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Reactive gases and aerosols are produced by terrestrial ecosystems,processed within plant canopies, and can then be emitted into theabove-canopy atmosphere. Estimates of the above-canopy fluxes are needed forquantitative earth system studies and assessments of past, present andfuture air quality and climate. The Model of Emissions of Gases and Aerosolsfrom Nature (MEGAN) is described and used to quantify net terrestrialbiosphere emission of isoprene into the atmosphere. MEGAN is designed forboth global and regional emission modeling and has global coverage with~1 km<sup>2</sup> spatial resolution. Field and laboratory investigations ofthe processes controlling isoprene emission are described and data availablefor model development and evaluation are summarized. The factors controllingisoprene emissions include biological, physical and chemical drivingvariables. MEGAN driving variables are derived from models and satellite andground observations. Tropical broadleaf trees contribute almost half of theestimated global annual isoprene emission due to their relatively highemission factors and because they are often exposed to conditions that areconducive for isoprene emission. The remaining flux is primarily from shrubswhich have a widespread distribution. The annual global isoprene emissionestimated with MEGAN ranges from about 500 to 750 Tg isoprene (440 to 660 Tgcarbon) depending on the driving variables which include temperature, solarradiation, Leaf Area Index, and plant functional type. The global annualisoprene emission estimated using the standard driving variables is ~600 Tgisoprene. Differences in driving variables result in emissionestimates that differ by more than a factor of three for specific times andlocations. It is difficult to evaluate isoprene emission estimates using theconcentration distributions simulated using chemistry and transport models,due to the substantial uncertainties in other model components, but at leastsome global models produce reasonable results when using isoprene emissiondistributions similar to MEGAN estimates. In addition, comparison withisoprene emissions estimated from satellite formaldehyde observationsindicates reasonable agreement. The sensitivity of isoprene emissions toearth system changes (e.g., climate and land-use) demonstrates the potentialfor large future changes in emissions. Using temperature distributionssimulated by global climate models for year 2100, MEGAN estimates thatisoprene emissions increase by more than a factor of two. This isconsiderably greater than previous estimates and additional observations areneeded to evaluate and improve the methods used to predict future isopreneemissions

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