Gaseous sulfuric acid (H₂SO₄) is a crucial precursor for secondary aerosol formation, particularly for new particle formation (NPF) that plays an essential role in the global number budget of aerosol particles and cloud condensation nuclei. Due to technology challenges, global-wide and long-term measurements of gaseous H₂SO₄ are currently very challenging. Empirical proxies for H₂SO₄ have been derived mainly based on short-term intensive campaigns. In this work, we performed comprehensive measurements of H₂SO₄ and related parameters in the polluted Yangtze River Delta in East China during four seasons and developed a physical proxy based on the budget analysis of gaseous H₂SO₄. Besides the photo-oxidation of SO₂, we found that primary emissions can contribute considerably, particularly at night. Dry deposition has the potential to be a non-negligible sink, in addition to condensation onto particle surfaces. Compared with the empirical proxies, the newly developed physical proxy demonstrates extraordinary stability in all the seasons and has the potential to be widely used to improve the understanding of global NPF fundamentally.