Quantification, diel variation and photochemistry of inorganic chlorine trace gases in continental Germany

Abstract

Understanding the sources, distribution, and lifetime of inorganic chlorine-containing trace gases is crucial to assessing their tropospheric impacts. We report in situ measurements of Cl₂, HOCl, ClNO₂, and ClONO₂ using iodide chemical ionization mass spectrometry during a 2.5-week campaign in June 2024 at a rural continental site in central Germany. Air masses that had passed over “marine-anthropogenic” regions (≈400 km distant) showed significantly higher mixing ratios of chlorine-containing gases than “continental-unpolluted” air masses. From the marine-anthropogenic period, we provide the first quantitative observations of ClONO₂ in the lower troposphere (up to 59.8 pptv during daytime). Persistent nonzero ClONO₂ at night implies a non-photochemical source of ClONO₂ or its precursor ClO, and/or that heterogeneous loss is slower than laboratory uptake coefficients suggest. ClNO₂ levels were consistent with production via N₂O₅ uptake on chloride-containing particles; both ClNO₂ and Cl₂ were enhanced when O₃-/N₂O₅-rich air entrained into the nocturnal boundary layer. Photolysis of ClNO₂, Cl₂, and HOCl yielded mean maximum Cl atom production rates of 1.0 × 10⁶ cm⁻³ s⁻¹ under marine-anthropogenic influenced air and 1.6 × 10⁵ cm⁻³ s⁻¹ under continental-unpolluted conditions. In the early morning, Cl production (due to ClNO₂ photolysis) exceeded primary–OH production from O₃ photolysis, while after noon HOCl photolysis was the dominant Cl source. At low solar zenith angles, HOCl photolysis contributed up to 40% of primary OH. These measurements indicate that Cl atoms can strongly influence hydrocarbon oxidation in similar rural regions, with potential regional and global implications (of up to 15%) for the methane lifetime.