10.15 Biogenic Emissions
Categories 784, 785, 786, and 787
10.15.1 Introduction
Categories 784, 785, 786 and 787 account for reactive organic gas (ROG) emissions from natural vegetation in the San Francisco Bay Area (SFBA). Living vegetation throughout the SFBA produce organic emissions as part of natural biological processes. Biogenic volatile organic compounds (BVOCs) emitted by plants play an important role for ecological and physiological processes, for example, as a response to stressors. Some of these BVOCs are emitted in surprisingly large amounts and have high enough chemical reactivity to significantly affect the chemistry of the atmosphere. BVOCs contribute greatly to the formation of aerosols and ozone in the urban space and hence are reported/accounted for in the BAAAQMD’s emissions inventory .
Chemical analysis of BVOCs has led to their classification into four separate chemical groupings. This detail is retained in the four categories developed for the current inventory and covered in this chapter. These emissions are considered to be part of the general group known as area sources, since emissions occur from numerous diffused sources from vegetation spread throughout the SFBA.
Emissions are classified under the following category numbers:
| Category # | Description |
|---|---|
| 784 | Isoprene |
| 785 | Methyl Butenol |
| 786 | Monoterpenes |
| 787 | Other Biogenic VOCs |
10.15.2 Methodology
BVOC emissions data are derived by inventory staff of the state’s chief air quality regulatory agency, the California Air Resources Board (CARB). This 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) inventory481. BAAQMD staff exports the emissions data directly from CEPAM into the District’s Base Year inventory package.
BVOC emissions are principally estimated using the MEGAN3.0 biogenic emissions model482. Model inputs include the California GAP Analysis Project, California Department of Water Resources crop reports, SCAG and SANDAG (councils of government) urban land use, and satellite-derived leaf area index (LAI) data. The MEGAN model is driven by temperature and solar radiation, BVOC emission factors and specific leaf weight factors.
(a) Activity Data / Throughput
The District does not have access or exposure to the activity data used in the MEGAN model and directly uses the CEPAM emissions for reporting.
(b) County Distribution / Fractions
The county-level emissions data were used to calculate county fractions for each of the species listed above. For three of four species (except monoterpenes), Napa County accounts for the most emissions, with 69% of SFBA Methyl Butenol emissions originating in Napa County. This is most likely due to prevalence of viticulture and wine cultivation in Napa County.
(c) Emission Factors
Since emissions are derived directly from CEPAM emissions inventory which is based on the MEGAN3.0 model2, the District does not have information on category-specific emission factors.
(d) Control Factors
Since BVOC emissions are naturally occurring, there are no direct regulations or control measures to reduce these emissions. It can be argued that since emission driver metrics like temperature (local and global) are being influenced by anthropogenic factors, regulations to curb greenhouse gas emissions and reduce global warming in the long-term will have an impact on regional BVOC emissions.
(e) Speciation
All BVOC emissions are considered highly reactive and subsequently part of ROG. The ROG:TOG ratio is thereby considered 1 for all four categories.
(f) Sample Calculations
Since emissions are derived from state’s CEPAM inventory directly, there are no sample calculations to demonstrate.
10.15.3 Changes in Methodology
In District’s prior inventories (e.g. base year 2011 inventory), the BVOC emissions were based on CARB’s Biogenic Emission Inventory processing model (BEIGIS)483. CARB has since moved on to the MEGAN3.0 model. The MEGAN model provides the most current and accurate land cover, but the model architecture is not significantly different from MEGAN v.2.04 for isoprene. The BEIGIS model shares MEGAN v.2.04 architecture but uses different land cover and vegetation specific emission factors. The BEIGIS model uses hourly temperature and solar radiation data sets gridded at 4 km2 while MEGAN computes the same metric at a 1 km2 resolution.
10.15.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
The TOG emissions for base year 2015 from the four BVOC categories in the SFBA total to ~69,000 tons/year, which makes it the single largest group of TOG emitters in the current emissions inventory. These emissions are, however, not considered anthropogenic and are thus not included in the base year emissions inventory Summary Report.
10.15.5 Trends
Note that the BVOC emissions are the same across all years in the CEPAM database. This is likely due to the assumption that vegetative land cover across the nine counties of SFBA does not change statistically over time.
10.15.6 Uncertainties
Please refer to information on MEGAN3.0 model484 to develop an understanding of uncertainties in the BVOC emissions estimates.
10.15.7 Contact
Author: Abhinav Guha and Stephen Reid
Reviewers: Tan M. Dinh and Yuan Du
Last Update: November 06, 2023
10.15.8 References & Footnotes
CARB. 2016. CEPAM Inventory. https://www.arb.ca.gov/app/emsinv/fcemssumcat/fcemssumcat2016.php↩︎
CARB. 2020. CARB Biogenic Emissions Inventory Model. [accessed 2020 Jan]. https://ww2.arb.ca.gov/biogenic-emissions-inventory↩︎
Scott, K. I. and Benjamin, M. T. 2003. Development of a biogenic volatile organic compounds emission inventory for the SCOS97-NARSTO domain. Atmospheric Environment, 37, S39–S49. https://doi.org/10.1016/S1352-2310(03)00381-9↩︎
MEGAN 3.0. 2020. [accessed 2020 Jan]. Model of Emissions of Gases and Aerosols from Nature. https://bai.ess.uci.edu/megan↩︎