Source identification of episodic rain pollutants by a new approach: Combining satellite observations and backward air mass trajectories

In this study, various source apportionment methods, viz., enrichment factor, relative source contribution, and factor analysis in addition to cluster analysis coupled with satellite observations, were used to investigate the origin of rainwater pollutants in an arid urban area of Saudi Arabia. The rainwater samples were collected by an automatic wet-only sequential rain sampler and analyzed for their pH, electrical conductivity (EC), and major ions (Cl^–, NO_3–, SO₄ 2_–, HCO_3–, Na⁺, K⁺, NH₄₊, Ca²⁺, and Mg²⁺). The results revealed that the chemical composition was dominated by Ca²⁺, SO₄ 2_–, Cl^–, HCO_3–, and Na⁺, which represented 80% of the total ionic equivalent concentration. Ca²⁺, the most abundant ion, exhibited a mean concentration of 480 µeq L^–1 and accounted for 30% of the total ionic equivalent concentration. The study area received a total ionic wet deposition flux (F_WD) of 4.07 tons km^–2 y^–1, and SO₄ 2– and NO₃ – contributed 76% and 24%, respectively, of the rainwater acidity. Ca²⁺ was responsible for 80% of the rainwater’s neutralization. The source apportionment indicated that intense local human activity and medium-to long-range transport from Kuwait, Iraq, Qatar, Riyadh, and the Western Province produced the majority of the anthropogenic components (SO₄ 2_–, NO_3–, and NH₄₊), which represented 47% of the total ionic F_WD. However, moderate to high particulate matter loads over the southern part of Saudi Arabia and long-range transport from Iraq and Kuwait contributed the crustal components (Ca²⁺, HCO_3–, K⁺, and Mg²⁺), which accounted for 36% of the total ionic F_WD, whereas the Arabian Gulf was the primary source of the marine components (Na⁺ and Cl^–). We found combining satellite observations and cluster analysis of backward air mass trajectories to be an effective new approach for assessing the source-receptor relationships for atmospheric air pollutants.