Which of the following cities has been worst affected by urban smog in recent…

Concept — what smog is. The word smog is a blend of smoke and fog: in its classical form it is literally smoke-laden fog, and more generally it is the visible, chemically active haze that builds up over a city when emissions released near the ground cannot disperse — typically because a temperature inversion or a stagnant anticyclone caps the air above it. Two classical types are distinguished. Classical (London-type) sulphurous smog forms in cold, damp air from coal smoke, soot and sulphur dioxide (SO2), is chemically reducing, and does involve fog droplets. Photochemical (Los Angeles-type) smog forms in warm sunlight when nitrogen oxides and unburnt hydrocarbons from vehicle exhaust react to give ozone and peroxyacyl nitrates; it is chemically oxidising and needs no fog at all.

Concept — how "worst affected" is measured. The severity of a smog episode is judged not by historical fame but by the pollutant loading actually measured during the episode — chiefly the fine-particulate concentration PM2.5, alongside the gaseous pollutant that characterises the type (ozone in photochemical smog, SO2 in sulphurous smog) — together with the scale of emergency response the episode forces: school closures, grounded flights, driving bans and formal alert levels. When a measured value is set against a guideline, the averaging periods must match: the WHO guideline of 25 µg/m3 in force at the time of this paper is a 24-hour mean, so it is compared with a 24-hour mean and not with a one-hour peak.

Application to this question. The stem says "in recent times", so the yardstick above is applied to the years immediately preceding this 2014 paper rather than to any city’s historical reputation. In the January 2013 episode the 24-hour mean PM2.5 in Beijing reached about 569 µg/m3 — more than twenty times the WHO 24-hour guideline — while hourly readings at individual stations ran past 800 µg/m3. Flights were grounded, schools suspended outdoor activity, and Beijing adopted a colour-coded emergency air-quality alert system in October 2013 in response. Judged by measured particulate loading and by the emergency response required, the city worst affected by urban smog in recent times is Beijing.

Cross-check — the other cities named, by value.

• London: its defining episode, the Great Smog of December 1952, is historical. Coal smoke and SO2 trapped under a temperature inversion for four days caused thousands of excess deaths and led to the Clean Air Act, 1956. Domestic coal burning has since been tightly restricted and has declined sharply, so classical sulphurous smog is no longer London’s present-day condition.

• Los Angeles: the city where photochemical smog was first recognised, from the wartime episodes of 1943 onwards, and explained by A. J. Haagen-Smit in the 1950s. Ozone levels in the basin have fallen substantially since the 1970s under California’s vehicle-emission standards, so the problem there is far smaller than at its mid-century peak.

• Paris: a genuine and contemporaneous case — the March 2014 episode was severe enough to trigger alternate-day (odd–even) number-plate driving restrictions and free public transport. Its PM2.5 peak of about 185 µg/m3 was, however, several times lower than the Beijing figures of the same period, which is why it is not the worst-affected city among those listed.

Result: Beijing.