Instrument Support Level 3
Air Quality Design Inc, High Performance, 2 Channel NOx monitor
95 cm x 60 cm x 175 cm. 250 kg
95 cm x 60 cm x 175 cm. 250 kg
AQD High Sensitivity NOx
This instrument built by Air Quality Design inc. is designed to be deployed in situations where a high degree of sensitivity and a low limit of detection are required for NO and NO2 selectively – down to ~2 pptv. The instrument can run 24 hours unattended with automatic calibration carried out systematically and consistently on a daily basis. The instrument is built into a single rack/flight case assembly allowing for easy rapid deployment and for the instrument to be powered on within 5 minutes of being sited. The data computer runs a custom DAQ factory interface for the instrument providing simple button-based control, graphical read-out of current signals, and graphical read-out of time series. Data is logged constantly automatically in 24 hour block files in CSV format.
The instrument is a highly sensitive chemiluminescent NO/NO2 analyser.
It operates by detecting the photon liberated during the reaction of a nitric oxide molecule with ozone.
NO + O3 → NO2 + O2 + hv
This photon is detected by a photomultiplier tube which is cryogenically cooled to reduce the dark current in the detector and thus improve sensitivity/limit of detection and reduce noise. As can be seen in the reaction, the number of photons is directly proportional to the number of NO molecules, thus a quantitative measurement can be made. This is done under a vacuum of 2-4 torr. The vacuum also drives the sample flow through the instrument of 1.0 L/min per channel.
In order to detect NO2 it must first be converted to NO. In the AQD NOx instrument, this is achieved with a photolytic blue light converter (BLC) which irradiates the incoming air sample at 380-410 nm in order to reduce the NO2 to NO. This NO is then detected as above.
NO2 + hv → NO + O
The conversion of this reaction is typically ~50% whereas the selectivity is ~100%. The BLC design avoids the drawbacks of older catalytic designs which reduce higher NOy species to NO also resulting in an overestimation of the true NO2 mixing ratio.
The AQD-highsens-NOx instrument has an internal automated calibration system capable of calibrating for both NO and NO2. The calibration system also ascertains the magnitude of any signal artefact due to dark current in the detectors. The automated calibration system logs all these parameters in an output file at the time of calibration.
The full list of parameters is:
- Sensitivity of channel 1 in cps/ppt
- Sensitivity of channel 2 in cps/ppt
- NO2 → NO converter efficiency in %
- NO artefact in ppt
- NO2 artefact in ppt
- NO limit of detection at 1/s, 1/min & 5/min
- NO2 limit of detection at 1/s, 1/min & 5/min
This calibration can be carried out in zero air or in ambient conditions if the background NOx concentration is not changing eg. Nighttime.
Calibration can be automated to occur periodically at a convenient time or can be on-demand of the user.
The AQD-highsens-NOx instrument also has a highly sophisticated zero system where a zero reading for each channel is taken every 5 minutes during normal operation. This ensures that and change in the background is accurately tracked thus contributing to the instruments’ sensitivity and accuracy.
The instrument is calibrated using a cylinder of 5 ppm NO in N2 (BOC, UK). A small flow (5sccm) is of the calibration gas is added to the sample flow (1000sccm), allowing dilution of the calibration gas to around 25 ppb and calibration at equivalent humidity to ambient air. The calibration cylinder is itself calibrated to an NPL cylinder held in the COZI laboratory in York. NO2 is calibrated using gas-phase titration of the NO to NO2 upon addition of a small amount of O3.
The AQD-highsens-NOx instrument is essentially maintenance free in normal operation barring changing gases. Periodic cleaning of the photolytic NO2 NO converter is required when the conversion efficiency falls below acceptable limits (~25%). The interval between cleaning will vary depending on the particulate content of the air sampled. This requires several hours of downtime – though NO data can still be recorded with only a brief interruption.
Vacuum tip seals of the vacuum pump require periodic replacement (yearly).
- The instrument requires constant pressurized oxygen (O2) flow to operate.
- Operating a single channel requires 100ml/min or ~ 150L per day
- Operating both channels required 200ml/min or ~ 300L per day
- Zero grade compressed O2 delivered by a regulator is acceptable as is oxygen supplied by an O2 concentrator provided moisture, NOx and VOC traps are employed. Oxygen inlet pressure should be ~ 2 Bar. Gases, regulators and/or traps should be supplied by the applicant.
- Calibration of the instrument requires a flow of Zero Air (ZA). Each calibration requires 115L of ZA supplied at 2.5L/min.
- Zero air can be supplied from a compressed cylinder via a regulator or from a Pure Air Generator (PAG) capable of delivering 2.5L/min. Both gas cylinders or PAG should be supplied by the applicant.
- Calibration also requires a flow of a Nitric Oxide (NO) standard. A nominal 5ppm standard as usual.
- An NO standard can be supplied with a regulator to accompany the instrument at the agreement of both parties. Else, the applicant should supply their own standard cylinder of known concentration and accompanying regulator. In the case that the applicant supplies their own standard, the set-up time may be extended to accommodate extra calibration steps and modification to the control software.
- The main parts of the instrument that are subject to degradation during use are the diodes for the NO2 converter (replacement cost £8000), seals for the pump (replacement cost £500) and the ozonizer units (replacement cost £3000). A typical 4-week deployment should cover replacement costs for each of these
- Instrument insurance
- This system must be insured by the user for £100K and covers loss, theft or damage to the instrument: damage is that over and above general wear and tear. The system has been designed to be rugged and autonomous. Even so, the end-user must respect the fact that the system is a precision optical instrument that must be treated with great care. The user is responsible for the instrument from the time it leaves to the time it is returned and signed off as in an acceptable operating condition by the instrument scientist. This will be done as soon as is possible on its return.
- Public liability insurance
- We are not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the instrument scientist.
- Shipping expenses
- The user is liable for all costs arising from the shipping of the instrument both to and from deployment.
- IS T&S
- The user is responsible for coving the travel and subsistence expenses of the instrument scientist while attending the instrument.
The system when packed ready for shipping consists of a single flight case that serves as a shock-mounted instrument rack. All of the 4 rubber castors have brakes. Two lifting handles per side are provided.
Shipping dimensions: 95 cm (L) x 60 cm (D) x 175 cm (H)
Shipping weight: 250 kg
Due to the size and weight (250 Kg) of the instrument it is advised only to wheel the instrument into position; therefore, the site must have access which allows this (Ramps, elevators, forklift etc). Carrying the instrument is not recommended.
The instrument requires ventilation due to the heat it produces so should be situated somewhere which allows unrestricted airflow. The instrument is not weather proofed however and is fitted with a domestic 3-pin power connector so should be sited accordingly.
- Care when moving instrument due to high weight (250 Kg)
- Precautions must be observed in the use, handling and storage of compressed gases. Refer to the site regulations for the proper procedures.
- Care must be taken in the connection and use of pure oxygen which is an accelerant.
When unpacked the instrument has the following physical specification
Footprint: 95cm (L) x 60 cm (W) x 175 cm (H)
Weight (not including shipping case): 250 kg
Power: 240V ~ 6 Amps / 1500W – single domestic outlet.
Operation temperature: -10°C to 35°C ambient
Extra floor space is required for gas cylinders etc.
The data computer sits on a tray extending 30cm in front of the instrument extending the length to 125 cm in operation.
Data is provided at 1 minute, 5 minute and 1 hour averages of 1hz data.
- The instrument produces a range of out files and all are text format.
- The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.
- Data is provided in NetCDF files following the AMOF data standard
- Files contain no more than 24hr of data.
- Instrument name is
- The data product(s) associated with this instrument:
- Example data file