Winter's Foggy Mystery: How City Smog Forms

In the dead of winter, Beijing's air quality often takes a nosedive. The city's notorious smog is a complex mix of pollutants. One key player in this mix is sulfur-containing aerosols. These tiny particles come from the chemical reactions happening in the air. But how do they form? Let's dive into the science behind it. Fossil fuels are a big part of the problem. When burned, they release harmful gases into the air. One of these gases is sulfur dioxide (SO2). In the winter, when the air is cold and damp, SO2 can undergo chemical changes. These changes turn SO2 into sulfate, a major component of smog. But there's more to the story. Nitroaromatic compounds (NACs) also play a role. These compounds are found in fossil fuel emissions. In the presence of water vapor, NACs can break down. This breakdown produces nitrous acid, which speeds up the conversion of SO2 to sulfate. The process doesn't stop there. Sulfate can react with other compounds in the air. These reactions lead to the formation of aliphatic organosulfates. These are another type of sulfur-containing aerosol. They contribute to the haze that chokes Beijing's skies. All of this happens in the aqueous phase. That's a fancy way of saying it happens in water droplets in the air. The high humidity in winter provides plenty of water for these reactions to occur. This is why Beijing's smog is often worse in the winter. So, what can be done? Understanding the chemistry behind smog is a start. But more needs to be done to reduce fossil fuel emissions. This is a complex problem that will require innovative solutions. The stakes are high. The health of millions of people is at risk.