Conclusions
The new technique developed is simple and uses widely available materials. The need for only small sample/reagent volumes (around 300 µL) reduces the generation of waste and enables the determination of NO2 at concentrations typically found in moderately polluted atmospheres.
The method uses simple and inexpensive equipment that could readily be substituted by other equivalent items, depending on local availability. The portability of the image acquisition and processing system means that measurements can be made in situ, even in remote regions. It would also be possible to develop software suitable for use with different measurement platforms, including cell phones and tablet computers.
As an alternative to gray scale, digital images also can be evaluated using multiple linear regression (MLR) or partial least squares (PLS) with possible improving of coefficient of correlation and low standard error of prediction [35].