Cup of Carbon: smartphone-based analysis of dissolved organic carbon in water for use in citizen science

Dissolved organic carbon (DOC) is an important component of the global carbon cycle which influences water properties such as colour and acidity. Standard methods for quantification of DOC use instruments such as Total Organic Carbon (TOC) analysers, UV/Vis spectrometers, or portable colorimeters. However, the need for specialist equipment may be a barrier to the accessibility of DOC measurement in resource limited settings. The "Cup of Carbon" method is presented as a low-cost and accessible method for the estimation of the DOC concentration of water by digital image colorimetry, using only a mug with a white interior, a laminated piece of white paper, and a smartphone digital camera. The blue pixel intensity of RGB data from digital images of water samples in mugs was used to estimate DOC concentration of water samples, and a novel white-subtraction image processing step improved the accuracy of the measurement, although also causing a decrease in measurement precision. A free R Shiny computer app was created for the fast estimation of DOC concentration from Cup of Carbon images. The Cup of Carbon method enabled good estimates of the DOC concentration of water samples from around the Silver Flowe peatland in Galloway, Southwest Scotland, from images taken by multiple users with different smartphones. Estimated DOC concentrations were within 8.2 ± 26.2% (-0.24 ± 2.07 mg L^-1) of the DOC concentration measured by UV/Vis spectroscopy. The simplicity and low-cost of the Cup of Carbon method make it ideally suited for the estimation of water DOC concentration in citizen science, outreach, or education settings.