Sustainability 101: Greenhouse (GHG) Emissions - Quantification & Analysis

Greenhouse gases (GHGs) trap heat in the Earth's atmosphere, contributing to climate change. Different GHGs have varying capacities to trap heat, expressed as their global warming potential (GWP) relative to carbon dioxide (CO_2).

Greenhouse Gas Equivalencies Calculator

1. Relative Global Warming Potentials

When considering the greenhouse effect, CO_2 is used as the baseline. The relative effects of other important GHGs are significantly higher than CO_2 on a gram-per-gram basis over a 100-year time horizon. For detailed and updated GWP values, consult reputable sources like the Intergovernmental Panel on Climate Change (IPCC) and the EPA's website on Greenhouse Gas Emissions.

Based on the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) values, common GHGs have the following approximate relative effects:

Carbon Dioxide ( $CO_2$ ): x (baseline)
Nitrous Oxide ( $N_2 O$ ): 298x (meaning 1 gram of N_2O has 298 times the warming potential of 1 gram of CO_2)
Methane ( $C H_4$ ): 25x (a major GHG and its GWP is approximately 25 times that of CO_2 over 100 years, according to IPCC AR4. It's crucial to include for a comprehensive understanding).

Note on GWP values: It's important to note that GWP values are periodically updated by the IPCC based on new scientific understanding. For the most current and widely accepted GWP values, refer to the latest IPCC Assessment Reports. The figures of 298x for NO_2 and 120x for NO are not standard for GWP; N_2O has a GWP of 298. Nitrogen oxides (NO_x), including NO and NO_2, are precursors to tropospheric ozone (a GHG) and contribute to particulate matter, but their direct GWP is not typically quantified in the same way as CO_2, CH_4, and N_2O. For further scientific insights into climate change and GHGs, you can explore resources from NASA's climate change resources and access scientific articles through databases like PubMed or Google Scholar.

2. Greenhouse Gas Emissions from Vehicles

Vehicle emissions are a significant contributor to overall GHG emissions. It's crucial to distinguish between tailpipe emissions and upstream emissions, especially for electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).

Vehicle tailpipe emissions are the GHGs your car produces when driving.

Upstream emissions are the GHGs associated with the production and distribution of gasoline and electricity.

2.1. Greenhouse Gas Equivalencies Calculator and Vehicle Emissions Analysis

The "Greenhouse Gas Equivalencies Calculator" and "Beyond Tailpipe Emissions Calculator" are tools designed to estimate the total GHG emissions from driving EVs and PHEVs.

Greenhouse Gas Equivalencies Calculator: This calculator helps translate abstract GHG emission measurements into concrete terms. Access it here: EPA Greenhouse Gas Equivalencies Calculator and find details on its calculations and references at Greenhouse Gas Equivalencies Calculator - Calculations and References. A widget version is also available: EPA Greenhouse Gas Equivalencies Calculator Widget.
Beyond Tailpipe Emissions Calculator: This tool estimates the total GHG emissions, including those from electricity production for EVs/PHEVs. Access it here: EPA Beyond Tailpipe Emissions Calculator (fueleconomy.gov).

2.1.1. Key Concepts in Vehicle Emissions:

Tailpipe Emissions: These are the GHGs directly emitted from a vehicle's exhaust pipe during operation. For gasoline vehicles, these include CO_2, N_2O, and CH_4. EVs produce zero tailpipe emissions.
- Formula (Conceptual): E_tailpipe = Fuel Consumption × Emission Factor_gasoline
- Where E_tailpipe represents the tailpipe emissions, Fuel Consumption is the amount of gasoline used, and Emission Factor_gasoline is the GHG emitted per unit of gasoline burned.
Upstream Emissions: These are the GHGs associated with the production and distribution of the fuel or electricity used to power a vehicle.
- For Gasoline Vehicles: Upstream emissions include those from crude oil extraction, refining, and transportation of gasoline. While there might be some regional variability, tailpipe emissions dominate the total CO_2 emissions for gasoline vehicles.
  - Formula (Conceptual): E_upstream,gasoline = Fuel Consumption × Emission Factor_upstream,gasoline
- For Electric and Plug-in Hybrid Vehicles (EVs/PHEVs): Upstream emissions primarily come from the generation of electricity at power plants and its transmission. These emissions depend heavily on the electricity generation mix in a specific region. For a tool to profile power generation in your area, consider using EPA's Power Profiler (eGRID).
  - Formula (Conceptual for EVs): E_upstream,EV = Electricity Consumption (kWh) × Emission Rate_grid (kg CO2e/kWh)
  - Where E_upstream,EV is the upstream emissions for an EV, Electricity Consumption is the electricity used by the EV, and Emission Rate_grid is the average GHG emissions per kilowatt-hour of electricity in a given region.

2.1.3. Factors Influencing Emissions Calculation:

Geographic Location (ZIP Code): Your ZIP Code is critical for EV/PHEV emission calculations because the GHG intensity of electricity generation varies significantly by region. Electricity produced from coal-fired power plants will result in higher upstream GHG emissions than electricity generated from renewable sources like wind or solar.
Vehicle Model Year and Type: These factors influence the vehicle's efficiency and, for PHEVs, the balance between electric and gasoline-powered driving.

