California Air Resources Board

Natural Sources are non-manmade emission sources, which include biological and geological sources, wildfires, windblown dust, and biogenic emissions from plants and trees.

Areawide Source Methodologies

Find out how ARB calculates emissions from natural source categories.

• Biogenic Sources
• Wildfire Sources
• Smoke Emission Estimation
• Geogenic Sources

Biogenic Emissions Inventory

ARB estimates emissions of biogenic volatile organic compounds (BVOCs) from vegetation for natural areas, crops, and urban vegetation. BVOC emissions are functions of the species leaf mass, emission factors, temperature, and light conditions.

In 1997, the United States Environmental Protection Agency (U.S. EPA) adopted the 24-hour fine particulate matter (PM2.5) standard of 65 micrograms per cubic meter (µg/m3). In 2006, this standard was lowered to a more health-protective level of 35 µg/m3. The South Coast Air Basin (South Coast) is designated nonattainment for both the 65 and 35 µg/m3 24-hour PM2.5 standards (24-hour PM2.5 standards). In 2020, monitored data demonstrated that the South Coast attained both 24-hour PM2.5 standards. The South Coast Air Quality Management District (District) has developed the 2021 Redesignation Request and Maintenance Plan for the 1997 and 2006 24-hour PM2.5 Standards for the South Coast Air Basin (2021 PM2.5 Redesignation Request) demonstrating that the South Coast has met the requirements to be redesignated to attainment for the 24-hour PM2.5 standards. On November 5, 2021, the District will consider approval of the 2021 PM2.5 Redesignation Request. CARB staff has reviewed the 2021 PM2.5 Redesignation Request and has concluded that it meets the requirements of the Clean Air Act for redesignation of the South Coast to attainment for both the 65 μg/m3 and 35 μg/m3 24-hour PM2.5 standards. On December 4, 2021, CARB will consider adoption of the 2021 PM2.5 Redesignation Request Plan for inclusion into the California State Implementation Plan.

CARB Documents

• Submittal Letter to U.S. EPA (pending)
• Staff Report (October 29, 2021)
• Notice of Public Hearing (October 29, 2021)
• Resolution (pending)

District Documents

• Draft Final 2021 Redesignation Request and Maintenance Plan for the 2006 and 1997 24-Hour PM2.5 Standards for South Coast Air Basin

Phase Down

In 2003, the California Legislature passed Senate Bill (SB) 705 (Florez) which aimed to phase out open agricultural burning in the San Joaquin Valley (Valley) between 2005 and 2010. As allowed under SB 705, the San Joaquin Valley Air Pollution Control District (District) could postpone the burn prohibition for some crop categories and materials if the District determined the postponement was necessary based on criteria delineated in SB 705, and CARB concurred. Since adoption of SB 705 in 2003, the District requested, and CARB provided concurrence on, postponements of the requirements of SB 705, in 2005, 2007, 2010, 2012, 2015, 2021, and 2024.

The District’s 2020 Staff Report and Recommendations on Agricultural Burning (2020 Report) provided a roadmap toward a full phase-out with its new agricultural burn prohibitions, while requesting CARB concurrence on proposed postponements of burn prohibitions for certain crop categories and materials for which alternatives are more challenging to implement. CARB concurred with the District’s burn prohibition postponements through August 31, 2021, and delegated the Executive Officer the authority to provide concurrence for an additional period beyond the first six-month concurrence period, until January 1, 2025, provided the District implemented additional measures in support of the near-complete phase-out of agricultural burning in the Valley, with CARB support. The District adopted the revised 2020 Report on June 17, 2021, and CARB provided concurrence on June 18, 2021, effective until January 1, 2025. Through these District and CARB Board actions, only very limited open burning of agricultural material will be allowed in the Valley. The District's 2024 Staff Report and Recommendations on Agricultural Burning (2024 Report) describes the limited burning that will be allowed to continue. CARB concurred on the 2024 Report on December 23, 2024, effective until January 1, 2030.

In response to the near-complete phase-out of agricultural burning adopted by CARB and the District, the California Legislature appropriated $180 million in the Budget Act of 2021 (SB 129, Skinner) to CARB to grant to the District to support incentives for alternatives to agricultural burning in the Valley. This significant funding has been critical to support the rapid transition from open burning to alternative practices for disposal of agricultural biomass.

CARB Documents

Limited Concurrence (January 3 - February 24, 2021) (January 2021)

February 25, 2021, Board Hearing

• Staff Report: San Joaquin Valley Agricultural Burning Assessment
• CARB Resolution 21-4: Limited Concurrence (February 25 - August 31, 2021)
• Other materials available here

CARB Concurrence (until January 1, 2025) (June 2021)

Principles for Alternatives to Agricultural Burning Grant (August 2021)

Agricultural Burning Alternatives Analysis Report (October 2021)

CARB Concurrence (until January 1, 2030) (December 2024)

District Documents

2020 Staff Report and Recommendations on Agricultural Burning (December 2020)

Supplemental Report and Recommendations on Agricultural Burning (June 2021)

2024 Staff Report and Recommendations on Agricultural Burning (November 2024)

Other materials available at District webpage