Here's an answer from Clarke Knight who's studying pollen accumulation rates at Berkeley:: "Pollen is not typically 'measured' in air, per se. Pollen can be collected with terrestrial traps (often made of pvc pipes) or moss pollsters. Traps would be buried flush with the ground and pollen could be collected over the course of a season. Pollen can also be collected from lakes by taking lake core sediments. Because pollen is microscopic (10-150 microns typically), you would also need additional equipment to sort out the pollen and then count it."