Ozone Levels in 2015

In fact, 2014 was our cleanest year on record.  In 2015, however, we saw increased ozone in the region.

The greater Houston region has been working for many years to meet EPA’s health-based standard for ozone.  Over the past 15 years, we have seen significant decreases in both ozone levels and the number of days that the standard was exceeded in Houston.  In fact, 2014 was our cleanest year on record.  In 2015, however, we saw increased ozone in the region.  The levels of ozone were slightly higher, and the number of days we exceeded the standard was higher as well.  Scientists are still working to understand the reason for the higher ozone.  This article will discuss Houston’s ozone levels in 2015.  In two future articles, we will discuss some potential reasons why ozone levels were higher in our region.

Days_monitors_exceeded

Graph Courtesy of Houston Regional Monitoring Corporation

This graph shows the number of ozone exceedance days for EPA’s new, more stringent 70ppb standard, as well as from previous levels of the standard from 1998 through 2015.  As you can see, in 2015 there was a bump up in the number of ozone exceedance days, departing from a long-term historical downward trend.  There are two other important things to note about this graph.  First, the number of days when Houston had a monitor that exceeded the ozone standard is clearly much lower in recent years than even 10 years ago.  This is because even during a time of population and economic growth, we have put in place many regulatory and voluntary measures that reduce ozone’s building blocks generated by industry, cars and trucks, non-road sources like forklifts and lawnmowers.

Another thing we notice from this graph is that in some years ozone is higher than in other years, even while we are making progress.

Another thing we notice from this graph is that in some years ozone is higher than in other years, even while we are making progress.  Ozone is formed in a complex chemical reaction that is driven by sunlight and other weather phenomena.  Some years have more days in which the weather is more favorable for forming ozone than others.  In very hot, dry summers like 2010 and 2011, there were more days in which the weather favored the formation of ozone, contributing to a bump up in ozone levels.  It’s possible that 2015 was another year in which there were more days that were hot, dry, and less windy.

So, how does 2016 data look?  Ozone can form in Houston through October or November, so we likely won’t have a full understanding of our ozone season until later this fall.  However, the data so far shows that as of September 26, 2016, we have had far fewer days that exceeded the 70ppb standard than in 2015.  The graph below shows the cumulative number of days that exceeded the standard in 2015 (in purple) versus 2016 (in yellow).  Despite an earlier start to the ozone season in 2016, ozone levels remain lower so far.  If this trend continues through the fall, we can expect our long-term downward trend in ozone to continue.

ozone-chart

Based on sites common to both periods.  Source:  U.S. EPA AirData https://www.epa.gov/air-data

To Explore Other Ways of Looking at Ozone Data Visit:  https://www.epa.gov/outdoor-air-quality-data