Nitrogen containing compounds could be the “missing link” in the formation of haze particles that have plagued China. It is yet another typical morning, your alarm clock goes off and you slowly get up, ready for your morning routine. All of a sudden, your nose detects a strong, acrid burning smell, as if your neighbour is starting up his charcoal grill. You march up to the window and fling the curtains open, ready to lecture your pesky neighbour. Instead of your usual view, something far more sinister greets you. A fog-like entity drapes the landscape, dimming the morning sun and you can almost see dust-like particles floating in the air. This is no ordinary fog, it’s haze.
This is a sight that people living in North China Plain (NCP) would have to face annually during winter. Haze is a concoction of multiple toxic substances and would adversely affect people who have chronic heart or lung diseases. Its severity is based on the concentration of particular matter of less than 2.5 micrometre (otherwise known as PM2.5). That means that haze blanketing the cities consist of ultrafine particles that damages your lungs should you inhale it in large quantities. During the 2013 haze event in NCP, daily PM2.5 concentration soared to hazardous levels, forcing Chinese residents to take desperate measures to protect themselves.
A group of Chinese researchers led by Yafang Cheng, an academic researcher at the Max Plank Institute for Chemistry, found that current air quality models are unable to accurately measure another toxin, sulfates. Sulfates are known to be a major component of haze particles. Current models measure sulfate concentrations when it’s produced through photochemical reactions (chemical reactions that occur in the presence of light) of airborne molecules. During the 2013 haze, much of the light had been blocked out by haze, reducing photochemical reactions. This should have lowered sulfate production, which is supposed to be good news. Instead, astonishingly high sulfate concentrations were observed by Cheng’s team as compared to concentrations recorded by current models. This meant we would be exposed to significantly more toxic air than reported levels. How and why did this happen? “(The) increased sulfate production suggests the existence of a missing sulfate production pathway but the chemical mechanism for this has not yet been identified”, explained Cheng. Without knowledge of this unknown reaction, current air quality models weren’t able to factor these values in, resulting in under-reported sulfate concentrations.
In search of the “missing link”, Cheng and her team began the process of sampling and analysing the polluted air using state-of-the-art equipment. This equipment could be thought as your everyday vacuum cleaners with multiple sensors embedded, allowing them to take accurate PM2.5 concentrations in the polluted air. Together with advanced weather prediction software, they were finally able to piece together the “missing link”. This came in the form of a chemical model between nitrogen-containing compounds, primarily Nitrogen Dioxide, NO2 (produced when fuels or coal are burnt at high temperatures) and clouds.
Taking into account the reduced photochemistry as well as rising PM2.5 concentrations in NCP, a high concentration of NO2 was found. This led to NO2, being the primary pollutant, to be subjected to a chemical reaction with clouds to form sulfate particles. Though this reaction was suggested by earlier studies to occur in fogs under polluted conditions, no fog was observed during the 2013 haze by Cheng and her team. This meant that “a new and more comprehensive conceptual model of sulfate formation” has been discovered.
Known for burning large amounts of coal annually, China is infamous for their air pollution. The New York Times have reported that air pollution in China accounts for 366,000 premature deaths in 2013. This is no mere coincidence as 2013 was also the year where sulfate and PM2.5 concentrations skyrocketed in NCP, prompting the researchers to carry out this study. With this model, Cheng remarked that reducing NO2 emissions should reduce sulfate and PM2.5 concentrations much more than predicted by current air quality models. China should now be able to implement policies using this knowledge in hopes of allowing their residents to enjoy clear skies.
Other countries, primarily those affected by haze, could also study this model, preventing their citizens from suffering the same fate as NCP. The scientific community could gleam new insights from this model, allowing them to calibrate air quality models to improve its accuracy, allowing us to plan our outdoor activities accordingly.Before you rush out to buy cartons of industrial-grade masks in preparation of haze, remember that we can also do our part to reduce NO2 emissions. Some ways include taking the public transport instead of driving. Sacrificing some of these luxuries prevent us from wearing a mask or carrying an oxygen tank every time we go out.