# Isotopic constraints on the origin of reactive chlorine in the troposphere

## TL;DR

This study uses isotopic analysis to distinguish oceanic and anthropogenic sources of reactive chlorine in the troposphere, revealing their distinct signatures and impact on air pollution.

## Contribution

The study identifies distinct isotopic signatures of ClNO2 from oceanic and anthropogenic sources, enabling better source attribution in the troposphere.

## Key findings

- ClNO2 isotopic values (δ37Cl) range from −21 to +39‰ in China, broader than previously known.
- Sea-salt emissions show δ37Cl of −9 ± 4‰, while anthropogenic combustion sources show +20 ± 7‰.
- Oceanic chlorine influences air pollution, affecting both coastal and inland regions.

## Abstract

Oceanic and anthropogenic processes, such as sea-salt emissions and combustion activities, release substantial amount of reactive chlorine into the troposphere, affecting air quality, ozone depletion, and climate change. However, distinguishing between these sources for reactive chlorine remains challenging. Here, we establish isotopic constraints on the origin of tropospheric reactive chlorine using chemical ionization mass spectrometry to analyze nitryl chloride (ClNO2). Field observations from four regions in China reveal a much broader isotopic value (δ37Cl) range for ClNO2 (−21 to +39‰) than previously documented for Earth’s chlorine reservoirs. Notably, significant δ37Cl differences for ClNO2 from sea-salt emissions (−9 ± 4‰) and anthropogenic combustion sources (+20 ± 7‰) were identified. These distinct isotopic signatures, combined with field data, highlight the important role of oceanic chlorine in air pollution, with its chemical cycling affecting not only coastal regions but also extending into inland areas. This research advances our understanding of chlorine’s behavior and cycling in the troposphere.

A large difference in δ37Cl between oceanic and anthropogenic origins provides isotopic constraints on tropospheric chlorine.