Isoprene emissions and impacts in an ecological transition region inferred from tall tower measurements

Lu Hu1,2, Dylan B Millet3, Munkhbayar Baasandorj2, Timothy J Griffis3, Peter A Turner2, Dr. Detlev Helmig4, Abigale Curtis4 and Hueber Jacques4, (1)Harvard University, School of Engineering and Applied Sciences, Cambridge, MA, United States, (2)University of Minnesota Twin Cities, Minneapolis, MN, United States, (3)University of Minnesota Twin Cities, Department of Soil, Water, and Climate, St. Paul, United States, (4)University of Colorado at Boulder, Institute of Arctic and Alpine Research, Boulder, CO, United States
Abstract:
We present a full year of continuous in-situ measurements of isoprene and its oxidation products methyl vinyl ketone and methacrolein (MVK+MACR) by PTR-MS from a 244 m tall tower in the US Upper Midwest (KCMP tall tower). The tower is located at an ecological transition between isoprene-emitting deciduous forest to the north and east, and predominantly non-isoprene-emitting agricultural landscapes to the west and south. Based on independent cartridge measurements and a source-tracer analysis, we estimate that anthropogenic interferences (or anthropogenic isoprene) contribute on average 20% of the observed PTR-MS m/z 69 signal during daytime in summer at the KCMP tall tower (and up to 80% at night). Interferences for MVK+MACR at m/z 71 are small (7%). After removing these interferences, the observed isoprene and MVK+MACR mixing ratios show pronounced seasonal cycles, reaching maxima of 2540 pptv (isoprene) and 2790 pptv (MVK+MACR) during summer. The KCMP tall tower is impacted both by nearby isoprene sources (with transport time within an isoprene lifetime) and more distant regional isoprene sources (with transport time exceeding an isoprene lifetime), as indicated by daytime enhancements of isoprene (but little MVK+MACR) under southwest winds, and enhancements of MVK+MACR (but little isoprene) under transport from other directions. We find that the GEOS-Chem chemical transport model driven with the MEGANv2.1 biogenic inventory can reproduce the observed isoprene mixing ratios to within model uncertainty once improved land cover and temperature estimates are implemented in the model. However, a model low bias in MVK+MACR of (25% - 66%) cannot be resolved, even across diverse model assumptions for chemistry, atmospheric mixing, and land cover. This suggests that, while isoprene emissions in the immediate vicinity of the KCMP tall tower are adequately captured, the model is still underestimating emissions across the broader region. Using the loss of HOx radicals relative to the loss of NOx radicals (LHOx/LNOx) in the model as an indicator, we find that this region experiences a strong seasonal shift between VOC-limited chemistry in the spring and fall and NOx-limited or transitional chemistry in the summer, and that this transition is driven by the temporal and spatial distribution of isoprene emissions.
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