Low-cost spectrophotometer for measuring nitrogen dioxide (NO2) air pollution

Palmes diffusion tubes are widely used as a low-cost method for measuring ambient nitrogen dioxide (NO₂) air pollution. Based on the principle of molecular diffusion, ambient NO₂ accumulates as nitrite at the closed end of the tube. After a typical four-week exposure period, the tubes are returned to a laboratory, where the nitrite is dissolved in water, reacted with a colorimetric reagent, and quantified by measuring the resulting color change using a spectrophotometer.

Despite their effectiveness and affordability, Palmes diffusion tubes are still rarely used in Africa. A major reason is that tube preparation and analysis are typically must be carried out in laboratories outside the continent. One key barrier to establishing local Palmes laboratories is the high upfront cost of spectrophotometers required for sample analysis.

While conventional spectrophotometers can measure absorbance across a wide range of wavelengths, most reagent-based colorimetric analyses require only a single wavelength. For Palmes tube analysis, absorbance is measured at 540 nm, corresponding to the maximum absorption of the Griess reagent. Since green LEDs emit light within a narrow waveband close to this absorption peak, they offer a low-cost alternative light source.

We present and will live-demonstrate a simple device that replaces the spectrophotometer in the Palmes tube analysis workflow. The device consists of a 3D-printed light-tight cuvette holder housing a green LED for illumination and a photodiode to measure transmitted light. Measurement results are displayed directly on the device. The system can determine nitrite concentrations with an accuracy of 3 µg/L, corresponding to approximately 0.1 µg/m³ of ambient NO₂ for a four-week exposure period—well below the intrinsic uncertainty of the Palmes diffusion method.

Costing only a fraction of a conventional spectrophotometer, this device has the potential to greatly expand in-situ monitoring of NO₂ pollution in sub-Saharan Africa without substantially increasing costs. Moreover, it provides a promising proof of concept for developing similar low-cost instruments for other air and water quality applications based on colorimetric measurements.