Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)

Abstract

Abstract. Reactive gases and aerosols are produced by terrestrial ecosystems, processed within plant canopies, and can then be emitted into the above-canopy atmosphere. Estimates of the above-canopy fluxes are needed for quantitative earth system studies and assessments of past, present and future air quality and climate. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) is described and used to quantify net terrestrial biosphere emission of isoprene into the atmosphere. MEGAN is designed for both global and regional emission modeling and has global coverage with ∼1 km 2 spatial resolution. Field and laboratory investigations of the processes controlling isoprene emission are described and data available for model development and evaluation are summarized. The factors controlling isoprene emissions include biological, physical and chemical driving variables. MEGAN driving variables are derived from models and satellite and ground observations. Tropical broadleaf trees contribute almost half of the estimated global annual isoprene emission due to their relatively high emission factors and because they are often exposed to conditions that are conducive for isoprene emission. The remaining flux is primarily from shrubs which have a widespread distribution. The annual global isoprene emission estimated with MEGAN ranges from about 500 to 750 Tg isoprene (440 to 660 Tg carbon) depending on the driving variables which include temperature, solar radiation, Leaf Area Index, and plant functional type. The global annual isoprene emission estimated using the standard driving variables is ∼600 Tg isoprene. Differences in driving variables result in emission estimates that differ by more than a factor of three for specific times and locations. It is difficult to evaluate isoCorrespondence to: A. Guenther (guenther@ucar.edu) prene emission estimates using the concentration distributions simulated using chemistry and transport models, due to the substantial uncertainties in other model components, but at least some global models produce reasonable results when using isoprene emission distributions similar to MEGAN estimates. In addition, comparison with isoprene emissions estimated from satellite formaldehyde observations indicates reasonable agreement. The sensitivity of isoprene emissions to earth system changes (e.g., climate and land-use) demonstrates the potential for large future changes in emissions. Using temperature distributions simulated by global climate models for year 2100, MEGAN estimates that isoprene emissions increase by more than a factor of two. This is considerably greater than previous estimates and additional observations are needed to evaluate and improve the methods used to predict future isoprene emissions.

Monoterpene and Sesquiterpene Emission Estimates for the United States

“…After close examination of the underlying data we concluded there was no justification for finer discrimination of emissions rates by PFT, species, or sampling date, location, or season, due to the limited size and high unexplained variability of the data. Table 1 provides a summary of the data for SQT and MT emission factors (EF), including the mean and standard deviation of the EF across plant samples in the four PFT categories, with EF determined using a BER within a canopy environment model (14) at standard conditions with a representative biomass density (gdw m -2 ) for the specified PFT. Since the assumed biomass density for each PFT was a fixed value (500, 750, 500, and 500 gdw m -2 for BT, NT, SB, and GC, respectively (16)), the standard deviations of the EF in Table 1 reflect only the variability in BER across plant samples.…”

“…MEGANv2.02 driving variables and parametrizations for isoprene have been described previously (14). The MEGANv2.02 approach for MT and SQT is described in this manuscript, and parameters for other compounds will be described elsewhere.…”

Observational Insights into Aerosol Formation from Isoprene

“…5,9 Isoprene is emitted from vegetation and is the single largest source of VOCs to the atmosphere with global emissions of ∼500 Tg C yr −1 . 10 In the atmosphere, isoprene reacts predominantly with OH 11 forming SOA with a yield of 1−4% under low NO x (NO x < 0.1 ppb), 12 1−6% under high NO x (NO x > 1 ppb), 13,14 and up to 29% in the presence of highly acidic seed aerosols, 15 making it an important source of SOA. In smog chamber experiments, SOA yields were observed to be dependent on NO x concentrations, the VOC/NO x ratio, the NO 2 /NO ratio and aerosol acidity, 7,9,13,15 consistent with an anthropogenic−biogenic coupling.…”

SOA Formation Potential of Emissions from Soil and Leaf Litter

“…14 Much effort has gone toward developing parametrizations of VOC emissions from vegetation sources, such as those contained in the model of emissions of gases and aerosols from nature (MEGAN). 15,16 While not considered a major global source of terpenoid emissions, soil and leaf litter still contribute significantly to VOCs in the atmosphere, particularly in spring and fall. 17,18 The research focus for soil and litter emissions has been primarily on methanol, acetone, acetaldehyde, and other VOCs with photoproducts that are too volatile to contribute significantly to SOA production.…”