Aviation as an Emerging Urban Air Quality Challenge in Megacities

China is poised to become the world's largest civil aviation market, yet reducing both CO2 and air pollutant emissions remains a major challenge. Here, we develop a city-resolved projection framework linking origin-destination air travel demand with urban socioeconomic characteristics, fleet turnover, and route-level operations to project aviation activity and its spatial redistribution across Chinese cities through 2035. Our results show that aviation NOX and PM emissions increase by 65-124% and 59-149% across different scenarios, between 2019 and 2035, despite technological and operational improvements. Although megacities remain major emission hotspots, their share of total aviation NOx declines from over 40% to about 30%, driven by rapid growth in medium-sized cities, where route-level emissions more than double. Newly built airports, many in western China, contribute an additional 17% of aviation NOX emissions by 2035. Notably, in several megacities, aviation NOX emissions at low altitude are projected to surpass those from road transport, making aviation a dominant local transportation-related pollution source. Atmospheric simulations indicate that aviation-attributed PM2.5 and O3 could contribute up to 9 and 14% of ambient concentrations in developed urban regions by 2030. Together, these findings identify aviation as an emerging urban air quality challenge that requires city-targeted mitigation strategies.