JP

Quantifying Mercury Surface Fluxes by Combining Atmospheric Observations and Models

Mercury (Hg) is a critical environmental concern. Although an important component of its biogeochemical cycle, large uncertainties still exist in the estimates of surface fluxes of mercury. Three projects presented in this thesis improve our understanding of mercury surface fluxes at different spatial scales by combining atmospheric observations and models. First, a global scale inverse model study uses observations at multiple ground-based stations and simulations from a three-dimensional chemical transport model (GEOS-Chem) to obtain a total mercury emission of about 5.8 Gg yr0 (gaseous elemental mercury). The optimized Asian anthropogenic emissions (0.7-1.8 Gg yr0. The inversion also suggests that the legacy mercury releases tend to reside in the terrestrial system rather than in the ocean. Second, the comparison of nested grid GEOS-Chem model simulations with aircraft observations support results from the global inversion, and further suggests that the Northwest Atlantic Ocean is a net source of Hg0, with high evasion fluxes in summer (related to the high precipitation rates and deposition fluxes of oxidized mercury), whereas the terrestrial ecosystem in the eastern United States is likely a net sink of Hg0 during summer. Third, a one-dimensional chemical transport model is built and used to simulate the mercury diurnal variabilities observed at Dome Concordia on the Antarctic plateau. The model simulation best reproducing the Hg0 observations shows that in summer mercury is rapidly cycled between the shallow atmospheric boundary layer and the surface snowpack. A two-step bromine initiated scheme oxidizes Hg0. Oxidized mercury is deposited, photoreduced in the surface snow, and reemitted as Hg0 back into the atmosphere.

Quantitative Assessment of Parametric Uncertainty in Northern Hemisphere PAH Concentrations

We quantitatively examine the relative importance of uncertainty in emissions and physicochemical properties (including reaction rate constants) to Northern Hemisphere (NH) and Arctic polycyclic aromatic hydrocarbon (PAH) concentrations, using a computationally efficient numerical uncertainty technique applied to the global-scale chemical transport model GEOS-Chem. Using polynomial chaos (PC) methods, we propagate uncertainties in physicochemical properties and emissions for the PAHs benzo[a]pyrene, pyrene and phenanthrene to simulated spatially resolved concentration uncertainties. We find that the leading contributors to parametric uncertainty in simulated concentrations are the black carbon-air partition coefficient and oxidation rate constant for benzo[a]pyrene, and the oxidation rate constants for phenanthrene and pyrene. NH geometric average concentrations are more sensitive to uncertainty in the atmospheric lifetime than to emissions rate. We use the PC expansions and measurement data to constrain parameter uncertainty distributions to observations. This narrows a priori parameter uncertainty distributions for phenanthrene and pyrene, and leads to higher values for OH oxidation rate constants and lower values for European PHE emission rates.

© 2015 American Chemical Society

Radiative effects of interannually varying vs. interannually invariant aerosol emissions from fires

Open-burning fires play an important role in the earth's climate system. In addition to contributing a substantial fraction of global emissions of carbon dioxide, they are a major source of atmospheric aerosols containing organic carbon, black carbon, and sulfate. These “fire aerosols” can influence the climate via direct and indirect radiative effects. In this study, we investigate these radiative effects and the hydrological fast response using the Community Atmosphere Model version 5 (CAM5). Emissions of fire aerosols exert a global mean net radiative effect of −1.0 W m−2, dominated by the cloud shortwave response to organic carbon aerosol. The net radiative effect is particularly strong over boreal regions. Conventionally, many climate modelling studies have used an interannually invariant monthly climatology of emissions of fire aerosols. However, by comparing simulations using interannually varying emissions vs. interannually invariant emissions, we find that ignoring the interannual variability of the emissions can lead to systematic overestimation of the strength of the net radiative effect of the fire aerosols. Globally, the overestimation is +23 % (−0.2 W m−2). Regionally, the overestimation can be substantially larger. For example, over Australia and New Zealand the overestimation is +58 % (−1.2 W m−2), while over Boreal Asia the overestimation is +43 % (−1.9 W m−2). The systematic overestimation of the net radiative effect of the fire aerosols is likely due to the non-linear influence of aerosols on clouds. However, ignoring interannual variability in the emissions does not appear to significantly impact the hydrological fast response. In order to improve understanding of the climate system, we need to take into account the interannual variability of aerosol emissions.

