Context
Urban atmospheric pollution is a very important field of research since most people live in cities. In particular, acid fogs are a major health concern because the droplets are small enough to be efficiently inhaled. A comprehensive knowledge of urban atmospheric chemistry is therefore an essential prerequisite to risk assessments and pollution control strategies.

Sampling
Fogwater samples were collected at eight different sites in the city of Brussels, depending on the presence of fog. Fogwater was sampled using a Screen collector (Fig. 1). Droplets are collected by inertial impaction on an angled screen made of six layers of polyamide strands. Droplets coalesce rapidly on these strands and flow downward into a funnel and a polyethylene collection bottle. A collection rate of approximately 55 ml/h is yielded in a fog with a typical Liquid Water Content (LWC) of 0.1g/m³. About fifty fogwater samples corresponding to twenty fog events were collected during a 17 month-period.

Fig.1

Analysis
The conductivity and the pH are measured after the filtration of samples. Ammonium (NH₄⁺) and nitrates (NO₃^-) ions are analysed by visible colorimetry on an autoanalyser. Chloride (Cl^-) and sulfates (SO₄⁼) anions are measured by high-pressure liquid chromatography. Major cations (Ca⁺⁺, Mg⁺⁺, Na⁺, K⁺) are determined by flame atomic absorption spectrophotometry, and some trace metals (Al, Fe, Mn, Pb, Cd, Zn, Cu) are analysed by graphite furnace atomic absorption spectrophotometry.

Main results
Fogwater consists of a very concentrated ionic solution compared to rainwater and dew. An inverse relationship between the conductivity and the Liquid Water Content is observed, reflecting a dilution effect. The fogwater composition is dominated by NH₄⁺ and SO₄⁼, and to a lesser extent by NO₃^- and Cl ^-
NH₄⁺ and SO₄⁼ concentrations show a very good correlation, which means that the equivalent proportion between NH₄⁺ and SO₄⁼ is remarkably constant. The fogwater pH value is 4.2 on average, but it is occasionally very acidic (pH £ 3). The samples showing the lowest pH’s values are the most concentrated in Cl ^-, which reflects the presence of HCl in the atmosphere most probably emitted by the domestic waste incinerator.
Thus, acidity is partially compensated by ammonium in most Brussels fog samples. The NH₄⁺ cation, acting as the main acid-neutralizing component, originates from gaseous NH₃ whose main sources are intensive cattle breeding and agricultural activities. A preliminary study of the trace metals composition shows that aluminium, iron, zinc and lead concentrations can reach 1 ppm. Strong correlations observed between typical crustal (Fe, Al, Mn) and anthropogenic (Pb, Cd) species reveal that the small particles emitted by the combustion of fossil fuels can constitute a common source to these metals. The composition of samples collected at different times and places is found to be highly variable because it depends on a great number of chemical, physical and meteorological parameters such as temperature inversion, adjacent local sources of a specific pollutant, and liquid water content. Brussels fogwater can be largely assigned to a solution of ammonium salts such as (NH₄²) SO₄ and NH₄HSO₄. These salts are primarily present in the aerosol phase and act as condensation nuclei for the growth of cloud droplets, where they are subsequently partially dissolved. The dissolved fraction of fogwater is mainly controlled by the incorporation of the fine fraction of aerosols.

Contact for further information: sfally@ulb.ac.be