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Emissions Calculations

Emissions resulting from project construction and operation comprise three categories: “criteria” pollutants, toxic air contaminants (TACs), and greenhouse gases (GHGs).  Criteria pollutants are those for which specific ambient air quality standards have been set by federal and state agencies, and include volatile organic compounds (VOCs), nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO2), respirable particulate matter (PM10), fine particulate matter (PM2.5), and lead (Pb).  Of these, VOCs and NOx react photochemically to form ground-level ozone (O3), which is the principal criteria air pollutant of concern in many regions in California.

Emissions from project construction and operation generally reflect the information provided in the project description.  The estimated quantities of criteria pollutants, TACs, and GHGs emitted by a project, in combination with other factors, demonstrate whether a project could have a significant impact on air quality or climate change.  Unlike air permitting, which generally only focuses on the direct stationary source operation and related fugitive emissions, a CEQA analysis must also include construction emissions and indirect emissions, such as employee travel and deliveries to and from the project site.

Construction Emissions

Construction emissions analyses are typically performed using the California Emissions Estimator Model™ (CalEEMod), the official statewide land use computer model designed to provide a uniform platform for estimating potential criteria pollutant and GHG emissions associated with both construction and operation of land use projects under CEQA.  CalEEMod was developed by the California Air Pollution Control Officers Association (CAPCOA) in collaboration with the 35 California air districts, which provided default data to account for local requirements and conditions.

CalEEMod quantifies mobile source emissions from on-road vehicles and off-road construction equipment by incorporating the California Air Resources Board (CARB) EMission FACtors (EMFAC) and OFFROAD models, respectively.  On-road vehicles are characterized using project-specific information or default data for emission factors, trip frequencies and lengths, meteorology, vehicle fleet mix (type and age), and the Pavley and Low Carbon Fuel standards.  Off-road (non-road) equipment is characterized by type, age, horsepower rating, daily running hours, and average load factor.  In addition to engine exhaust, CalEEMod estimates fugitive dust generated by earthmoving activities and road traffic for both paved and unpaved surfaces using methodologies developed by the EPA.  The model also contains mitigation measures to reduce fugitive dust, criteria pollutant, and GHG emissions along with calculating the benefits achieved from the selected measures.

Operational Emissions

Operational emissions from common stationary sources, such as boilers, heaters, gas turbines, firewater pumps, and emergency generators, are quantified using source-specific emission factors published by the EPA, CARB, or local air districts. Alternatively, rule-based emissions standards or manufacturer data are used to derive emission factors for particular equipment. Emissions of criteria air pollutants, TACs, and GHGs can be calculated with a high degree of confidence using emission factors. For non-standard stationary sources where emission factors are not available, mass emissions are estimated using appropriate emission estimation techniques such as source tests, material balance, or engineering calculations.

Direct operational emissions from mobile sources (on-road vehicles and non-road equipment, as described above) and area sources (natural gas combustion, painting, consumer products, and landscape maintenance) are estimated using CalEEMod. The model is also used to estimate indirect emissions, such as GHG impacts from energy use (electric power generation), solid waste disposal (collection and landfilling), vegetation planting and/or removal (sequestration), and water use (pumping energy and wastewater treatment).  The model also identifies mitigation measures to reduce criteria pollutant and GHG emissions, along with calculating the benefits achieved from the selected measures.

Relation to Health Risk Assessment

For projects where CalEEMod is used to estimate construction emissions, off-road engine exhaust PM10 is usually treated as diesel particulate matter (DPM) for on-site risk assessment purposes, particularly for the site preparation and grading phases, which involve the use of heavy earthmoving equipment in a defined area relative to receptors. DPM is a composite TAC as defined by the California Office of Environmental Health Hazard Assessment (OEHHA), comprising several organic and inorganic contaminants expressed as a single value.

Operational stationary source emissions may include diesel-powered emergency generators and firewater pumps required for safety, or gas-fired commercial/industrial utility boilers. As applicable, TAC emissions from stationary equipment are quantified using source-specific emission factors and operating data. Emissions from emergency engines are estimated based on tier rating, nameplate horsepower, and annual operating hours allowed under the statewide Airborne Toxic Control Measures (ATCM) regulation and other operational restrictions, such as proposed permit conditions.

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