Reconciling reported and unreported HFC emissions with atmospheric observations

We infer global and regional emissions of five of the most abundant hydrofluorocarbons (HFCs) using atmospheric measurements from the Advanced Global Atmospheric Gases Experiment and the National Institute for Environmental Studies, Japan, networks. We find that the total CO2-equivalent emissions of the five HFCs from countries that are required to provide detailed, annual reports to the United Nations Framework Convention on Climate Change (UNFCCC) increased from 198 (175–221) Tg-CO2-eq⋅y–1 in 2007 to 275 (246–304) Tg-CO2-eq⋅y–1 in 2012. These global warming potential-weighted aggregated emissions agree well with those reported to the UNFCCC throughout this period and indicate that the gap between reported emissions and global HFC emissions derived from atmospheric trends is almost entirely due to emissions from nonreporting countries. However, our measurement-based estimates of individual HFC species suggest that emissions, from reporting countries, of the most abundant HFC, HFC-134a, were only 79% (63–95%) of the UNFCCC inventory total, while other HFC emissions were significantly greater than the reported values. These results suggest that there are inaccuracies in the reporting methods for individual HFCs, which appear to cancel when aggregated together.

© 2015 National Academy of Sciences

Reducing CO2 from cars in the European Union

The European Union (EU) recently adopted CO2 emissions mandates for new passenger cars, requiring steady reductions to 95 gCO2/km in 2021. We use a multi-sector computable general equilibrium (CGE) model, which includes a private transportation sector with an empirically-based parameterization of the relationship between income growth and demand for vehicle miles traveled. The model also includes representation of fleet turnover, and opportunities for fuel use and emissions abatement, including representation of electric vehicles. We analyze the impact of the mandates on oil demand, CO2 emissions, and economic welfare, and compare the results to an emission trading scenario that achieves identical emissions reductions. We find that vehicle emission standards reduce CO2 emissions from transportation by about 50 MtCO2 and lower the oil expenditures by about €6 billion, but at a net added cost of €12 billion in 2020. Tightening CO2 standards further after 2021 would cost the EU economy an additional €24–63 billion in 2025, compared with an emission trading system that achieves the same economy-wide CO2 reduction. We offer a discussion of the design features for incorporating transport into the emission trading system.

Relationship between future changes in mean climate and changes in extreme events

In this study, we investigate possible future climate change over Northern Eurasia and its impact on extreme events. Northern Eurasia is a major player in the global carbon budget because of boreal forests and peatlands. Circumpolar boreal forests alone contain more than five times the amount of carbon of temperate forests and almost double the amount of carbon of the world's tropical forests. Furthermore, severe permafrost degradation associated with climate change could result in peatlands releasing large amounts of carbon dioxide and methane. Meanwhile, changes in the frequency and magnitude of extreme events, such as extreme precipitation, heat waves or frost days are likely to have substantial impacts on Northern Eurasia ecosystems. For this reason, it is important to quantify the possible climate change over Northern Eurasia under different emissions scenarios, while accounting for the uncertainty in the climate response and changes in extreme events. We analyze 45 simulations with the Massachusetts Institute of Technology (MIT) Integrated Global System Model (IGSM) framework, an integrated assessment model that couples an earth system model of intermediate complexity (with a 2D zonal-mean atmosphere) to a human activity model. Regional change is obtained by linking the MIT IGSM with the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM). The IGSM-CAM simulations were carried for three values of climate sensitivity (low, median and high), three emissions scenarios and five different initial conditions. We analyze the relationship between future changes in mean climate and changes in extreme events for temperature and precipitation over Northern Eurasia.

Renewables Intermittency: Operational Limits and Implications for Long-TermEnergy System Models

In several regions of the world, the share of intermittent renewables (such as wind and solar PV) in electricity generation is rapidly increasing. The current share of these renewable energy sources (RES) can still more or less be handled by existing systems and flexibility, benefiting from remaining excess capacity of dispatchable (backup) generation and links to other grids that can balance the intermittency. However, often higher levels of intermittent RES are envisaged for the future, posing significant challenges on system operation and planning. In assessing possible energy futures, long-term energy system models are typically used. The representation of RES in such models needs careful attention, as intermittent RES come with a number of specific characteristics, making them different from conventional dispatchable generation. This paper focuses on technical implications related to systems trying to achieve high shares of renewable electricity. The relevance of demand and RES generation profiles are demonstrated. After some threshold, a sharp decreasing relationship between installed RES capacity and marginal contribution in terms of generation is identified; therefore, even with perfect backup, a technical limit exists on achievable RES shares. The impact of RES on net demand peak reduction is also addressed. In the absence of system flexibility, substantial backup is required to ensure reliable electricity provision. The role of different flexibility instruments is explored and is found to be significant. Reflections are provided on options to include these aspects in long-term energy system models.

