Isoprene emissions and impacts over an ecological transition region in the U.S. Upper Midwest inferred from tall tower measurements

Abstract

We present 1 year of in situ proton transfer reaction mass spectrometer (PTR-MS) measurements of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) from a 244 m tall tower in the U.S. Upper Midwest, located at an ecological transition between isoprene-emitting deciduous forest and predominantly non-isoprene-emitting agricultural landscapes. We find that anthropogenic interferences (or anthropogenic isoprene) contribute on average 22% of the PTR-MS m/z 69 signal during summer daytime, whereas MVK + MACR interferences (m/z 71) are minor (7%). After removing these interferences, the observed isoprene and MVK + MACR abundances show pronounced seasonal cycles, reaching summertime maxima of >2500 pptv (1 h mean). The tall tower is impacted both by nearby and more distant regional isoprene sources, with daytime enhancements of isoprene (but little MVK + MACR) under southwest winds and enhancements of MVK + MACR (but little isoprene) at other times. We find that the GEOS-Chem atmospheric model with the MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic inventory can reproduce the isoprene observations to within model uncertainty given improved land cover and temperature estimates. However, a 60% low model bias in MVK + MACR cannot be resolved, even across diverse model assumptions for NO x emissions, chemistry, atmospheric mixing, dry deposition, land cover, and potential measurement interferences. This implies that, while isoprene emissions in the immediate vicinity of the tall tower are adequately captured, they are underestimated across the broader region. We show that this region experiences a strong seasonal shift between VOC-limited chemistry during the spring and fall and NO x -limited or transitional chemistry during the summer, driven by the spatiotemporal distribution of isoprene emissions. Isoprene's role in causing these chemical shifts is likely underestimated due to the underprediction of its regional emissions.

Aerosol Optical Depth Over India

“…The biomass burning emissions were from the Global Fire Emissions Database version 4 (Giglio et al, 2013). The biogenic VOC emissions were from the Model of Emissions of Gases and Aerosols from Nature version 2.1 inventory of Guenther et al (2012) as implemented in GEOS-Chem by Hu et al (2015). Emissions from other natural sources (e.g., lightning and volcanoes) were also included (Fisher et al, 2011;Murray et al, 2012).…”

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“…As discussed in Jiang et al (2015) several previous studies have suggested that the MEGANv2.0 isoprene emissions are biased high over North America. Hu et al (2015) found that using the MEGANv2.1, together with an improved land cover distribution, in GEOS-Chem successfully reproduced isoprene observations in North America. However, the MEGANv2.1 inventory is not yet available in the GEOS-Chem adjoint model.…”

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