Fog and rain water chemistry in a tea plantation of northern Taiwan

Tea plantations are expanding globally and many are in mountainous areas with frequent fog but few studies have examined fog chemistry in these areas. We examined chemical composition of fog and rain water at a tea plantation in northern Taiwan. Fog water was collected using a Kroneis passive cylindrical fog-water collector and rain water was collected using a 20-cm-diameter funnel. The most abundant ions were Cl⁻ and Na⁺ in both fog and rain waters due to the proximity of the site to the coast. The order of abundance of other ions was NO₃⁻ > Mg²⁺ > SO₄²⁻ > Ca²⁺ > NH₄⁺ > K⁺ > H⁺ in fog water and SO₄²⁻ > K⁺ > NO₃⁻ > NH₄⁺ > Ca²⁺ > Mg²⁺ > H⁺ in rain water. The concentration enrichment ratio (fog to rain) ranged between 2.2 (K⁺) and 22 (Mg²⁺) lying between sites near major emission sources and sites in remote areas, possibly because the immediate surrounding landscape is covered with secondary forests although it is near large cities. Factor analysis highlights the influences of sea-salt aerosols on the variation of fog and rain water chemistry. Sea-salt corrections using Na⁺ as the sea salt tracer led to negative concentrations of Cl⁻ and Mg²⁺ suggesting that assumptions involved in sea-salt corrections were not satisfied. Agriculture influence is identified as a unique factor for explaining variance of K⁺, NH₄⁺, and dissolved organic nitrogen (DON) concentrations in fog water but not rain water. Ion concentrations in fog and rain water were generally higher in the weekly samples associated with air trajectories passing through the continental East Asia than those associated with oceanic trajectories pointing to the role of regional pollution sources in affecting local fog and rain water chemistry. Our study highlights greater effects of tea agriculture on fog than rain water chemistry.