What Methods are Used in the Compilation of Air Toxics Emission Inventories?
The development of the Minnesota portion of the regional air toxics emission inventory follows the instructions illustrated in the protocol document for the Great Lakes Regional Air Toxics Emission Inventory Project and uses the Regional Air Pollution Inventory Development System (RAPIDS) to estimate emissions. Presented in the following sections are methodologies used in each principal source category on data acquisition, emission estimation, quality assurance and quality control plans, and uncertainties inherent in the inventory.
Point Sources
To estimate emissions of air toxic pollutants from Minnesota point sources, first, the Minnesota criteria pollutant emission inventory data are converted into the RAPIDS computing environment. This approach provides information on each device/process at each facility, including identification, activity data, and emissions for criteria pollutants (carbon monoxide, nitrogen oxides, particulate matter, particulate matter smaller than 10 microns, lead, sulfur dioxide, and volatile organic compounds) and then estimates emissions for each device/process. The following describes the available emission estimation methods and their prioritization for use in the emission inventory.
The following hierarchy is used to estimate emissions.
- Direct Reporting Values
All Minnesota companies are encouraged to submit data to the air toxics emission inventory. The MPCA sends letters to selected Minnesota companies for collecting emission inventory data prior to establishing each emission inventory database. As an increasing number of companies have provided data to the MPCA, the Minnesota state inventories have become more reliable. Letters for collecting 2005 air toxics emission inventory data can be found on the Guidance on Data Submittal Web page.
The reported information is examined and appropriate emissions are used. In addition, lead (Pb) emissions are provided by companies for the criteria emission inventory. These values are adapted to the air toxics emission inventory in order to maintain the consistency in these two MPCA inventories. - Emission Factors
An emission factor is defined as a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. Emission factors can be either source-specific or generic. In the 2005 emission inventory, the emission factors from the EPA’s online version of the Factor Information Retrieval (FIRE) Data System, were used as generic emission factors. In most cases, these emission factors are derived from actual measurements of the emissions from representative sources/processes, and are assumed to be the long-term averages for all facilities in the source category. The source-specific emission factors are derived from source-specific emission testing, mass balance, or chemical analysis. Therefore, they are preferred for estimating emissions from a source.
The MPCA has worked with certain industrial sectors to develop source-specific emission factors. Source-specific emission factors are developed based on the information in facility permit applications and stack testing reports. Metal mining/iron ores process and electric services/coal burning facilities are examples of this special effort.
A set of state-specific emission factors has also been developed by the MPCA from augmentation of generic emission factors to similar processes that didn’t have generic emission factors. - Toxics Release Inventory (TRI) Data
The TRI is compiled by the Minnesota Department of Public Safety for manufacturing point sources with certain reporting thresholds. The emissions data are facility-based and of unknown accuracy. For many facilities reporting to the TRI, the emission estimates appear to be incomplete in terms of the number of pollutants included. However, when emissions could not be obtained from the above two methods, TRI emissions were used for some facilities. - National Emission Inventory (NEI) Data
The NEI is prepared by the EPA's Emission Inventory Group with input from numerous state and local air agencies, from tribes, and from industry. MPCA staff review NEI data. Additional emission estimates are verified and adopted if they are appropriate.
Nonpoint Sources
The categories of nonpoint sources have been determined by the MPCA after reviewing the Source National Emission Inventory for Hazardous Air Pollutants (NEI), Emission Inventory Improvement Program (EIIP) documents and other available information. The emissions data for nonpoint sources are obtained from surveys, literature, and the submittals for the National Emission Standards for Hazardous Air Pollutants (NESHAP). There are 38 categories included in the 2005 Minnesota emission inventory. All of these categories along with activity data and information sources are listed in Nonpoint Sources.
Mobile Sources
For onroad mobile sources, toxics emissions were obtained from the U.S. EPA who used the MOBILE6.2 Vehicle Emission Modeling Software. The inventory includes 28 vehicle types and 12 roadway types (Onroad Mobile Sources).
For all nonroad equipment, MPCA used estimates from the EPA who used the NONROAD 2005 model. The inventory covers 9 equipment types. (See Nonroad Mobile Sources.)
Airport emissions are further categorized to commercial aircraft, air taxis, general aviation, ground support equipment, and aircraft auxiliary power units. The MPCA uses landing and takeoff (LTO) data for each airport and the Federal Aviation Administration (FAA) emissions modeling software or available emission factors to estimate hydrocarbon emissions. Air toxics emissions are estimated by a speciation method.
For locomotives, the MPCA collects operational information from railroads. The operational information includes the consumption of diesel fuel in line haul and yard locomotive operations, and the counties of operation. Emission estimates are calculated using emission factor and speciation methods.
Commercial marine vessel emissions are divided into underway and in port emissions. The 2005 emissions for commercial marine vessels were estimated based on the 2002 emissions with an adjustment of port activities in 2005 vs. 2002. The 2002 commercial marine vessel emissions were prepared by CENtral States Regional Air Partnership (CENRAP) using state-specific data.
Quality Assurance and Quality Control
To develop a reasonable and comprehensive air toxics emission inventory, procedures have been developed to provide quality assurance/quality control (QA/QC) of the data throughout the entire process of emission inventory development. Quality assurance is a planned set of external activities that are conducted by personnel not directly involved in the development of the inventory to evaluate data quality. On the other hand, quality control is a planned set of internal activities conducted by inventory development personnel to ensure data accuracy and completeness.
First, the RAPIDS software provides feedback on missing data during emission calculation and “out of range” errors when importing or entering data via the input screens.
For point sources, the Minnesota emission inventory data for criteria pollutants are used. It minimizes errors in the activity data because these data are the basis for emission fees. For this reason, facilities pay close attention to the quality of these data. MPCA also examines and validates the direct reported air toxics emissions for the reasonableness and completeness. The process-level draft air toxics emission estimates are sent to selected companies for their voluntary validation. The selected companies are those having source-specific emission factors. The information and comments received are incorporated into the emission inventory.
For nonpoint sources, the emissions are calculated using the RAPIDS software and spreadsheets. The results from these two approaches are compared and evaluated until a perfect match is reached.
For mobile sources, Minnesota estimates are compared with EPA’s estimates.
Minnesota emission inventory data also undergo QA/QC activities at a regional level. Emissions from Minnesota are compared with other Great Lakes states with regard to emissions by pollutant and by source category (point, nonpoint, mobile, etc.). Emissions in the current inventory are compared with those in previous inventories to determine the reasonability of differences among inventories. Emissions are ranked by county for each pollutant to check for outliers. Pollutants in similar processes are identified to find missing or additional air toxics.
Finally, after Minnesota emissions data are submitted to the National Emission Inventory (NEI), the EPA performs a national-level QA/QC on the data. MPCA will review the identified problems and make necessary corrections.
