Ozone Levels in 2015: What Can We Learn from the Data?

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.  We know that weather is a key factor that drives ozone formation.  Some years have more days with hot, dry, stagnant weather that can contribute to more days with high ozone.  Wildfires and agricultural burning, both nearby and far away, can also contribute to high background levels of ozone entering the area.  We know from the National Interagency Fire Center data that 2015 was a particularly bad wildfire season in the U.S.

Houston’s network of air pollution monitors does not just measure ozone.  Many sites also have the ability to measure the amount of ozone’s building blocks, or precursors.

There is another source of data that scientists will examine to determine why 2015 had higher ozone levels.  Houston’s network of air pollution monitors does not just measure ozone.  Many sites also have the ability to measure the amount of ozone’s building blocks, or precursors.  Remember that ozone is not directly emitted into the air.  It’s formed by a complex chemical reaction that occurs under favorable weather conditions.  Two different kinds of compounds come together to form ozone.  The first group is called oxides of nitrogen, or NOx.  NOx is emitted by all kinds of combustion processes, from industrial processes, to automobiles, other engines, and even your backyard barbecue.  The other group is volatile organic compounds, or VOCs.  VOCs are found in many chemicals, gasoline, household paints and cleaners, and in plants.  That smell when you slice a lemon is limonene, a naturally occurring VOC found in the peel.

Let’s look at how NOx and VOCs in Houston have changed over time.

The chart below shows the average annual level of NOx from 1985 to 2015.  As you can see, NOx emissions have dropped significantly—48% since 1998!  This is particularly remarkable given the tremendous population, vehicle, and economic growth in the region over that time period.  Looking closely, it appears that average NOx may have increased slightly in 2015.

average_nox_2015

Graph Courtesy of Houston Regional Monitoring Corporation

How about VOCs?  Since 2004, the Texas Commission on Environmental Quality (TCEQ) has regulated industry’s emissions of a special class of VOCs, known as highly reactive VOCs or HRVOCs.  This group of VOCs is important because they are especially efficient at forming ozone.  As you can see, average annual concentrations of HRVOCs has decreased significantly since the program began just a decade ago—a 57% drop!  Much like NOx, though, there appears to be year to year variation in the average concentration, and a slight uptick in 2015.

Average_hrvoc_2015

Graph Courtesy of Houston Regional Monitoring Corporation

Scientists will look for the cause of these slight increases and the role they might have played in higher ozone in 2015.

Scientists will look for the cause of these slight increases and the role they might have played in higher ozone in 2015.  Were they due to population growth?  More cars on the road or port traffic?  Changes or increases in routine or episodic industrial processes?  Or the wildfires we mentioned above?

So, how does the 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 late October 2016, we have had far fewer days that exceeded the 70ppb standard than in 2015, and overall levels have been lower even on exceedance days.  Check back here this fall for an updated discussion of our overall long-term progress in reducing ozone in Houston.

To Track 2016 Ozone Data:  https://www.tceq.texas.gov/cgi-bin/compliance/monops/8hr_exceed.pl