Worsening air pollution from winter coal burning: This "new" class of halogen free radicals is worth watching

When talking about halogen free radicals (especially chlorine free radicals), one must think of the stratospheric ozone hole caused by halogen elemental compounds (freon, etc.).However, over the last 10 years of studying the troposphere, scientists have gradually discovered that halogen radicals play a different role.They can affect the concentration of classical free radicals (OH, HO2.RO2) in the atmosphere by rapidly oxidizing a range of hydrocarbons, thereby promoting the production of ozone and controlling the concentration of a variety of atmospheric pollutants.At present, most studies on halogen free radicals focus on nitacyl chloride (CINO2) and chlorine gas (CI2). The chlorine free radicals produced by their photolysis can affect the atmospheric oxidation.But are there other halogen activation channels in contaminated areas that contribute halogen radicals? How big is their impact? None of this is clear.


In the winter of December 2017, our research team carried out a comprehensive field observation experiment on active halogens (Cl2, Br2, HOCI, HOBr and BrCI) in China in the North China Plain (Wangdu, Hebei Province), and unexpectedly found a large number of unexpected active halogens in this region. The most active Brcl (the highest concentration of Brcl was 482ppt).BrcI can be photolysis in atmosphere and release bromine radical and chlorine radical.Thus, BrCl is both an active bromine compound and an active chloride. This study is the first ground observation in the world to report a persistent high BrCl concentration, which is 10 times greater than the maximum observed on the ground in the Arctic region, except the Arctic study. The discovery was shocking.In addition, in March 2018, the research team also observed a high BrcI(up to 190 PPT) at the top of Mount Taishan (1465 Ma.S. 1.), 300km from the observation site in Hebei Province. But where do such high concentrations of active halogens come from?

The analysis of the observed data shows that there is a strong correlation between the active bromine substance and the emission indicator of coal burning. Based on further analysis of the observed data, the researchers confirmed that coal burning in the region during winter can emit large amounts of chlorine and bromine particulate matter and active bromine compounds.Furthermore, by further calculating the chlorine and bromine content of loose coal reported in China in 2017 and the literature, the high concentration of active bromine compounds and chlorine and bromine particulate matter observed in the North China Plain in winter of 2017 can be reasonably explained by the amount of loose coal burned in the region where the observation was made.The heterogeneous reaction between active bromine compounds and chlorine-containing particles leads to the generation of BrCl, which contributes both bromine and chlorine free radicals through photolysis.

In addition, a nitrate-related photoactivation pathway was found to maintain the diurnal concentration of active bromine substances. Therefore, high concentration of active halogen substances (especially BrCl) may be prevalent in coal burning areas in northern China in winter.

What effect does such a high concentration of active halogens have on the atmosphere?

In order to further explore the contribution of active halogen compounds to atmospheric oxidation and ozone generation, the research team independently built and developed a chemical box model containing chlorine and bromine. This model is the most comprehensive model including chlorine bromide chemical vapor phase mechanism in the world. The study found:

  1. The contribution of BrCl to chlorine and bromine free radicals in the atmosphere is more than 50%. In previous studies, nitryl chloride is mainly activated at night, so the contribution of daytime to chlorine free radicals is limited. However, BrCl has a diurnal activation mechanism and can continuously release chlorine and bromine free radicals, which can not be ignored.
  2. The two free radicals chlorine and bromine can greatly enhance the oxidation rate of volatile organic compounds (alkanes up to 180%, aldehydes and ketones up to 90%), at the same time can increase the average concentration of classical oxidants (OH, HO2, RO2) in the atmosphere by 25%, 50% and 75%, and increase the net ozone generation rate by 55%. The enhancement of atmospheric oxidation can accelerate the generation of secondary particulate matter.
  3. In addition, the resulting bromine free radicals can greatly enhance the oxidation of elemental mercury to divalent mercury. Oxidized divalent mercury is toxic, soluble in water, and easy to enter the ground and ecosystem through deposition. The North China Plain is one of the regions with high concentrations of elemental mercury in the world. Therefore, divalent mercury generation and rapid deposition facilitated by high concentrations of bromine free radicals may significantly increase the risk of toxicity to humans and ecosystems in the areas of discharge.


As a result, the study suggests that in the future, in addition to carbon dioxide, particulate matter, and sulfur-nitrogenous substances from coal-burning activities, close attention should be paid to halogens and mercury. This study provides a new scientific basis for controlling rural coal burning in northern China and promoting clean heating work such as “coal to gas” and “coal to electricity”.


The co-first authors of the paper are Xiang Peng and Weihao Wang, PhD students in the group of Professor Tao Wang of the Hong Kong Polytechnic University, who is the corresponding author of the paper.Jianmin Chen, Fudan University; Yujing Mou, Ecology Center, Chinese Academy of Sciences; Likun Xue, Shandong University; Jinhe Wang, Shandong Jianzhu University; Academician A.R.Ankara, USA; Alfonso Saiz-Lopez, Spain; and Christian de France Professor George and other institutions and many scientists participated in this research, which provided important observational data and analysis support for this research.