This article is simply an attempt to discuss the proposed observation altitudes for MISO, a Laser Heterodyne Radiometer (LHR) satellite mission proposed to observe CH4 isotopes (¹³C in CH4 and ¹²C in CH4) in the UTLS. ADFA-UNSW are proposing 3 observation altitudes for the satellite, centred at 14, 18 and 22km, giving 4km vertical resolution. These altitudes are chosen from available absorption lines that provide sufficient absorption at higher altitudes without saturating at lower altitudes, as well as the available wavelength range for the detectors/lasers. The question at the moment is whether these altitudes are sensible to include in the instrument simulator from a scientific perspective. This article will discuss some of the considerations for those choices. Some points up front: - We don’t know how strictly constrained the possible observation altitudes are. - This article therefore assumes that any observation range is possible. - We also do not fully know (remember?) the precision/accuracy of the proposed measurements with the LHR technique, but assume that they are insufficient to resolve the small isotopic signals in the troposphere. - We are concentrating only on δ¹³C in CH₄, not on δD (i.e. CH₃D). - It would be instructive if a calculation of the expected averaging kernels could be provided given the satellite observing geometry, instrument resolution, and selected lines. Have the proposed observing altitudes been selected on the basis of such calculations?

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INTRODUCTION Background Australia releases the most dust in the southern hemisphere, with global reaching affects such as (but not limited to) phytoplankton growth, air pollution, and soil enrichment. Australian dust could have very important biogeochemical effects including New Zealand farming and southern oceanic iron supply. The contribution of Australian dust to New Zealand is not well established. Measurements of dust deposition in peat bogs near NZ estimated that 30-90% was Australian . Global average yearly dust emission estimates vary widely from X(TODO) to 2150 Tg yr^-1 . Yearly dust emission variance can be extreme, for example north African dust emissions range from 400 to 2200 Tg yr^-1 . In Australia around 100 Tg yr^-1 of dust are released anually(TODO CITE), however the estimate is rough and the large confidence intervals of dust emissions present problems when trying to determine direct radiative effect (DRE) of dust globally. The importance of the Eyre basin and recent increases in dust loading over the region has been shown using the Bureau of Meteorology’s (BOM’s) aerosol optical depth (AOD), Angstrom exponent (AE), and scattering coefficient data . Australian dust emission is more periodic than emissions from the northern hemisphere, with droughts and extreme weather systems combining to form massive dust storms like the 2009 ’red dawn’ dust storm over Sydney . In 2002 a dust storm was estimated to have shifted almost 96 Tg over one 24 hour period . This combined with relatively few long term Australian dust studies leads to lower confidence in simulated Australian dust properties. One estimate of anthropogenic dust emissions in Australia is 75%, based on 30% or more of each grid box being used . Aims GEOS-Chem modeled dust simulation is largely untested over Australia and New Zealand. We compare GEOS-Chem dust simulations with data from both AERONET and CoDii Dustwatch stations based in Australia. We examine Australian dust dynamics and seasonality as well as examine how well the model represents large dust events. We also look at how much of a role El Nino Southern Oscillation (ENSO) effects have on Australian dust sources and transport.