Urban Growth Fuels Hidden Air Chemistry in China

Land use has changed a lot between 2001 and 2020 in China, especially as cities grew and forests were planted. Scientists wanted to see how these changes affect the way certain chemicals form on surfaces and later in the air. They used a computer model that simulates air quality, keeping weather and pollution sources the same for all runs. Only the amount of built‑up area (PURB) and the leaf cover index (LAI) were changed to mimic different landscapes. The study found that reactions on the ground are the main source of nitrous acid (HONO), making up more than half of its total levels. When cities expand, HONO rises sharply: Beijing‑Tianjin‑Hebei saw a 50. 3 % jump, while the Yangtze River Delta, Pearl River Delta and Sichuan Basin increased by 48. 4 %, 45. 2 % and 38. 1 % respectively. Changes in vegetation had a smaller effect, usually less than 5 %.More HONO changes the chemistry of total nitrate (TNO₃). In daytime, higher HONO breaks down into hydroxyl radicals, which speed up the conversion of NO₂ to nitric acid (HNO₃). This boosts nitrate levels by 0. 8, 0. 6 and 0. 3 µg/m³ in the three regions mentioned. At night, however, HONO competes with another pathway that uses nitrate ions, leading to a slight drop in nighttime nitrate by 0. 2 and 0. 3 µg/m³. The shift in land cover also slightly alters weather patterns, but this effect on HONO is small. It tends to counteract the nitrate build‑up by about 0. 12 µg/m³. A separate test that lets the surface take up more or less HONO depending on sunlight showed clear day‑night differences: daytime levels go higher and nighttime levels lower than when a fixed uptake rate is used. Overall, the research shows that building more cities changes how chemicals form on surfaces and in the air. It highlights the need to include real‑time land use changes and better surface chemistry rules in regional air quality tools.