The bromine atom (Br•) has been known to destroy ozone (O3) and accelerate the deposition of toxic mercury (Hg). However, its abundance and sources outside the polar regions are not well-known. Here, we report significant levels of molecular bromine (Br2)─a producer of Br•─observed at a coastal site in Hong Kong, with an average noontime mixing ratio of 5 ppt. Given the short lifetime of Br2 (∼1 min at noon), this finding reveals a large Br2 daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br2 is generated by the photodissociation of particulate nitrate (NO3-) and that the reactive uptake of dinitrogen pentoxide (N2O5) on aerosols is an important nighttime source. Model-calculated Br• concentrations are comparable with that of the OH radical─the primary oxidant in the troposphere, accounting for 24% of the oxidation of isoprene, a 13% increase in net O3 production, and a nearly 10-fold increase in the production rate of toxic HgII. Our findings reveal that reactive bromines play a larger role in the atmospheric chemistry and air quality of polluted coastal and maritime areas than previously thought. Our results also suggest that tightening the control of emissions of two conventional pollutants (NOx and SO2)─thereby decreasing the levels of nitrate and aerosol acidity─would alleviate halogen radical production and its adverse impact on air quality.
Keywords: Molecular bromine; air quality; anthropogenic pollution; atmospheric oxidation; mercury; sea sprays.