Seasonal trends and source apportionment of water-soluble inorganic ions in PM_2.5 in Makkah, Saudi Arabia

Makkah, a major religious hub in Saudi Arabia that hosts millions of pilgrims annually, has experienced rapid urban expansion, intensified traffic, and extensive construction factors that collectively worsen the air quality. This study examined the concentrations and sources of water soluble inorganic ions (WSIIs) in PM₂.₅ collected from five urban sites: Alharam, Ar Rusayfah, Alshoqiyah, Alhajj, and Herra Hospital. The concentrations of average PM₂.₅ were as follows: 109 ± 59.6 μg m⁻³, 75.9 ± 50.5 μg m⁻³, 59.3 ± 39.3 μg m⁻³, 63.2 ± 39.4 μg m⁻³, and 63.6 ± 53.9 μg m⁻³ in Alharam, Ar Rusayfah, Alshoqiyah, Alhajj, and Herra Hospital, respectively. These values exceeded the World Health Organization's 24 h guideline (15 μg m⁻³) by factors of approximately 4.0 to 7.3. Seasonal patterns showed that PM₂.₅ loads climbed to a springtime maximum before tapering off to their winter minimum. Across every sampling site, sulfate (SO₄²⁻) emerged as the most abundant ion, followed by ammonium (NH₄⁺) and nitrate (NO₃⁻), collectively accounting for approximately 19 % of PM₂.₅ mass. Strong relationships between NH₄⁺, SO₄²⁻, and NO₃⁻ suggested that secondary aerosols were significantly formed, mostly in the form of ammonium nitrate (NH₄NO₃) and ammonium sulfate (NH₄)₂SO₄. The concentrations of WSIIs varied significantly across the five sites, with higher levels observed in urban and high-traffic areas, indicating site-specific emission strengths. Spatially, secondary ions such as SO₄²⁻ and NO₃⁻ were dominant in densely populated zones, while natural sources influenced the suburban sites. Positive Matrix Factorization (PMF) identified the key sources of emissions, including the combustion of fuel, construction activities, biomass burning, and secondary mineral dust, differing in proportion depending on local activities and meteorological conditions. Elevated levels of water-soluble ions can enhance the hygroscopicity of particles, reduce visibility, and pose substantial health risks. The persistent exceedance of international air quality standards underscores the urgent need for targeted emission-control strategies, especially during the height of the pilgrimage season, to protect the public health and ensure environmental sustainability.