Mexico City Air Pollution

I frequently get asked how bad the air pollution is in Mexico City. Well, it depends on the time of year but this past week has been pretty bad. Let me give you an idea.

I will be talking about the pollutant ozone. The former head of the NOAA lab where I worked referred to "the good, the bad, and the ugly" of ozone. If ozone is located in the stratosphere (i.e. the ozone layer), ozone is incredibly good because it prevents very harmful sun rays from getting to the Earth's surface thus allowing us to inhabit the planet. However, if ozone is located in the troposphere (the layer of the atmosphere closest to the Earth' surface) it is bad because it causes respiratory illnesses and it also damages plants. The ugly part of ozone is that when ozone is in the troposphere, it also acts as a greenhouse gas. Needless to say, due to ozone's effects on human health, the US Environmental Protection Agency (EPA) just reduced the ozone standard in the US from 80 ppbv (parts per billion by volume) to 75 ppbv average over an 8-hour period. That is, over an 8-hour period the average concentration of ozone cannot exceed 75 ppbv.

So let's see how bad the ozone pollution was in Mexico City this past Saturday, May 10th, which happened to be mother's day here. The highest consecutive ozone concentrations in the part of the city I live in on Saturday were as follows.

1:00pm 111 ppbv
2:00pm 118 ppbv
3:00pm 145 ppbv
4:00pm 155 ppbv
5:00pm 172 ppbv
6:00pm 159 ppbv
7:00pm 145 ppbv
8:00pm 92 ppbv

The 8-hour average is 137 ppbv, almost double the EPA's standard! So yes, the pollution is pretty bad here and Mother's day was one of the worst days I have seen recently (although I think the city breaks the EPA standard almost daily). One of the reasons that the ozone levels were so high on Mother's Day is that the mother is the most important figure in a Mexican family. Therefore, the 22 million people that live in Mexico City were all trying to get to their mothers. I am not joking, the traffic was horrible, worse than during the week. This greatly contributed to the high ozone levels. Kind of crazy.