A portable, low-cost, smartphone assisted methodology for on-site measurement of NO2 levels in ambient air by selective chemical reactivity and digital image analysis

Abstract In this work, we propose a simple, sensitive, selective and inexpensive colourimetric method based on compact and portable instrumentation to monitor nitrogen dioxide (NO2) levels in ambient air, applicable to on-site measurements when needed. The NO2 concentration was determined by monitoring the Griess-Saltzman colourimetric reaction on filter paper pads exposed to ambient air, where NO2 was collected by active sampling (portable pump) on pads coated with triethanolamine (TEA). Digital images of the pads were captured by a smartphone camera inside a box under controlled lighting conditions. The images were analysed without any extraction procedures, using a dedicated smartphone application developed by our research group. The linear working range of the proposed method was from 19.1 to 191.0 µg m-3 with a correlation coefficient of 0.996. A limit detection of 4.85 µg m-3 was achieved using a sampling time of 60 min and a sampling flow rate of 400 mL min-1. The reproducibility of the proposed methodology was also studied, achieving a relative standard deviation value of 9 % (n = 5). The colourimetric method was tested in ambient air for the on-site determination of hourly NO2 concentrations in an urban area of Badajoz (Spain). The field measurements were validated by orthogonal regression against the reference method, with significant correlation at the 95 % confidence level, and a slope and intercept statistically equal from expected values. An expanded relative uncertainty of 15% was estimated according to European guidelines for the demonstration of equivalence of ambient air monitoring methods.