The speed of chemical reactions is expressed as a rate of change in the concentration of a participating species with respect to time. The reaction rates of chemical species in the atmosphere are extremely important parameters when considering sources and sinks of gas-phase components. For instance, the reaction rate for the reaction of hydroxyl radical with atmospheric propane in the troposphere is very fast, yielding an alkyl radical that goes through a series of reactions to yield ozone in urban environments. Conversely the reaction rate of propane with the nitrate radical ("NO₃ dot") is.