Chloramine chemistry as a missing link in atmospheric chlorine cycling

Chlorine radicals (Cl·) profoundly impact atmospheric oxidation capacity. Chloramines, especially trichloramine (NCl3), are emerging precursors of Cl·. However, their sources and roles in the atmosphere remain elusive. This study presents field evidence of primary emissions and explicit secondary production pathways of atmospheric NCl3 in Beijing, supplemented by observations from New Delhi and reanalysis of measurements in Toronto. We demonstrate that the sequential chlorination reactions initiated by molecular chlorine (Cl2) and ammonia (NH3) in atmospheric aerosols are a major source of NCl3. The NCl3 produced in aerosols is a source of gaseous NCl3 and serves as an intermediate during the conversion from Cl2 to Cl·, while direct NCl3 emissions constitute a previously overlooked source of Cl·. Overall, chloramine chemistry alters the Cl· production mechanism and represents a crucial missing pathway to Cl· worldwide.
, , # Chloramine chemistry as a missing link in atmospheric chlorine cycling

Dryad DOI: 10.5061/dryad.c866t1gmf

All field observation data used and codes developed for this manuscript and Table S15.

## Description of the data and file structure

Beijing_obs_data.csv
Hourly averaged data, forward averaging is applied, e.g., the timestamp for the average of measurements from 2022/1/22 00:00 to 2022/1/22 01:00 is 2022/1/22 00:00.
Time indicates local Beijing time

1. Time series of reactive chlorines mixing ratios, in units of part per trillion (ppt)
2. Time series of trace gases and volatile organic compounds mixing ratios, in units of part per billion (ppb)
3. Meteorological parameters:
Temp refers to temperature in units of degrees Celsius
RH refers to relative humidity in units of %
WD refers to wind direction in units of degrees
WS refers to wind speed in units of meters per second
Precipitation in units of millimeters
4. Aerosol composition data (chloride, nitrate, sul...,