Yes. Several reasons:<p>A) The diurnal cycle means that pollution production has been reduced for roughly twelve hours, so it has had a chance to disperse.<p>B) the cooler morning temperatures have allowed some precipitation of excess moisture in the air which condenses around dust particles and thus 'washing' them out of the air.<p>C) Cooler temperatures due to lack of turbulence-producing solar heating mean less dust lifted into the air by thermals.<p>As an ex-pilot, I must state that flying is far more pleasant in the early mornings, as the air is cleaner/clearer and there are very few annoying turbulent thermals at that hour of the day.