Abstract
Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model:TES regressions are generally consistent with the model:aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS >0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1-2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 0.26, 0.12 and 3.0 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.5 for expanding canopies with leaf area index <1.2) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of both the IASI and TES measurements.
Highlights
Methanol (CH3OH) is the most abundant non-methane volatile organic compound (VOC) in the atmosphere, with a global burden of 3–4 Tg, and is an important precursor of CO, HCHO, and O3 (Tie et al, 2003; Millet et al, 2006; Duncan et al, 2007; Choi et al, 2010; Hu et al, 2011)
We use in situ data from recent North American aircraft campaigns to evaluate the space-borne methanol retrievals: MILAGRO (Singh et al, 2009; Kleb et al, 2011) over Mexico, the Gulf of Mexico, and southern Texas (March 2006); INTEX-B (Singh et al, 2009; Kleb et al, 2011) over the Pacific Ocean and western US (April/May 2006); ARCPAC (Brock et al, 2011) over the US; ARCTAS (Jacob et al, 2010) over Canada and the western US (June/July 2008, which comprised the latter phase of the study); and TexAQS-II (Parrish et al, 2009) over the Houston area (September/October 2006)
The Infrared Atmospheric Sounding Interferometer (IASI) data exhibit consistently lower slopes than the Tropospheric Emission Spectrometer (TES):model and aircraft:model comparisons; this may be because the IASI sensitivity to methanol peaks higher in the atmosphere than does that of TES (Beer et al, 2008; Razavi et al, 2011), but it may be partly due to the influence of retrievals with low degrees of freedom for signal (DOFS) that are by necessity retained in the comparison
Summary
Methanol (CH3OH) is the most abundant non-methane volatile organic compound (VOC) in the atmosphere, with a global burden of 3–4 Tg, and is an important precursor of CO, HCHO, and O3 (Tie et al, 2003; Millet et al, 2006; Duncan et al, 2007; Choi et al, 2010; Hu et al, 2011). We use aircraft measurements and a global chemical transport model (GEOS-Chem CTM) to evaluate new space-based observations of tropospheric methanol, and interpret the satellite data in terms of their constraints on the seasonality of biogenic methanol emission fluxes. We use aircraft measurements from an ensemble of field campaigns (Intercontinental Transport Experiment-Phase B, INTEXB; Megacity Initiative: Local and Global Research Observations, MILAGRO; the second Texas Air Quality Study, TexAQS-II; Arctic Research of the Composition of the Troposphere from Aircraft and Satellites, ARCTAS; Aerosol, Radiation, and Cloud Processes affecting Arctic Climate, ARCPAC) with the GEOS-Chem CTM to (i) evaluate atmospheric methanol retrievals from the Tropospheric Emission Spectrometer (TES), and (ii) interpret the TES and IASI space-borne observations in terms of the seasonality of biogenic emissions from major plant functional types in midlatitude ecosystems
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