No abstract

This is a repository copy of Acceleration of global N2O emissions seen from two decades of atmospheric inversion.

Abstract. We present over one year (January 2010-February 2011 of continuous atmospheric methanol measurements from the University of Minnesota tall tower Trace Gas Observatory (KCMP tall tower; 244 m a.g.l.), and interpret the dataset in terms of constraints on regional methanol sources and seasonality. The seasonal cycle of methanol concentrations observed at the KCMP tall tower is generally similar to that simulated by a global 3-D chemical transport model (GEOS-Chem, driven with MEGANv2.0 biogenic emissions) except the seasonal peak occurs ∼1 month earlier in the observations, apparently reflecting a model underestimate of emission rates for younger versus older leaves. Based on a source tracer approach, which we evaluate using GEOS-Chem and with multiple tracers, we estimate that anthropogenic emissions account for approximately 40 % of ambient methanol abundance during winter and 10 % during summer. During daytime in summer, methanol concentrations increase exponentially with temperature, reflecting the temperature sensitivity of the biogenic source, and the observed temperature dependence is statistically consistent with that in the model. Nevertheless, summertime concentrations are underestimated by on average 35 % in the model for this region. The seasonal importance of methanol as a source of formaldehyde (HCHO) and carbon monoxide (CO) is highest in spring through early summer, when biogenic methanol emissions are high but isoprene emissions are still relatively low. During that time observed methanol concentrations account for on average 20 % of the total CO and HCHO production rates as simulated by GEOS-Chem, compared to 12 % later in the summer and 12 % on an annualCorrespondence to: D. B. Millet (dbm@umn.edu) average basis. The biased seasonality in the model means that the photochemical role for methanol early in the growing season is presently underestimated.

SUMMARY: Isoprene is the dominant non-methane organic compound emitted to the atmosphere 1 – 3 . It drives ozone and aerosol production, modulates atmospheric oxidation, and interacts with the global nitrogen cycle 4 – 8 . Isoprene emissions are highly uncertain 1 , 9 , as is the non-linear chemistry coupling isoprene and the hydroxyl radical, OH—its primary sink 10 – 13 . Here we present the first global isoprene measurements from space, using the Cross-track Infrared Sounder (CrIS). These isoprene measurements, together with observations of its oxidation product formaldehyde, provide new constraints on isoprene emissions and atmospheric oxidation. We find that isoprene:formaldehyde relationships measured from space are broadly consistent with current understanding of isoprene-OH chemistry, with no indication of missing OH recycling at low-NO x . We analyze these datasets over four global isoprene hotspots in relation to model predictions, and present a first demonstration of isoprene emission quantification based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural NO x emissions bias modeled OH and hence isoprene. Over southern Africa, we find that a prominent isoprene hotspot is missing from bottom-up predictions. A multi-year analysis sheds light on interannual isoprene variability, and suggests the role of El Niño.

Nitrous oxide (N2O) emissions within the US Corn Belt have been previously estimated to be 200–900% larger than predictions from emission inventories, implying that one or more source categories in bottom‐up approaches are underestimated. Here we interpret hourly N2O concentrations measured during 2010 and 2011 at a tall tower using a time‐inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42 to 58% and from 200 to 525% for direct and indirect components, respectively. Optimized agricultural N2O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg N yr−1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg N yr−1 in 2011. We attribute the interannual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom‐up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom‐up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014 to 0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom‐up field studies.