Drexel University characterizes ozone production using mass flow control

The tropospheric air pollutant ozone poses serious risks to human health. It can damage the lungs, worsen chronic respiratory diseases, and increase susceptibility to respiratory infections. Unfortunately, due to continued emissions of its precursors from sources such as cars and industry, ozone concentrations are still considered unhealthy in many parts of the country. In order to combat these high levels of atmospheric ozone, it is important to be able to detect and precisely quantity ozone levels and characterize how it is formed in the atmosphere.

Challenge: Ozone measurement solutions

Ozone is not released directly into the air, rather it results from chemical reactions between already emitted pollutants in the air. Experts have identified a few of these pollutants, including volatile organic compounds, nitric oxide, nitrogen dioxide, and peroxy radicals. It is common for experts to simulate these pollutant concentrations with photochemical air models. While these mathematical models provide valuable information, direct measurements of the key processes greatly assist in the formulation of ozone regulatory strategies.

Solution: The ECHAMP technique, with precise mass flow control

Dr. Wood, a professor of atmospheric chemistry at Drexel University, is developing a novel way to quantify the rate at which ozone is formed in the atmosphere with higher accuracy. His technique, the Ethane CHemical AMPlifier (ECHAMP), is an improvement upon the existing peroxy radical measurement technique of chemical amplification. Compared to the older methods, ECHAMP is both more accurate and safer for users, with lower sensitivity to relative humidity and no use of toxic gases.

The ECHAMP technique follows a few basic steps:

  1. The ECHAMP instrument precisely mixes sampled air with nitric oxide and ethane.
  2. Each sampled peroxy radical produces 25 molecules of nitrogen dioxide.
  3. Determine the peroxy radical concentration from the number of nitrogen dioxide molecules.
  4. Determine the ozone formation rate using this peroxy radical data and other supporting measurements.

Ambient air, nitric oxide, nitrogen, and ethane must be flowed into the ECHAMP chambers at very precise flow rates, to ensure correct measurement of the peroxy radicals. Dr. Wood decided to use Alicat mass flow controllers to meet the precise flow rate requirements of the ECHAMP technique. In building the ECHAMP machine, he utilized nine Alicat mass flow controllers to maintain the flow in and out of the two chambers.

Alicat mass flow controllers in the ECHAMP setup

Alicat mass flow controllers in the ECHAMP setup

These controllers provided Dr. Wood with several important benefits essential for precise ECHAMP measurements:

  • High-accuracy measurements to 0.6% of reading
  • 30 ms response times
  • A controllable range of 0.1% to 100% of full scale
  • The option to switch between 98 selectable gases
  • No warm up times

Future plans for the ECHAMP technique

The utility of the ECHAMP technique has been demonstrated in multiple field sampling events across the United States. During a field sampling event in Indiana, data from the ECHAMP was compared to measurements from other peroxy radical measurement techniques and found to be within an acceptable range of error.

The ECHAMP in use at a field sampling event

The ECHAMP in use at a field sampling event

In another study in Texas, peroxy radical concentrations determined from the ECHAMP data were used to characterize ozone production at different sites around the San Antonio region. These results found that ozone formation in this region was limited by nitrogen oxides, and ozone policies should focus on those pollutants.

Dr. Wood hopes to continue using the ECHAMP technique to determine ozone production rates at locations around the country, as data from these field studies can be used to better direct air quality policies aimed at reducing ozone pollution.

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