as VOC and NOx. However, in the Houston area, there are multiple sources of such emissions (industrial activity, vehicle exhaust, etc.), and mixing with those sources challenges the quantification of regional contributions to locally measured concentrations. This is important because air pollution control measures impact the industrial activity in the area. While anhydrosugars have not been used in this study to help constrain regional background O3 and NOx, they open an unexplored pathway for future studies that can build on the additional work presented in Chapter 4 of this thesis, such as the estimation of regional background O3 and NOx using statistical analysis of O3, NOx and meteorology measured in the Houston-Galveston-Brazoria (HGB) region. This study used four different approaches based on principal component analysis (PCA). Three of these approaches consist of independent PCA on both O3 and NOx for both 1-h and 8-h levels to compare the results with previous studies and to highlight the effect of both temporal and spatial scales. In the fourth approach, O3, NOx and meteorology were co-varied. Results show that the estimation of regional background O3 has less inherent uncertainty when it was constrained by NOx and meteorology, yielding a statistically significant temporal trend of -0.68 ± 0.27 ppb y-1. Likewise, the estimation of regional background NOx trend constrained by O3 and meteorology was -0.04 ± 0.02 ppb y-1 (upper bound) and -0.03 ± 0.01 ppb y-1 (lower bound). The best estimates of 17-y average of season-scale background O3 and NOx were 46.72 ± 2.08 ppb and 6.80 ± 0.13 ppb (upper bound) or 4.45 ± 0.08 ppb (lower bound), respectively. Average background O3 is consistent with previous studies and between the approaches used in this study, although the approaches based on 8-h averages likely overestimate background O3 compared to the hourly median approach by 7-9 ppb. Similarly, the upper bound of average background NOx is consistent between approaches in this study but overestimated compared to the hourly approach by 1 ppb, on average. The study likely overestimates the upper bound background NOx due to instrument overdetection of NOx and the 8-h averaging of NOx and meteorology coinciding with maximum daily eight hours average O3. Regional background O3 and NOx in the HGB region both have declined over the past two decades. This decline became steadier after 2007, overlapping with the effects of controlling precursor emissions and a prevailing southeasterly-southerly flow.