Organic aerosols (OA) are ubiquitous in the troposphere with profound impacts on public health, climate, and the global biogeochemical cycle. However, these environmental impacts of OA remain uncertain because the global sources, abundances, atmospheric evolution, depositions, and radiative properties of OA are not well quantified. In particular, OA are complex mixtures that evolve through atmospheric aging, making them difficult to identify and quantify. Due to measurement limitations, the sources of the absorptive components of OA are not well understood. This thesis aims to quantify the global sources of the absorptive components in atmospheric OA to better assess their climate effects. Biomass burning is a significant source of absorptive OA in the atmosphere. While previous studies have used levoglucosan to quantitatively assess the contribution of biomass burning to ambient OA, they did not consider levoglucosan’s chemical degradation in the atmosphere. To address this limitation, I developed the first global simulation of atmospheric levoglucosan that explicitly accounted for its chemical degradation, with the goal of evaluating the impacts on the use of levoglucosan as a tracer in quantitative aerosol source apportionment. Levoglucosan was emitted into the atmosphere from the burning of plant matter in open fires (1.7 Tg yr
-1) and as biofuels (2.1 Tg yr
-1). Atmospheric sinks of levoglucosan included aqueous-phase oxidation (2.9 Tg yr
-1), heterogeneous oxidation (0.16 Tg yr
-1), gas-phase oxidation (1.4 × 10
-4 Tg yr
-1), and dry and wet deposition (0.27 and 0.43 Tg yr
-1, respectively). The global atmospheric burden of levoglucosan was 19 Gg with a lifetime of 1.8 days. Observations showed a sharp decline in levoglucosan’s concentrations and its relative abundance to organic carbon aerosol (OC) and particulate K
+ from near-source to remote sites; such features could only be reproduced when levoglucosan’s chemical degradation was included in the model. Using model results, I developed statistical parameterizations to account for the atmospheric degradation in levoglucosan measurements, improving their use for quantitative aerosol source apportionment. Atmospheric particulate organic nitrogen (ON
p) is thought to be the dominant colored component of atmospheric brown carbon aerosol (BrC), which affects the radiative balance of Earth’s climate system. Atmospheric deposition of ON
p) is also a significant process in the global nitrogen cycle and may be pivotally important for N-limited ecosystems. However, the global environmental impacts of atmospheric ON
p) are still unclear because past models largely overlooked the spatial and chemical inhomogeneity of atmospheric ON
p) and were severely deficient in assessing global ON
p) impacts. To address this gap, I constructed a comprehensive global model of atmospheric gaseous and particulate organic nitrogen (ON), which includes the latest knowledge on emissions and secondary formations. The model successfully simulated the global abundance and deposition fluxes of atmospheric ON
p) and estimated the global atmospheric ON deposition to be 26 Tg N yr
-1, predominantly in the form of ON
p) (23 Tg N yr
-1) and originating primarily from wildfires (37%), oceans (22%), and aqueous productions (17%). Globally, ON
p) contributed as high as 40% to 80% of the total N deposition downwind of biomass burning regions. Atmospheric ON
p) deposition thus constituted the dominant external N supply to the N-limited boreal forests, tundras, and the Arctic Ocean, and its importance may amplify in a future warming climate. Based on my global ON
p simulation, I evaluated the contribution of absorptive ON
p components, hereafter referred to as brown nitrogen (BrN), to the global radiative impacts of absorptive organic aerosols, hereafter referred to as brown carbon aerosols (BrC). Previous studies of the radiative effects of BrC did not account for the chemical origins of BrC’s evolving optical properties and thus were unable to attribute the sources of the global radiative effects of BrC. I showed that BrN accounted for 76% of the light absorption of BrC in North America when atmospheric aging was considered. Globally, BrN contributed 15% and 6% of the total BrN and BC absorptive aerosol optical depth at 440 nm and 675 nm, respectively. The clean-sky direct radiative effect of BrN was estimated to be 0.03 W m
-2, with biomass burning BrN being the dominate contributor (0.015 W m
-2), followed by secondary imine BrN (0.009 W m
-2). The global mean ratio of the direct radiative effect of BrN versus that of black carbon (BC) was 17% (range 3% to 59%). My findings suggested that BrN has a significant impact on the direct radiative forcing of aerosols in regions dominated by biomass burning, such as the northern boreal forests and the Southern Hemisphere.
| Date of Award | 2023 |
|---|
| Original language | English |
|---|
| Awarding Institution | - The Hong Kong University of Science and Technology
|
|---|
| Supervisor | Jianzhen YU (Supervisor) & Tzungmay Fu (Supervisor) |
|---|