Browsing by Subject "GEOS-Chem"

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    Constraints on the sources and impacts of volatile organic compounds (VOCs) over North America from tall tower measurements
    (2014-06) Hu, Lu
    This dissertation presents the first-ever in-situ tall tower measurements of volatile organic compound (VOC) concentrations. The data span August 2009 through July 2012, and provide new constraints on seasonal and long-term controls on VOC sources and their atmospheric effects. The 200 m sampling height provides a large-scale footprint, while the tall tower location, near the intersection of the main North American ecosystems and at times downwind of the Twin Cities, affords information on natural emissions from some of the most important US landscapes as well as on anthropogenic sources.I interpret the dataset using an atmospheric chemical transport model (GEOS-Chem CTM), with a focus on several key atmospheric VOCs. This dissertation finds that current models underestimate methanol emission rates for younger versus older leaves. This biased seasonality means that the photochemical role for methanol early in the growing season is presently underestimated. A Bayesian inverse analysis of the tall tower observations reveals that the prior estimate of North American anthropogenic acetone sources (based on the US EPA's NEI05 inventory) is accurate to within 20%. However, biogenic acetone emissions from broadleaf trees, shrubs, and herbaceous plants are presently underestimated (~37%), while emissions from needleleaf trees plus secondary production from biogenic precursors are overestimated by a similar amount (~40%). Model-measurement comparisons imply that isoprene emissions in the immediate vicinity of the tall tower are accurately captured by the MEGANv2.1 biogenic inventory, but that larger-scale regional emissions are underestimated, reflecting the heterogeneous land cover in this transitional landscape. Isoprene emissions play a key role in seasonal shifts between VOC-limited chemistry in the spring and fall and NOx-limited or transitional chemistry in the summer.A Bayesian inverse analysis based on the tall tower measurements suggests that: i) the RETRO global emission inventory significantly overestimates (> two-fold) US C6-C8 aromatic emissions; ii) the US EPA's NEI08 inventory likewise overestimates the toluene flux by a factor of 3, partly reflecting a bias in the estimated non-road emissions; and iii) total annual emissions of benzene and C8 aromatics in the EPA's NEI08 are accurate to within the analysis uncertainty, but with some seasonal biases for on-road emissions.
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    Global nitrogen deposition (2°×2.5° grid resolution) simulated with GEOS-Chem for 1984-1986, 1994-1996, 2004-2006, and 2014-2016
    (2018-05-31) Ackerman, Daniel E; Chen, Xin; Millet, Dylan B; dackerma@umn.edu; Ackerman, Daniel E; Ecology, Evolution, and Behavior Department, University of Minnesota
    Atmospheric deposition of inorganic nitrogen is critical to the function of ecosystems and elemental cycles. During the industrial period, humans have doubled the amount of inorganic nitrogen in the biosphere and radically altered rates of atmospheric nitrogen deposition. Despite this rapid change, estimates of global nitrogen deposition patterns generally have low, centennial-scale temporal resolution. Lack of information on annual- to decadal-scale changes in global nitrogen deposition makes it difficult for scientists researching questions on these finer timescales to contextualize their work within the global nitrogen cycle. Here we use the GEOS-Chem Chemical Transport Model to estimate wet and dry deposition of inorganic nitrogen globally at a spatial resolution of 2°×2.5° for 12 individual years in the period from 1984 to 2016. During this time, we found an 8% increase in global inorganic nitrogen deposition from 86.6 TgN yr-1 to 93.6 TgN yr-1, a trend that comprised a balance of variable regional patterns. For example, inorganic nitrogen deposition increased in areas including east Asia and Southern Brazil, while inorganic nitrogen deposition declined in areas including Europe. Further, we found a global increase in the percentage of inorganic nitrogen deposited in chemically reduced forms from 30% to 35%, and this trend was largely driven by strong regional increases in the proportion of chemically reduced nitrogen deposited over the United States. This study provides spatially explicit estimates of inorganic nitrogen deposition over the last four decades and improves our understanding of short-term human impacts on the global nitrogen cycle. We provide all output from these GEOS-Chem simulations related to atmospheric deposition. We provide all output from these GEOS-Chem simulations related to atmospheric deposition.