2.1.3. Calculator Functionality:

These calculators allow users to:

Input their ZIP Code, vehicle model year, and specific vehicle.
Estimate both tailpipe and upstream emissions for EVs and PHEVs.
Understand how local electricity generation impacts the overall GHG footprint of their vehicle.

3. AVERT: A Tool for Analyzing Avoided Emissions

AVERT (Avoided Emissions and Generation Tool) is a free, user-friendly tool developed by the EPA. It's designed to help evaluate emissions changes resulting from energy efficiency, renewable energy, electric vehicle, and energy storage programs, policies, and projects.

3.1. Purpose and Applications of AVERT:

AVERT primarily helps:

Quantify Emission Reductions: Estimate county, state, and regional emission changes (NO_x, SO_2, CO_2, PM_2.5, VOCs, NH_3) from fossil-fueled power plants and displaced fuel-burning vehicles.
Support Clean Air Act Plans: Assist state air quality planners in incorporating quantifiable emission benefits into Clean Air Act plans to meet National Ambient Air Quality Standards (NAAQS) and other clean air goals.
Inform Policy Decisions: Provide data for energy offices and public utility commissions to estimate and promote the air quality benefits of their energy policies.
Analyze Specific Programs: Evaluate the impacts of various energy interventions, including:
- Onshore and offshore wind energy
- Rooftop-scale and utility-scale photovoltaic (PV) installations
- Rooftop PV-plus-storage and utility-scale PV-plus-storage (available since 2023 data)
- Portfolio energy efficiency and uniform energy efficiency programs
- Electric vehicle deployment
- Energy storage resources

3.2. AVERT's Methodology and Modules:

AVERT uses a peer-reviewed methodology to analyze electric power sector impacts on an hour-by-hour basis, producing approximations of marginal emission rates for each of the 14 AVERT regions. For a comprehensive understanding of how AVERT works, refer to How AVERT Works and the AVERT User Manual (v4.3). You can also find a general overview at AVERT Overview and a tutorial homepage at AVERT Tutorial Homepage.

Access AVERT:

AVERT Overview Page: EPA AVERT
AVERT Web-based Tool: AVERT Main Module (Web Edition)
AVERT Downloadable Excel Tool: Available from the Download AVERT page.

3.2.1. AVERT Modules:

AVERT consists of three main modules, which can be used together or separately depending on the analysis needs:

AVERT's Statistical Module:
- Functionality: Performs statistical analysis on historical generation, heat input, and emissions data (PM_2.5, SO_2, NO_x, CO_2, VOCs, NH_3) from EPA’s Air Markets Program Data (AMPD) and National Emissions Inventory (NEI).
- Input: Uses hourly "prepackaged" data. Can also analyze user-modified data created in AVERT’s Excel-based Future-Year Scenario Template.
- Output: Produces regional data files that are input files for AVERT’s Main Module.
AVERT's Future-Year Scenario Template (Excel-based):
- Functionality: Allows users to create user-modified data for future-year scenarios. This module is necessary for analyses extending beyond the historical baseline (typically more than five years).
AVERT's Main Module (Excel-based and Web-based):
- Functionality: Calculates emission impacts based on hourly electric generating unit information in regional data files and user-entered energy changes. For guidance on running scenarios, refer to How to Run Scenarios in AVERT.
- Input: Users select one of 14 AVERT regional data files and input data on the type of program to analyze (e.g., MWh or MW savings for energy efficiency, total capacity for renewables, number and type of EVs, energy storage capacity and charging patterns).
- Avoided Emissions = Sum of (Hourly Change in Generation × Hourly Marginal Emission Rate) for all 8760 hours in a year
  - Avoided Emissions: This is the total amount of emissions reduced over a year. Think of it as the environmental benefit of your energy program.
  - Hourly Change in Generation: This represents how much the power plants change their output (in megawatt-hours, MWh) during a specific hour because of your intervention. For example, if you install solar panels, this would be the electricity those panels produce, reducing the need for grid power. If you implement energy efficiency, this is the power saved.
  - Hourly Marginal Emission Rate: This is the crucial part. It's the amount of pollution (like CO2, NOx, etc.) released per unit of electricity by the power plants that are most likely to adjust their output in response to changes in demand. These are usually the less efficient, "dirtier" plants that fire up or down to meet hourly electricity needs.
- Output: Provides county-level emissions and generation tables and charts. It also produces SMOKE-formatted data for advanced air modeling and data compatible with EPA’s COBRA tool for quantifying and monetizing public health impacts.
- Processing Time (Excel-based): Calculations can take up to 10 minutes, depending on the region and computer capabilities.

