Smoggy days are often blamed on regional pollution and weather, but at least one recent scientific study has shown that the city can, under the right conditions, “form its own smog”. The study by Hong Kong and Macau air scientists argued that a rapid build-up of particulate matter in the air – a key component of smog – was possible even in the absence of northerly winds that can transport pollutants from afar. Watch: Drone footage of smog in Hong Kong The evidence boiled down to at least one particular sunny September day in Hong Kong in which a “land-sea breeze” pattern formed along with weak winds far below average speeds. Smog from China shrouding Hong Kong poses ‘very high’ health risk The scientists observed a rapid rise of photochemical activity during mid-afternoon, in which ozone and nitrogen dioxide skyrocketed along with increasing sunshine. “It is clear that there was a rapid increase in particulate matter (PM) concentration on this day when we were not really affected by external meteorological conditions. It’s not easy to argue in this case that winds were blowing PM to Hong Kong from the region ,” said co-author Professor Chan Chak-keung, dean of City University’s school of energy and environment. The culprits, he said, were most likely local sources such as vehicles or industrial emissions, which contain nitrogen oxides and volatile organic compounds. The latter pollutant is also found in products such as organic solvents, paints and printer inks. Chan’s team investigated “episodes” – days with high PM concentrations – in one-month periods in each of the four seasons from 2011 to 2012 at the University of Science and Technology’s air quality research supersite. Other episodes across the seasons were also observed with high local photochemical activity, but those days also came under the influence of transported air from the north, making it less clear what was actually local or regional. The paper was published in scientific journal Atmospheric Chemistry and Physics in November. “Of course, regional sources play a role but [this research] shows that under the right conditions, PM can build up and Hong Kong can form its own photochemical smog.” Hong Kong’s official air quality index failing to warn on deadly health hazard Photochemical smog is created when nitrogen oxides react with volatile organic compounds in the air under sunlight. It leads to the formation of ozone. This hazardous pollutant facilitates the formation of the tiny particles, small enough to be inhaled deep into the lungs and even into the bloodstream. The particulate matter in the air lowers visibility, turning the sky smoggy and gives it a lurid orange tint at dusk. The Environmental Protection Department usually points to meteorological influences such as northeast monsoons when the air quality health index hits “very high” health risk levels. During a bout of high pollution last Thursday, it said: “Hong Kong is being affected by an airstream with higher background pollutant concentrations. The light wind hinders effective dispersion of air pollutants.” It added that the formation of ozone and fine particulates during the daytime resulted in high pollution in the region. Chan said most smoggy days were doubtless a result of regional factors or pollution. But he said the study’s findings warranted more research on how PM was formed and pinpointing its sources.