Methods and apparatus for determining levels of gaseous elements and optionally utilizing the determined levels to calibrate one or more sensors of an air purifier. For example, in some implementations a first image is captured of a colorimetric sensor device at the start of a sensing period and a second image is captured of the colorimetric sensor device at the end of the sensing period. The colorimetric sensor device includes at least one colorimetric sensor configured to change colors in response to reaction with a gaseous pollutant. Values may be determined based on the colors of the colorimetric sensor in the first and second images and the values may be utilized to determine a pollution value indicative of the amount of the gaseous pollutant to which the colorimetric sensor was exposed during the sensing period.