¹ Atmospheric abundances for trace gases are reported here as the mole fraction (molar mixing ratio) of the gas relative to dry air (ppm = 10^-6, ppb = 10^-9, ppt = 10^-12); whereas the burden is reported as the total mass of that gas in the atmosphere (e.g., Mt = Tg = 10¹² g). For most trace gases in this chapter, the burden is based on the total weight of the molecule; for the N-containing gases, it includes only the mass of the N; and for some VOC budgets where noted, it includes only the mass of the C.