10.4 Accidental Vegetation Fires

Category 1580

10.4.1 Introduction

Category 1580 accounts for criteria pollutants (particulate matter, organic, NO_x, SO_x, and CO) emissions from accidental vegetation fires, i.e., wildfires burned on a variety of vegetation land cover such as woodland, timber, brush and grass. Wildfires are often caused by natural events (e.g., lightning strikes), unintentional human activities (e.g., equipment use, escaped prescribed burns), or unknown ignitions. Planned or prescribed fires such as weed burning, field crops and prunings burning, range improvement burning and forest management are not wildfires and covered in categories 315-319.

Starting from the current base year inventory, the category was treated as “CARB Source Categories” and the process was described below.

10.4.2 Methodology

For certain categories in the base year inventory, emissions data are derived by inventory staff of the state’s chief air quality regulatory agency, the California Air Resources Board (CARB). This significant emissions dataset, sorted by county, is published every few years and is a product of the State Implementation Plan (SIP) emissions document formally known as the California Emissions Projection Analysis Model (CEPAM) inventory. For related sets of categories, such as airport ground support equipment (GSE), ships, structures coatings, where independent data collection and derivation of emissions are both cost- and time-prohibitive and likely a redundant effort, BAAQMD staff export the emissions data directly from CEPAM into the District’s Base Year inventory package. This calculation approach and collection of categories is internally termed as “CARB Source Categories”.

The CEPAM provides historical emissions as well as forecasts emissions for major emission source classifications including –

1. on-road mobile sources [from Emissions Factor (EMFAC) model],
2. off-road mobile sources (OFFROAD model), and,
3. stationary and areawide sources - For these major source classifications, CEPAM combines facility level /area source emissions data reported to the California Emissions Inventory Development and Reporting System (CEIDARS) for multiple years by various regional air quality agencies (including the BAAQMD).

The current base year inventory uses the 2016 CEPAMv1.05 to estimate emissions from “CARB Source Categories”. This version of the CEPAM derives emissions from a 2012 base year inventory and contains backcasts and forecasts from year-2000 to year-2035. All applicable regulatory and technological controls are assumed to be built into the CEPAM dataset during CARB staff’s inventory computation work. After the emissions data are exported, the inventory for CARB Source categories is taken through a quality assurance (QA) process.

In the QA process, BAAQMD staff perform a systematic crosswalk between CEPAM’s source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids). Based on the scope of emissions covered, individual EIC or a group of EICs are mapped to a single cat_id. This process also addresses issues when cat_ids have no matching EICs or there are discontinued EICs that need to be investigated. Following this, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using certain mathematical methods, as described in the Trends section. Finally, the emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated, and CARB staff are consulted for explanation of any observed anomalies in trends.

For more details on how CARB estimated emissions for accidental vegetation fires, please refer to CARB’s CEPAM Inventory documentation. ⁴⁴⁴

10.4.3 Changes in Methodology

Comparing to the previous base year inventory, the category was processed as a “CARB Source Categories” under the current base year inventory, where county and year specific emissions were ported over from CARB CEPAM inventory directly instead of processed as a area source where emissions were estimated following CARB methodology by the District staff.

10.4.4 Emissions

The accidental vegetation fires is a significant source category for all pollutants it emits. A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

10.4.5 Trends

The emissions data from CARB CEPAM inventory only covers year 2000 to 2035. Different approaches were applied to estimate emissions for years 1990 to 1999 and 2036 to 2040 as described below. Please refer to CARB’s CEPAM Inventory Documentation ¹ for more information.

(a) Historical Emissions / History

Three different methods are used for backcasting to project historical emissions prior to year-2000 going back to year-1990, which is the starting year of the current base year inventory (referred to as BY2015). Following preliminary evaluation of the data available for initial years (post-year 2000) in the CEPAM inventory, BAAQMD staff recommends a backcasting approach. The default backcasting approach is defined by a formula here –

\[ \begin{eqnarray} \text{BY2015 emissions for year X for county Y} = \text{BY2011 total emissions for year X} \\ \\ \times \dfrac{\text{BY2015 year-2000 emissions}}{\text{ BY2011 year-2000 emissions}} \\ \\ \times \dfrac{\text{county Y fraction}}{\text{ fractional total}} \end{eqnarray} \]

This calculation is referred to as the BY2011 scaling method (based on the previous base year inventory). The second calculation approach is to derive the linear regression best fit of the year 2000-2008 CARB Sources emissions data for a category, species, and county, and extrapolate that data back in time. If this backcasting results in negative emissions prior to a certain year, the lowest positive emission value is held constant from that particular year back to year-1990. The third possible calculation approach is to use a specific growth profile for the category, provided by the in-house staff expert. These custom growth profiles are often derived from surrogate data provided by another agency or resource, or available activity data that is assumed to scale with the emissions for that specific CARB Source category.

For accidental vegetation fires, the BY2011 scaling method is used to derive emissions for years 1990 through 1999.

(b) Future Projections / Growth

Accidental vegetation fire category accounts for wildfire events which in nature are hard to predict until they occur. Emissions forecasting was done by holding the year-2035 emissions data constant through and up to year-2040, which is consistent with CARB’s no-growth assumption. At the time of developing 2016 CEPAM v1.05, CARB has estimated wildfire emissions based on actual annual wildfire activity up to year 2017. For any year beyond 2017, CARB assumed its wildfire emissions are constant. The average emissions of past ten years (year 2007 to 2016 while 2017 is considered as exceptional) are hold straight to year 2035⁴⁴⁵. As a results, it hasn’t accounted for the most recent large fires from 2018 to 2020 in Bay Area (e.g., LNU Lightning Complex fires, Glass fire), which are likely be included in the next release of CEPAM inventory.

10.4.6 Uncertainties

Wildfire events are unpredictable, large in spatial scale, and could last for days and even months. The accuracy and resolution of the underlying wildfire perimeters and vegetation fuel beds data, the assumption to use fuel moisture value of the ignition start date and the centroid of the wildfire polygon while compute fuel consumption, and the assumption that all area within the wildfire perimeters experienced fires all contributed to the unknown magnitude of uncertainties in emissions estimates for the category. There are even higher uncertainty in future year emissions which are assumed to the same as the average emissions of the past ten years.

10.4.7 Contact

Author: Yuan Du

Reviewer: Abhinav Guha, Tan Dinh

Last Update: November 06, 2023

10.4.8 References & Footnotes

---

444. CARB. Criteria Pollutant Emission Inventory Information (CEPAM) . [accessed 2023 Mar 13]. https://ww2.arb.ca.gov/criteria-pollutant-emission-inventory-data↩︎

445. CARB. November 2003. CEPAM Inventory Documentation Section 9.3 Wildfires. https://ww3.arb.ca.gov/ei/areasrc/onehtm/Wildfires_2019.pdf↩︎