Scaling Compliance with Coverage? Firm-level Performance in China's Industrial Energy Conservation Program

Industrial energy conservation programs in China form a cornerstone of China’s energy and environmental management efforts, engaging thousands of major energy-using enterprises, and targeting hundreds of million tons of annual coal-equivalent energy savings during the Eleventh and Twelfth Five-Year Plans (2006 to 2015). An important question in China and other developing countries is to understand how compliance systems develop and perform, especially in settings where regulators have limited prior experience and resources to support evaluation and enforcement. We use detailed, newly-released compliance reports, combined with industrial census data on participating firms, to identify the drivers of compliance at the firm level. We find evidence consistent with manipulation of reported compliance data during the Eleventh Five-Year Plan (2006–2010), but not during the expanded program under the Twelfth Five-Year Plan (2011–2015). We show that the non-compliance rate increased with the expansion of the program, and publicly-reported reasons for non-compliance vary widely. We find that firms that are large, and new program entrants, as well as firms in cities with low growth exhibit higher non-compliance rates after program expansion. Our findings demonstrate that although expanding coverage increases potential energy savings, regulators must grapple with increased heterogeneity in firms’ internal energy-saving opportunities and capabilities as well as in the degree of external accountability to regulators. Introducing a market for energy saving or CO2 emissions may help to solve the problem of uneven abatement costs, but differences in the strength of accountability relationships could undermine performance.

Scenarios of Global Change: Integrated Assessment of Climate Impacts

Using the MIT Integrated Global System Modeling (IGSM) framework, we assess the climate impacts of emission scenarios exhibiting global mean surface temperatures ranging between 2.4°C and 4.3°C above pre-industrial by 2100. We compare the outcomes from these forward-looking scenarios against the common goal described by the target-driven scenario of 2°C. Without further policy measures, the agreement at COP-21 in Paris is projected to result in a 3.5°C increase in global temperature in 2100 relative to pre-industrial levels. Scenarios developed by Shell International (called Mountains and Oceans) exhibit a substantial movement towards temperature stabilization, as they result in increases of only 2.4–2.7°C by 2100. Valuable components of these scenarios include a substantial shift to renewable energy and deployment of carbon capture and storage (CCS). These scenarios are successful in mitigating a large portion of water stress impacts and air pollution damages. They also significantly mitigate increases in ocean acidity. These projections show the significant value of policies that do not quite reach 2°C stabilization, but fall substantially close to that target by the end of the century. The challenge of meeting the Paris Agreement’s aspiration to limit warming to 1.5°C is monumental, yet may be desirable if societies see the 2°C impacts, described here, as running too much risk.

Sensitivity of inorganic aerosol impacts to US precursor emissions

I present work on the relationship between inorganic atmospheric aerosol impacts and their precursor emissions from the United States of America. The inorganic aerosol ions nitrate (NO ̶3), sulfate (SO2 ̶4), and ammonium (NH+4) form from emissions of nitrogen oxides (NOx), sulfur dioxide (SO2), and ammonia (NH3). Emissions of NOx and SO2 in the US have recently decreased, by 42% and 62% respectively for annual totals between 2005 and 2012, in response to economic, political, and technological developments. Under such large changes, the processes of aerosol formation may behave nonlinearly. The sensitivity of aerosol impacts to future emissions reductions – the change in a metric per unit change in emissions – can be very different from the sensitivity to past reductions. In this thesis, I use a chemical transport model to examine the sensitivities, changes in sensitivities, and the importance of nonlinear interactions for both health and climate impacts of inorganic aerosols.

The first section of this thesis focuses on surface concentrations of inorganic fine particulate matter (PM2.5), a relevant metric for human health. In winter, PM2.5 across the central US is primarily composed of ammonium nitrate, whose formation is highly dependent on thermodynamics. The recent NOx and associated total nitrate (HNO3+NO ̶3) reductions have made aerosol formation in this region limited by total nitrate availability. Future NOx emissions reductions will thus have a much larger impact than they would have in the past. In summer, SO2 ̶4 aerosols dominate PM2.5. The reduced NOx emissions lead to higher peroxide concentrations and faster aqueous SO2 oxidation, without increasing sulfate wet deposition to the same degree. With faster oxidation, a larger fraction of the emitted SO2 forms sulfate and particulate matter, increasing the sensitivity of surface aerosol concentrations to SO2 emissions even as emissions themselves have decreased. These results suggest that NOx and SO2 emissions reductions will continue to improve US air quality.

The second section of this thesis focuses on sensitivities of the direct radiative effect (DRE) of inorganic aerosols to US emissions, a key quantity for studying climate impacts. The DRE and changes in DRE in winter are largest over the ocean. The summertime DRE includes a long tongue of advected aerosols over the Atlantic as well as a broad area of large DRE over the eastern US. As with surface concentrations, sensitivity of DRE to NOx and SO2 emissions increased between 2005 and 2012, while sensitivity to NH3 emissions decreased. A simple scaling estimate of the DRE in the 2012 case from the 2005 DRE and sensitivities overestimates the magnitude of the DRE by 10.3mWm−2 in January and 21.4mWm−2 in July. These values are equivalent to underestimating the SO2 emissions reductions by 13.6% and 10.6%, respectively. These processes cause small errors for climate studies that assume scaling of aerosol radiative effects for current conditions, but greater errors could occur under future emission changes.

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