 Let me explain equation 415, which is a rate equation for methane. A rate equation is just a differential equation. The change of something with respect to time equals the production rate of something minus the fraction of something that is lost each unit of time multiplied by the amount of something. Note that the loss rate of something is always proportional to something. That something can be anything. It does not have to be a chemical concentration. It could be the amount of milk in your refrigerator or the number of socks in your drawer, both of which tend to disappear over time. In equation 415, it's the methane concentration, which has units of molecules per centimeter cubed. The production rate is in units of molecules per centimeter cubed per second. Remember, each term of the equation must have the same units. The last term is the loss rate. The reaction rate coefficient has units of centimeter cubed per molecule per second. But when we multiply it by the OH concentration, we get a product that has units of per second, which is a frequency. Now, OH varies from almost zero at night to a peak value of midday. However, we can take an average OH to find the average loss rate of methane. Note that if we assume that the production rate suddenly goes to zero, then we find a very simple equation, which has an exponential solution. We designate the time that it takes the exponential factor to go to minus one as a lifetime, which is just the inverse of a loss frequency.