Methanol from TES global observations: retrieval algorithm and seasonal and spatial variability.

K E Cady-Pereira,Y Xiao,M Luo,J Worden,D B Millet,K C Wells,V H Payne,M W Shephard

doi:10.5194/acp-12-8189-2012

Abstract

We present a detailed description of the TES methanol (CH3OH) retrieval algorithm, along with initial global results showing the seasonal and spatial distribution of methanol in the lower troposphere. The full development of the TES methanol retrieval is described, including microwindow selection, error analysis, and the utilization of a priori and initial guess information provided by the GEOS-Chem chemical transport model. Retrieval simulations and a sensitivity analysis using the developed retrieval strategy show that TES: (i) generally provides less than 1.0 piece of information, (ii) is sensitive in the lower troposphere with peak sensitivity typically occurring between ~900-700 hPa (~1-3 km) at a vertical resolution of ~5 km, (iii) has a limit of detectability between 0.5 and 1.0 ppbv Representative Volume Mixing Ratio (RVMR) depending on the atmospheric conditions, corresponding roughly to a profile with a maximum concentration of at least 1 to 2 ppbv, and (iv) in a simulation environment has a mean bias of 0.16 ppbv with a standard deviation of 0.34 ppbv. Applying the newly derived TES retrieval globally and comparing the results with corresponding GEOS-Chem output, we find generally consistent large-scale patterns between the two. However, TES often reveals higher methanol concentrations than simulated in the Northern Hemisphere spring, summer and fall. In the Southern Hemisphere, the TES methanol observations indicate a model overestimate over the bulk of South America from December through July, and a model underestimate during the biomass burning season.

Highlights

Global high-spectral resolution nadir measurements from the Tropospheric Emission Spectrometer (TES), a Fourier Transform Spectrometer (FTS) on National Aeronautics and Space Administration (NASA)’s Aura platform, enable the simultaneous retrieval of a number of tropospheric pollutants and trace gases, in addition to the TES standard operationally retrieved products such as carbon monoxide and ozone
We carried out an initial assessment of the TES retrieval performance by comparing all 2009 TES Global Surveys over land with the corresponding GEOS-Chem values (Fig. 11, left panels)
Each retrieval was matched with a collocated GEOS-Chem profile, to which the TES observational operator and Representative Volume Mixing Ratio (RVMR) weighting were applied

Summary

Introduction

Global high-spectral resolution nadir measurements from the Tropospheric Emission Spectrometer (TES), a Fourier Transform Spectrometer (FTS) on NASA’s Aura platform, enable the simultaneous retrieval of a number of tropospheric pollutants and trace gases, in addition to the TES standard operationally retrieved products such as carbon monoxide and ozone. Methanol (CH3OH) is one of the additional species that can be retrieved in conjunction with the TES standard products, and is important for local, regional, and global tropospheric chemistry studies. Oxidation of methanol is a major source of carbon monoxide (CO) and formaldehyde (HCHO) (Hu et al, 2011) and leads to production of tropospheric ozone (O3) (Tie et al, 2003). Methanol sources and sinks are poorly quantified, with estimated global emissions ranging from 120 to 340 Tg yr−1 (Millet et al, 2008). The predominant methanol sinks are photochemical oxidation by OH, ocean uptake, and deposition (Millet et al, 2008; Stavrakou et al, 2011)

Methods

Results

Conclusion