Projected westward advance of tropospheric formaldehyde over China

Economic development and population aggregation are driving the east-to-west spread of tropospheric formaldehyde (HCHO) in China; yet the historical trajectory and future threat of this westward migration remain poorly understood. Here, we analyze OMI satellite observations utilizing a novel surface-to-line differentiation approach and machine learning to project spatiotemporal distribution of tropospheric formaldehyde in China, measure the westward movement of formaldehyde using the classic Hu Huanyong Line (Hu Line). We show that high-HCHO boundary migrated from 132.7 km east of the Hu Line in 2005 to 130.2 km west of it in 2020, before a subsequent partial eastward retreat to 6.6-87.8 km. The westward migration rate peaked at 25.12 km yr^-1 during 2010-2014, then progressively slowed and became negative after 2020, reaching -8.48 km yr^-1. This trajectory reflects a fundamental transition from strong coupling to decoupling between economic expansion and pollution spread. Regionally, the strongest westward advance occurred over the Heilongjiang-Jilin and Shanxi-Shaanxi energy base, whereas the Beijing-Tianjin-Hebei region exhibited early contraction. Our research quantifies the risks of transregional HCHO transport and provides a scientific foundation for predictive air quality management and regional governance, and these findings also demonstrate that the Hu Line should be an environmentally dynamic boundary reflecting regional economic activity, providing a new perspective for assessing sustainable development patterns and human-environment interactions.