Aerosol acidity has broad significance in the atmosphere and ecosystems; however, a reliable way to quantify its changes over long time is lacking. Here, we propose a new approach calculating aerosol pH based on the nitrogen stable isotope composition (δ15N) of aerosol ammonium (NH4+) and for the first time reconstruct historical trajectory of aerosol pH over the last two centuries via δ15N of NH4+ achieved in a Tibetan ice core. We observed a significant decrease in δ15N of ice core NH4+ by 13‰ from the preindustrial to modern era, corresponding to a decreased in aerosol pH by 0.75 units. The decline in pH demonstrates a dominant role of anthropogenic emissions in acid gases over alkaline gases since Industrial Revolution. Our study also suggests that spatiotemporal patterns of aerosol acidity could be widely revealed by future nitrogen stable isotope of aerosol or ice core ammonium measurement, which in turn will promote the understanding in aerosol chemistry in the context of global environmental changes.y in the context of global environmental changes.