3.2.2. AVERT Versions:

Excel Version (Fully-featured): Offers comprehensive functionality, including access to different data years, custom load profiles, future scenarios, and additional outputs. Requires downloading files and software.
Web Edition (Streamlined): Provides similar functionality to the Excel tool but is online, requiring no downloads. It uses the most recent year of input data, creates commonly used output formats, and allows users to save results. It can also perform analyses for a selected state, even if it spans multiple AVERT regions. This version is suitable for quick estimates and basic analyses.

3.3. Key Considerations for Using AVERT:

3.3.1. When to Use AVERT:

Analyzing power sector and on-road vehicle emission impacts of energy policies and programs (energy efficiency, renewable energy, electric vehicles, energy storage).
Screening analyses to understand emission impacts.
Quantifying PM_2.5, NO_x, SO_2, CO_2, VOCs, and NH_3 benefits.
Examining regional, state, and county-level impacts based on temporal energy savings, hourly generation profiles, and vehicle types.
Exploring the net impacts of adding EVs to the grid (considering increased power use vs. displaced fuel-burning vehicles).
Assessing emission impacts from other energy policies that increase electricity demand.
Comparing emission impacts of different program types (e.g., wind vs. solar-plus-storage).
Understanding impacts during high electricity demand days.
Analyzing benefits in multiple states within an AVERT region.
Presenting location-specific benefits in tables and maps. For more information on the uses of AVERT, see Uses of AVERT.

3.3.2. When Not to Use AVERT:

For analyses extending more than five years from the baseline without using AVERT’s Statistical Module and Future-Year Scenario Template.
For examining emission impacts of major fleet adjustments (beyond the scope of typical energy programs).
For comprehensive mobile source regulatory analysis, including State Implementation Plan (SIP) and transportation conformity analyses. AVERT's vehicle emissions modeling is not intended for this purpose.

3.4. AVERT Data and Updates:

AVERT uses public, accessible, and auditable data.
The EPA has used AVERT to produce marginal emission rates for each of the 14 AVERT regions and a weighted average for the nation annually from 2007 to 2023. These rates can be used for quick estimates; these are available on the Avoided Emission Rates Generated from AVERT page.
Users should always check the EPA AVERT website for the latest version and updates. Past webinars can be found at AVERT Webinars. A summary of how AVERT has been used and cited is available at Publications Citing AVERT.

3.5. AVERT's Role in Clean Air Act Plans:

States and municipalities are increasingly adopting energy policies to lower customer costs, improve electric supply reliability, and diversify energy portfolios. These policies also offer significant potential for reducing criteria air pollutants and GHGs, particularly on high electricity demand days when air quality is often poor. AVERT helps state air quality planners quantify these emission benefits, enabling their inclusion in Clean Air Act plans for meeting NAAQS and other clean air goals.

4. Calculating True Net Emissions (Expanded Scope)

To calculate true net emissions, especially when considering policies and programs that shift energy consumption or generation, we need to account for both increased emissions in one sector and reduced emissions in another.

4.1. General Formula for Net Emissions:

Net Emissions = Total Emissions (Baseline Scenario) - Total Emissions (Intervention Scenario)

Or, more commonly, when focusing on the impact of a specific intervention:

$Net Emissions = (Emissions_{New Source/Increased Activity}) - (Emissions_{Displaced Source/Reduced Activity})$

4.2. Application to Electric Vehicles:

4.2. Application to Electric Vehicles:

For electric vehicles, calculating true net emissions involves considering the upstream emissions from electricity generation for EVs versus the tailpipe and upstream emissions avoided from gasoline vehicles.

Net Emissions from EV Deployment = (Upstream Emissions per EV × Number of EVs) – ((Tailpipe Emissions per Gasoline Vehicle + Upstream Emissions per Gasoline Vehicle) × Number of Displaced Gasoline Vehicles)

Where:

Upstream Emissions per EV ( $E_{upstream, EV}$ ): This is the emissions from generating the electricity an EV uses (as calculated in Section 2.1.1).
Tailpipe Emissions per Gasoline Vehicle ( $E_{tailpipe, gasoline}$ ): This is the emissions directly from the exhaust of a gasoline car.
Upstream Emissions per Gasoline Vehicle ( $E_{upstream, gasoline}$ ): This accounts for emissions from producing and distributing gasoline.

4.3. Application to Energy Efficiency/Renewable Energy:

For energy efficiency or renewable energy programs, the net emissions are primarily the avoided emissions from the power sector.

$Net Emissions_{EE/RE} = Δ Emissions_{power sector} (calculated using AVERT’s methodology, as in Section 3.2.1)$

4.4. Broader Carbon Accounting Tools:

Beyond AVERT, other EPA tools can assist in broader carbon accounting for various sectors:

Waste Reduction Model (WARM): The WARM tool helps solid waste planners and organizations estimate GHG emission reductions from various waste management practices. This includes a recycled content tool and a Policy and Program Impact Estimator. The WARM User's Guide provides detailed guidance.
GHG Reporting Program: The EPA's GHG Reporting Program requires large GHG emitters to report their emissions, providing a dataset for comprehensive analysis.

Calculating true net emissions across diverse sectors can be complex and often requires a combination of these tools and methodologies.