California Case Study of Wildfires and Prescribed Burns: PM2.5 Emissions, Concentrations, and Implications for Human Health.

Wildfires are a significant threat to human health, in part through degraded air quality. Prescribed burning can reduce wildfire severity but can also lead to an increase in air pollution. The complexities of fires and atmospheric processes lead to uncertainties when predicting the air quality impacts of fire and make it difficult to fully assess the costs and benefits of an expansion of prescribed fire. By modeling differences in emissions, surface conditions, and meteorology between wildfire and prescribed burns, we present a novel comparison of the air quality impacts of these fire types under specific scenarios. One wildfire and two prescribed burn scenarios were considered, with one prescribed burn scenario optimized for potential smoke exposure. We found that PM2.5 emissions were reduced by 52%, from 0.27 to 0.14 Tg, when fires burned under prescribed burn conditions, considerably reducing PM2.5 concentrations. Excess short-term mortality from PM2.5 exposure was 40 deaths for fires under wildfire conditions and 39 and 15 deaths for fires under the default and optimized prescribed burn scenarios, respectively. Our findings suggest prescribed burns, particularly when planned during conditions that minimize smoke exposure, could be a net benefit for the impacts of wildfires on air quality and health.

Direct Radiative Effect and Public Health Implications of Aerosol Emissions Associated with Shifting to Gasoline Direct Injection (GDI) Technologies in Light-Duty Vehicles in the United States.

Due to their enhanced fuel economy, the market share of gasoline direct injection (GDI) vehicles has increased significantly over the past decade. However, GDI engines emit higher levels of black carbon (BC) aerosols compared to traditional port fuel injection (PFI) engines. Here, we performed coupled chemical transport and radiative transfer simulations to estimate the aerosol-induced public health and direct radiative effects of shifting the U.S. fleet from PFI to GDI technology. By comparing simulations with current emission profiles and emission profiles modified to reflect a shift from PFI to GDI, we calculated the change in aerosol (mostly BC) concentrations associated with the fleet change. Standard concentration-response calculations indicated that the total annual deaths in the U.S. attributed to particulate gasoline-vehicle emissions would increase from 855 to 1599 due to shifting from PFI to GDI. Furthermore, the increase in BC associated with the shift would lead to an annual average positive radiative effect over the U.S. of approximately +0.075 W/m2, with values as large as +0.45 W/m2 over urban regions. On the other hand, the reduction in CO2 emissions associated with the enhanced fuel economy of GDI vehicles would yield a globally uniform negative radiative effect, estimated to be -0.013 W/m2 over a 20 year time horizon. Therefore, the climate burden of the increase in BC emissions dominates over the U.S., especially over source regions.

Air quality impacts of observationally constrained biomass burning heat flux inputs.

Biomass burning is a major contributor to ambient air pollution worldwide, and the accurate characterization of biomass burning plume behavior is an important consideration for air quality models that attempt to reproduce these emissions. Smoke plume injection height, or the vertical level into which the combustion emissions are released, is an important consideration for determining plume behavior, transport, and eventual impacts. This injection height is dependent on several fire properties, each with estimates and uncertainties in terms of historical fire emissions inventories. One such property is the fire heat flux, a fire property metric sometimes used to predict and parameterize plume injection heights in current chemical transport models. Although important for plume behavior, fire heat flux is difficult to predict and parameterize efficiently, and is therefore often held to fixed, constant values in these models, leading to potential model biases relative to real world conditions. In this study we collect observed heat flux estimates from satellite data products for three wildfire events over northern California and use these estimates in a regional chemical transport model to investigate and quantify the impacts of observationally constrained heat fluxes on the modeled injection height and downwind air quality. We find large differences between these observationally derived heat flux estimates and fixed model assumptions, with important implications for modeled behavior of plume dynamics and surface air quality impacts. Overall, we find that using observationally constrained heat flux estimates tends to reduce modeled injection heights for our chosen fires, resulting in large increases in surface particulate matter concentrations. While local wind conditions contribute to variability and additional uncertainties in the impacts of modified plume injection heights, we find observationally constrained heat fluxes to be an impactful and potentially useful tool towards the improvement of emissions inventory assumptions and parameterizations.