This is an attempt to survey the important radiatively active chemical species of the Earth's atmosphere as they might be perceived from distant space. While the distances involved imply that only half-disk reflected solar (stellar) radiation and full-disk thermally emitted radiation can be seen, the diurnal rotation of Earth would reveal some additional information, especially for species with reaction time-scales of 10-100 days like carbon monoxide. The survey progresses in order of descending mean atmospheric concentration, though mean concentration has only weak correlation with radiative signal until we reach the 10 ppb mixing-ratio level. For example, molecular oxygen (O.21 molecular ratio) may be marginally observable in the 1.2 mm reflected solar spectrum, while ozone (with the global equivalent concentration of 500 ppb) should be visible at many wavelengths, especially the strong 9.6-mm band. It would be more visible than oxygen in the reflected solar spectrum, in the Chapuis bands. A significant indicator of Earth's oxidant chemistry is the O₃/O₂ ratio, set to 2 x 10^-6. This value is essentially set by the Chapman ozone chemistry, and N/S differences are mostly determined by the arrangement of continents and especially mountains, which determine the transport of ozone in the slow-chemistry regions of the lower stratosphere. Effects of other catalytic ozone-destruction cycles would therefore be difficult to infer.

Carbon dioxide and water would also be easily detectable in the infrared. Given the right viewing geometry, the annual variation in the northern hemispheric abundance of CO₂ would be visible, since the variation runs to about 15 ppm out of 340 ppm, or 5 %. The mean abundance of methane (1.7 ppm) should be visible, but annual variations would be only a few percent with a smaller usable methane emission signal.

Nitrous oxide, at 300 ppb, might well be distinguishable in thermal IR spectra, though its ca. 300 year atmospheric lifetime implies that it is thoroughly mixed up to the tropopause. Detecting nitrous oxide's spectrum would require finesse, since its major CO₂-window emissions tend to be overlapped by methane. Methane and nitrous oxide would be particularly likely emissions in a living planet. Nitrous oxide would appear to accumulate in the atmospheres of planets with remotely Earth-like living conditions. That species (and maybe NO, nitric oxide) seem to be emitted due to multiple leaks in the redox chemistry performing the interchange of two vital forms of nitrogen in Earth-like living systems: ammonium and nitrate; terrestrial experience suggests that the intermediate +1/2 oxidation state cannot be bypassed completely, and so small leaks of the gas occur. The long lifetime of N₂O makes the gas observable. Somewhat similar arguments can be made for the "natural" tendency to emit methane in the redox environment of Earth-like life. For sulfur gases, the only accumulating analog is OCS, with approximately one-thousandth the concentration; what we know of the OCS emission process makes it appear more peripheral to vital biogeochemical processes.

Carbon monoxide is a fascinating species that could be observed. Its intermediate lifetime, due to OH concentrations averaging 10⁺⁵, means that it often has a longitudinal variation that would be seen from space as a diurnal variation in radiance of a factor of 2 to 3 (as shown by the NASA Langley MAPS instrument). The variation arises from the fact that many CO sources (biomass burning, the oxidation of natural and anthropogenic organic emissions) are strongly tied to the Earth's pattern of continents and oceans. Earth's continents are brighter than its dark oceans, so high CO would be correlated with high continental albedo. One difficulty is that the longitudinal variation of cloudiness and water vapor tend obscure CO's signal. Since prolonged averaging might be necessary to observe CO's relatively weak thermal IR features, time-aliasing techniques might be necessary to see the diurnal variation. Observation of the annual cycles of CO might be more feasible.

Other organic emissions include isoprene and the natural vegetative emissions, which rival or outweigh methane in contribution to the global carbon cycle. In total, CH₄, CO, and these natural organics contribute only a few percent of the larger CO₂ carbon cycle. The emissions of these natural organics appear to be highly specific to certain vascular plants, in distinction to the emission of CH₄ and N₂O. C₂ and C₃ non-methane hydrocarbons might accumulate to concentrations higher than their ppb levels in an Earth-like atmosphere with less OH. Acetone, with its clear carbonyl band, might be most visible; current concentrations are around 1 ppb. However, at some point, perhaps with OH at 10³ or higher, there would likely be competition in the oxidation/removal process from the halogen-based radicals Cl and BrO. On Earth, these arise from acid-displacement of Cl and other volatile halogen compounds from sea salt, but also might result from photochemical processes, especially in low-OH atmospheres. Methyl halides could also accumulate to some extent, but become susceptible to UV radiation in low-ozone atmospheres, and this process could contribute even more halogen radicals.