Our observations show that both local dust and dust carried over long distances can increase the number of particles that help form ice in clouds. Polluted air held fewer such particles and appears to weaken the ability of dust to initiate ice formation. In semi-arid cities, larger particles in the 1–2.5 µm range strongly indicate how many of these ice-forming particles are present. Understanding how different particles behave is essential for representing cloud glaciation and regional climate.