Pollutants in Rainwater: Analytical Advances, Key Challenges, and Environmental Significance

Rainwater plays a key yet often underestimated role in the atmospheric transport and deposition of environmental contaminants, contributing to the dissemination of both traditional and emerging pollutants across urban, industrial, and remote areas. For the first time, rainwater as an analytical matrix is systematically addressed, providing a critical synthesis of the objectives, strategies, and methodological approaches required for a comprehensive investigation of atmospheric contamination mediated by precipitation. This review aims to provide a comprehensive overview of the major classes of contaminants detected in rainwater and to critically examine the analytical strategies currently employed for their determination at trace and ultra-trace levels. The manuscript discusses the most widely applied sample preparation techniques, with particular emphasis on solid-phase extraction using polymeric and mixed-mode sorbents, alongside complementary approaches such as liquid-liquid extraction, solid-phase microextraction, and treatments of the particulate fraction. Instrumental methodologies based on gas chromatography and high-performance liquid chromatography coupled with mass spectrometry are presented as the gold standard for multiresidue analysis of organic pollutants, while ion chromatography and ICP-MS are highlighted for inorganic and metal contamination profiling. The review highlights current trends, methodological strengths, and limitations in rainwater analysis. Overall, this work underscores the importance of integrated and robust analytical approaches to achieve a comprehensive assessment of rainwater contamination and identifies existing gaps in linking analytical data with atmospheric processes, source attribution, and health risk assessment, thereby reinforcing the relevance of rainwater monitoring in environmental and public health studies.