Towards the end of both last week and this week, Philadelphia has had two heat waves that brought temperatures to the high nineties in Fahrenheit. This caused the plants to droop, which made it necessary to water multiple times each day. In addition to drying out the plants faster and making us sweat as we worked in the garden, the heat wave also caused a spike in ozone levels.
Last Friday, ozone levels peaked at over 100 ppb, which is an unhealthy level. A good level is considered about 55 ppb at maximum. The ozone level has not peaked as high this week yet, with the highest being just under 70 ppb, which is considered moderate. To view more data than shown on the graph below, see the GO3 Project’s website, where data from our ozone monitor is uploaded to.
Ozone levels tend to increase with temperature for three main reasons. High temperatures often exist when there is a high pressure system, meaning it is sunny and dry. Sunlight is needed for the chemical reactions that form ground level ozone. Without rain or wind, there is little to move the ozone out of an area. There also tends to be low mixing of the air on days with higher temperatures, which causes ozone and other pollutants to stay near the ground. The third reason we are seeing higher ozone levels during this heat wave is because the higher temperatures increase the reaction rate of the chemical reactions that form ozone. Therefore, ozone is forming faster and staying around the garden longer.
Ground level ozone harms your lungs when you breathe it in at high levels, and damages plants at lower levels. That is why we consider ozone sensitive plants to be early indicators, and study ozone damage on their leaves.