GAW Report No. 205 - IGAC Project

CHAPTER 1 - INTRODUCTION1.5.2 Satellite observationsGround, ship, and aircraft observations provide a detailed snapshot of atmosphericcomposition. Satellite-based observations provide a complementary global, continuousperspective, overcoming some of the temporal and spatial limitations of surface and aircraftmeasurements. Historically, satellites were most readily used to determine stratosphericatmospheric composition, largely because the presence of clouds and the overhead stratospheremake tropospheric measurement challenging. However, in recent decades, new instrumentstargeting tropospheric composition have been developed and deployed on satellites, rapidlyenhancing our ability to track global tropospheric composition. These observations can providecritical information for monitoring and forecasting of air quality, studying long-range transport ofpollution, and monitoring emissions of air pollutants and climate forcers. The majority oftropospheric measurements from space have employed nadir (downward looking) geometry.Occultation measurements offer much better sensitivity to trace species in the atmosphere;however detection is limited to the upper troposphere, which is generally less informative for airquality applications [Burrows et al., 2011].Solar backscatter measurements observe reflected and backscattered solar radiation in theultraviolet (UV), visible (VIS), and near infrared (NIR) spectral region and are typically sensitivedown to the lowest layers of the atmosphere, except in the case of significant cloud cover. Thisnear surface sensitivity and the narrow field of view obtained with nadir observations are importantpre-requisites for obtaining information about air pollutants. A number of key atmospheric tracegases absorb in the UV-visible range: O 3 , NO 2 , HCHO, SO 2 , BrO, and Glyoxal (CHOCHO).Retrievals of trace gas concentrations are a two-step process. First the slant column densities(SCD) are estimated along the radiation path using absorption spectroscopy and second thisquantity must be converted to the vertical column density (VCD) via the application of an air massfactor (AMF) (Figure 11). An AMF is the enhancement of the optical path length from the VCD dueto the viewing geometry of the satellite, the scattering properties of the atmosphere and surface,and the vertical distribution of the absorber. AMFs are commonly determined by combining tracegas profiles estimated from a chemical transport model with a radiative transfer model. Accountingfor clouds, aerosols and stratospheric contributions are non-trivial challenges associated withthese retrievals. Figure 12 illustrates recent satellite instruments that apply solar backscattertechniques to measure global tropospheric composition.Figure 11 - Simplified schematic of solar backscattering measurements15