Air pollution varies with the time of year.
Smog is usually a problem mostly in the summer months when there is a lot of sunlight; particulates are problem in dry months when wildfires are likely and during months when the wind blows across the desert.
Las Vegas: Particulates
There is a brown haze over Las, Vegas, Nevada and it is there because of particulates in the arid region. About 60% of the dust comes from new construction sites, dirt roads, and vacant land.
Haze from Afar
Pollution and air quality concerns are not restricted to only urban areas.
In spring of 2001, a white haze of dust from Mongolia arrived in North America. The haze affected one-forth of the US and could be seen from Canada to Mexico. The particles were known to cause respiratory problems. People believe that the haze was carried from Asia by winds across the Pacific Ocean.