Biomass burning in Amazonia [electronic resource] : an analysis of fire trends and aerosol-cloud-climate interactions based on remote sensing observations, a physical 3-D weather model, and ground-based measurements

Biomass burning is the largest source of anthropogenic aerosols in the Southern Hemisphere. In the Amazon Basin, burning is used to clear forests, remove crop residue, and mobilize nutrients. Over the last decade, trends in biomass burning over forest and savanna/agricultural lands in the Amazon have changed dramatically. We find that between the early 2000s and the late 2000s, the ratio of forest to savanna/agricultural fires more than halved over South America, in turn changing the optical properties of aerosols in the region. This change from forest to savanna burning is attributed in part to better forest fire management, changing agricultural practices along the Amazon frontier, and reduced deforestation rates. Interannual precipitation variability over forest and savanna lands is also shown to play an important role. Biomass burning aerosols over the Amazon have a substantial effect on cloud properties and the regional radiative balance. Remote sensing observations of aerosols and clouds over Brazil illustrate that meteorological variability and aerosol-cloud overlap, ignored in previous studies, must be accounted for to correctly determine aerosol-cloud interactions from satellite observations. When accounting for these confounding variables, we find that microphysical aerosol effects, which serve to increase cloud cover and optical thickness, dominate for low levels of aerosol loading (aerosol optical depth (AOD) < 0.3-0.5), whereas radiative effects, which serve to decrease cloud cover and optical thickness, dominate for higher levels of aerosol loading (AOD > 0.3-0.5). We find a similar result using high-resolution nested model simulations over the Amazon Basin, which include physical representations of direct, indirect, semi-direct, and cloud absorption effects. Simulations including and excluding biomass burning emissions are used to establish causation of the remotely sensed correlations. A two-regime relationship, defined by dominance of microphysical aerosol effects at low AODs and dominance of radiative effects at high AODs, is modeled for a variety of cloud variables including cloud optical thickness, cloud liquid droplet number, cloud fraction, and precipitation. These competing effects also exhibit a strong diurnal signal -- microphysical effects dominate in the early morning whereas radiative effects dominate in the late afternoon and night. By finding consistent relationships between remotely sensed observations and modeling results, we conclude that remotely sensed correlations between aerosols and clouds are not largely dominated by retrieval artifacts such as the hygroscopic growth of aerosol particles near clouds, brightening of aerosols near clouds, darkening of clouds below absorbing aerosols, and cloud contamination of aerosol retrievals over the Amazon, and that the complex aerosol-cloud relationships determined in this and previous studies over the Amazon can be attributed to genuine physical interactions between aerosols and clouds. In the Appendix, the same 3-D modeling tools used in the Amazon biomass burning study are applied to assess the health effect from the Fukushima nuclear disaster on March 11th, 2011. Radioactive emissions for the month following the accident are determined from worldwide observations by the Comprehensive Nuclear-Test-Ban Treaty Organization. Modeled worldwide airborne concentrations are used to determine inhalation and external atmospheric exposure, modeled deposition rates are used to determine external ground-level exposure, and ingestion exposure from contaminated food and water is extrapolated from previous Chernobyl studies all assuming a linear no-threshold model of human exposure. We estimate an additional 280 (30--2400) cancer-related mortalities and 390 (50--3800) cancer-related morbidities incorporating uncertainties associated with the exposure-dose and dose-response models used in the study. A hypothetical accident at the Diablo Canyon Power Plant in California, USA, with identical emissions to Fukushima, is studied to analyze the influence of location and seasonality on the impact of a nuclear accident. This hypothetical accident may cause up to ~45% more mortalities than Fukushima despite a lower local population density due to differing meteorological conditions.

Advisor: Jacobson Mark Z. (Mark Zachary) primary advisor. Thesis advisor
Hildemann Lynn M. (Lynn Mary) advisor. Thesis advisor
Ludwig F. L.
Street Robert