Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s−1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

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The bromine atom (Br • ) has been known to destroy ozone (O 3 ) 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 (Br 2 )�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 Br 2 (∼1 min at noon), this finding reveals a large Br 2 daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br 2 is generated by the photodissociation of particulate nitrate (NO 3 − ) and that the reactive uptake of dinitrogen pentoxide (N 2 O 5 ) 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 O 3 production, and a nearly 10-fold increase in the production rate of toxic Hg II . 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 (NO x and SO 2 )�thereby decreasing the levels of nitrate and aerosol acidity�would alleviate halogen radical production and its adverse impact on air quality.

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Abstract. Vehicular emissions contribute a significant portion to fine particulate matter (PM2.5) air pollution in urban areas. Knowledge of the relative contribution of gasoline- versus diesel-powered vehicles is highly relevant for policymaking, and yet there is a lack of an effective observation-based method to determine this quantity, especially for its robust tracking over a period of years. In this work, we present an approach to track separate contributions of gasoline and diesel vehicles through the positive matrix factorization (PMF) analysis of online monitoring data measurable by relatively inexpensive analytical instruments. They are PM2.5 organic and elemental carbon (OC and EC), C2–C9 volatile organic compounds (VOCs) (e.g., pentanes, benzene, xylenes, etc.), and nitrogen oxide concentrations. The method was applied to monitoring data spanning more than 6 years between 2011 and 2017 in a roadside environment in Hong Kong. We found that diesel vehicles accounted for ∼70 %–90 % of the vehicular PM2.5 (PMvehicle) over the years and the remainder from gasoline vehicles. The diesel PMvehicle during truck- and bus-dominated periods showed declining trends simultaneous with control efforts targeted at diesel commercial vehicles and franchised buses in the intervening period. The combined PMvehicle from diesel and gasoline vehicles by PMF agrees well with an independent estimate by the EC-tracer method, both confirming PMvehicle contributed significantly to the PM2.5 in this urban environment (∼4–8 µg m−3, representing 30 %–60 % in summer and 10 %–20 % in winter). Our work shows that the long-term monitoring of roadside VOCs and PM2.5 OC and EC is effective for tracking gaseous and PM pollutants from different vehicle categories. This work also demonstrates the value of an evidence-based approach in support of effective control policy formulation.