The concentration of an atmospheric constituent is the number of molecules of that chemical species per volume, and that varies with pressure and temperature. In planetary atmospheric science we usually refer instead to abundance, often specifying it by the species's mixing ratio: given a sample of the atmosphere, the fraction of the total number of molecules in the sample that are molecules of that species. In the absence of complications like condensation or chemical reactions, the mixing ratio doesn't vary with temperature or pressure. With regard to measuring the abundance of constituents in an atmosphere via remote sensing, so far spectroscopy is indeed the best way. The frequencies of absorption/emission lines observed (any gas's spectral lines can be seen in either absorption or emission, depending on the circumstances) identify which chemical species are there. But in spectroscopy not only do you measure line frequencies, you also measure the intensity of the lines, i.e. how much the constituent is absorbing or emitting whatever kind of electromagnetic radiation you're observing (light, radio, etc.), and the shape of the lines when plotted as intensity vs. frequency. The intensity tells you how much of that constituent is there. If you can measure the intensities of multiple lines from the same constituent, you can even estimate its temperature! The shape of those lines also changes with the pressure of the gas: as the pressure increases, the line width, the range of frequencies the line spans, also increases. This is called pressure broadening. If the lines are strong enough (you have a high enough signal-to-noise ratio, SNR), you can use the line widths to measure the atmospheric pressure at the altitudes where the absorption or emission is occurring. Usually the lines you observe are combinations over a range of altitudes in an atmosphere, and that means a range of pressures and temperatures. This makes for very complex line shapes. If the SNR is high enough, you can sort out which parts of the line's shape are due to molecules at which pressures, so you can roughly infer the temperature-pressure profile, the temperature as a function of atmospheric pressure.