Excerpt from © The Water Wise Home: How to Conserve, Capture, and Reuse Water in Your Home and Landscape (by Laura Allen)

City Rain: Clean or Not?
Rain is distilled water, free of pollutants. As raindrops fall through the atmosphere, they dissolve the carbon dioxide in the air, forming a weak acid with a pH of 5.7 or so. When rain falls through polluted air, it picks up particles of soot, dust, and smoke. If the air is polluted where you live, using rainwater catchment for irrigation is of no more concern than the rain falling directly from the sky and landing on your garden; the same pollutants will end up in the garden either way. However, with a roofwater catchment system, you should use a first-flush diverter (page 148) to minimize the introduction of additional pollutants from the roof to the garden. Any drinking water system requires additional filters to remove particulates.