Description of a Custom-Built Two- Channel NOx and NOy Analyzer (DRAFT)

Jeffrey Geddes’s dissertation in its entirety can be found here. Below is an outline I made as a personal reference. I am posting it here in case someone else wants a quick summary on this technique.

TODO: Add figures with annotations

• General Description and design

• chemiluminescence is the process of releasing a photon when an exciting electron returns to a ground state, the excess energy is provided by a chemical reaction
• Analysis of reactive nitrogens can be performed using this method e.g.
  • NO + O3 ->NO2* + O2
  • NO2* -> hv + NO2
    • hv wavelength > 600nm
• Aside: Olefin , ketone (alkanone)
• Ozone can react with olefin species to create excited ketones species that may chemiluminescence, produces wavelengths < 600 nm
  • This process can interfere with NO measurements, red glass filter helps minimize this effect
• A system that reduces all NOy species to NO will let you quantify the sum of the reactive nitrogen species but does not allow you to know the individual components contributions
• Two channels are needed to measure the mixing ratios of two nitrogen species like NO and NOx or NOx and NOy
• Instrument Design

• Described system, AQD NOxy, built by Air Quality Design, inc (airqualitydesign.com)
• AQD NOxy has two channels
  • 1) flow is directed to an LED photolysis cell that selectively potolyzes NO2 to NO
  • 2) flow is directed into the molybdenum (metal formally confused with lead) for total NOy
    • NOy —– (Moly ~ 300 C) —> nNO + products
      • n = moles in particular NO species
    • e.g. NO2O5 thermally decomposes to NO2 and NO3, which is expected to result in two NO after passing molybdenum chamber
• The LED in the NO2 photolysis chamber is blue. Blue is chosen because it hits the narrow absorption region for NO2 that has very little overlap with other nitrogen species and still has a sufficiently large quantum yield from photolysis
  • 8% interference with HONO, should be negligible relative no NOx mixing ratios

• The blue LED system uses very little energy compared to xenon or Hg lamps as light sources. Because the LED wavelength band is so narrow almost no power is needed. As a result the light emits very little heat to surroundings. This is a large benefit for measuring thermally sensitive species like PAN. Additionally the little demand for power makes this easy to maintain with low power and access.
• The downside is that in its current form it can only achieve ~30-50% conversion of NO2
  • This requires that correction factors be calculated during calibration.
  • This does have an impact on the detection limit of NO2 mixing ratios
• Toggling the blue LED converter on and off (in 30-60 second intervals) allows for interpolating between NO measurements
  • Channel 1 observes NO and NOx*
    • NOx* is combined signal from NO and some fraction NO2, determined by post-processing with the conversion efficiency, and NOy (in channel 2)
• Sampling, and NO2 and NOy conversion occurs, occurs in a detached weatherproof inlet system connected to the instrument calibration detection systems by a 40 meter umbilical containing all the electrical and ethernet connections in addition to the sampling and calibration lines
• The instrument calibration and detection systems must be kept in a climate controlled shed
• The NOx and NOy sampling and conversion needs to occur as close to top of tower or inlet location to reduce sampling losses of species such as HNO3, which stick to sample lines)
• Three modes of operation determined by inlet design
  • 1) Alternating NO/NOx-Continuous NOy mode
    • detects NO and NOx* in channel 1 by toggling LED on and off, interpolating between consecutive NO measurements in post-processing then correcting for the conversion efficiency to establish NO2 while Channel 2 continuously monitors NOy
  • 2) Continuous NOx-NO mode
    • Continuously achieves NOx* and NO detection by leaving the LED converter on in channel 1, channel 2 is bypassed so only NO is measured
    • eliminates need for interpolating between NO samples to determine NO2
  • 3) Continuous NO-NOy
    • Achieves continuous NO by leaving LED converter off in channel 1, while channel 2 continuously monitors NOy through the molybdenum converter
    • This method is not common since you lose NO2 measurement which is generally important to understand NOy behavior

• After conversion of NO2 and NOy in the inlet the flow from both channels travels to the instrument detection system
  • This phase has 2 channels each with pre-reactor and reactor volumes
  • before the reaction volume ozone is released from an ozonizer to meet the sample and cause the chemiluminescence reaction
  • This ozone can be emitted before the pre-reactor volume rather than the before the reactor volume, this allows a background signal to be determined
  • when the flow reaches the reaction volume it is assumed that most of the chemiluminescence is complete

Instrument Characterization