When winter smog takes over Asian mega-cities, more particulate matter is measured in the streets than expected. An international team has now discovered that nitric acid and ammonia contribute to the formation of additional particulate matter. Nitric acid and ammonia arise in city centers predominantly from car exhaust. Experiments show that the high local concentration of the vapors in narrow and enclosed city streets accelerates the growth of tiny nanoparticles into stabile aerosol particles.