Air Quality – Design Considerations

Air Quality Monitoring (AQM) using low-cost sensors requires unique considerations.

You will require a multi-disciplinary approach covering:

• Sensors
• Gas sampling
• Data processing
• Monitor design
• Atmospheric Science
• Regulations

1. Sensors: design, type, and calibration

First is to consider which sensors you need for your AQM, this will depend on what type of AQM you are considering, Air Quality demands low level capability below 50ppb.

All EURsense Air Quality electro-chemical sensors ppb can detect to much lower than this.

Environmental Sensors by Market:

Indoor Air (IAQ) :

CO, CO2, NOx, O3, HCHO, VOC’s, PM 10/2.5/UF

Outdoor Air (Altitude? Atmospheric Chemistry) :

CO, CO2, NOx, H2S, SO2, CH4, O3, HCHO, VOC’s, PM 10/2.5/1/UF

Outdoor Air (Rural Air) :

CO, CO2, NOx, H2S, SO2, O3, HCHO, VOC’s, PM 10/2.5/1/UF

Outdoor Air (Urban Air):

CO, CO2, H2S, SO2, O3, VOC’s, PM 10/2.5/1

Odours (Land Fill):

CO, CO2, H2S, SO2, CH4, VOC’s, PM 10/2.5/1

Odours (Wastewater):

CO2, H2S, SO2, CH4, VOC’s, PM 10/2.5/1

Odours (Oil & Gas):

CO, H2S, SO2, CH4, VOC’s, PM 10/2.5/1

Transportation (i.e. Fruit etc.)                           

CO2, C2H4O, C2H4, PH3, O2

Calibration is important, with a mesh network, you do not have the ability to use standard calibration techniques, so you need to rely on alternatives. One such method is to use co-location.

Co-location (a fair alternative to formal calibration):

Placement one of the AQMs (low-cost) close (within 1-metre) of a Reference Station (high cost and high maintenance), compare results of the AQM against the Reference AQM (preferably early morning, say 01:00 hours) and adjust the low-cost AQM according, then filter the results to the remainder of the mesh network.

Secondary Standard Sensor Technique

This technique involves switching sensors for an identical secondary reference sensor that is used only for cross checking sample readings on installed instruments.  This technique works particularly well with OPC particulate sensors, where checking installed instruments with standards is often impractical.

Simplest Method (works but not as accurate as co-location):

Place one of the AQMs close to a large body of water (lake etc), assume any pollutants are very small or zero, adjust the sensors accordingly and filter to the remainder of the mesh network.

Activated carbon filter for zero checking?  Zero point is the most important calibration setting?

2. Signal processing and algorithms

The need to acquire reliable scientific data is another driving force in the environmental sensors business.
Scientists now require more sophisticated monitoring programs to detect ecological changes that are occurring because of climate change. Related to this is the need for monitoring programs that yield data meeting specific scientific standards and quality control procedures.
Mobile environmental systems are being increasingly tested and proposed for urban areas, adding that such systems can identify and monitor urban air pollution events. Correlations can be made between resulting data and levels of local transport or industrial activity. These “rapid sensor grids” can enable real-time decisions, to reduce the impact of poor air quality on people and the environment.

In other words, you need good algorithms to convert the raw data collected to meaningful data for the end-use.

You can use the algorithms provided by Alphasense to compensate for temperature and humidity effects or develop your own algorithms.

3. Instrument design and development

Use GPS, GSM, then integrate into multiple low-cost boxes.

Advanced algorithms from the atmospheric chemists give us more and better data.

Need cooperation from authorities for fixed installations, or alternately use mobile versions     such as portables, vehicles, or drones.

There are fundamentally only two types of AQM, Fixed or Portable (including mobile).

Fixed, have continuous power, and are located at a specific site, they may be an individual unit or form part of  a mesh network.  These types of instruments would typically use our most sensitive B4 type of four electrode ppb electrochemical sensor.

Portable (mobile), typically are battery powered, although some scavenge power from other sources, solar panels, Wi-Fi signals etc. These types of instruments would typically use our A4 type of four electrode ppb electrochemical sensor due to their smaller size.

An example of an AQM used at London Heathrow Airport, had the following sensors: (A4 or B4?)

NO, NO₂, CO, CO₂, SO₂, O₃, VOCs and size-speciated Particle Monitor. 

Sampling

It is normal practice to have sensors pointing downwards assisting their protection from debris and direct exposure to sunlight, although they can be used in any orientation.

Likewise, if using a particle monitor within any instrument you need to protect the inlet and outlet, the outlet would normally be on the side or base of the AQM.

In all cases they have the following elements:

Sensors for various gases/VOCs etc.

Circuitry to drive the drive the sensors and provide the raw data.

Storage (internal flash drive or similar) or real time communications.

Displays/alarms, as necessary. 

Communications (Wi-Fi, Bluetooth etc).

Supplied or own use of Algorithms to improve linearity and provide compensation for temperature and humidity.

A purpose weather-proof design of enclosure with easy service access.

4. Atmospheric Science

If you are to use your own algorithms, you will need Atmospheric Scientists to assist or use Alphasense pre-defined Algorithms.

Atmospheric science is the study of the Earth’s atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on pollution and weather forecasting. 

Experimental instruments used in atmospheric science includes AQMs, sondes, drones, balloons etc.

5. Regulations (EPA approval or MCERTS?)

Every country is taking major efforts to move on to a more environmentally friendly manufacturing and production process and meeting international standards.

Increasing concerns about pollution control and safety has led governments and environment protection groups exerting pressure on industries to control pollution by adhering to environment regulations.

Major Legislations - Asia Pacific Examples

6. Summary

Fixed site low-cost Air Quality networks are here. But they do not replace reference AQMs.

PM < PID and electro-chemical PPB sensors like NO₂ sensors now work to requirement.

Validation, co-location, and good network management are needed.

Mobile and personal AQ points are being tested they are needed to complement fixed sites and for healthcare.

Wearables are still in the future.