Urban fires can burn large amounts of manmade structures and synthetic materials in densely populated areas, which potentially have different chemical characteristics to wildfires that mostly burn vegetation. Chemical composition data of urban fire plumes in real world are vital for evaluating the toxicity and health impacts on firefighters and vulnerable population; however, they are rarely available. Online monitoring of trace elements in ambient PM_2.5 at five sites throughout Hong Kong in 2021 provided unprecedented opportunities to observe the highly transient urban fire plumes. Sharp spikes of Br (up to 0.65 µg m^-3), Cl (7.0 µg m^-3), and certain metals such as Pb (0.97 µg m^-3), Zn (8.3 µg m^-3), and Cu (0.32 µg m^-3) during four urban fires were monitored, which showed ~10–6,000 times higher concentrations than pre-fire periods. These elements behaved declined concentrations with increasing distance from the fire location, but remained relatively constant elemental ratios that depended on burning materials in each fire. During fire-influenced hours, these elements exhibited 3–101 times enrichment relative to iron, an element that had no discernable increase in concentration. It was distinct to one monitored hill fire that was characterized of concentration spikes and high enrichment of biomass burning tracers, i.e., Rb (11.6 times) and K (6.5 times). We discussed the likely co-emission mechanism of Br and certain metals by urban fires from the view of melting and boiling points of metallic compounds. Reactive Br released from burning of the wide-spread brominated synthetic chemicals was suggested to facilitate the mobilization of toxic metals into the atmosphere. This mechanistic knowledge will help us understand and distinguish the elemental characteristics between urban fires and wildfires. The transient fire episodes with high emission intensity can hardly be resolved in source identification of ambient particles, but will interfere the source solution. Therefore, we further identified the sources of hourly trace elements monitored in the whole year, to evaluate the influence or contributions of fire episodes on their source apportionment.

Trace elements,Urban fire,Wildfires,Source apportionment