Pollen, considered to be one of the main causes of allergies and also a major contributor to pollution, is easily spread through the air and its impact on human health may increase due to climate change. The physical and optical properties of pollen also determine how much sunlight arrives on Earth's surface -- and the amount of sunlight reaching the surface has a major influence on our planet's temperature.

Dr Mueller, now at the University of Hertfordshire's School of Physics, Astronomy and Mathematics and previously at Leibniz Institute of Tropospheric Research (TROPOS) in Germany, said: "Together with colleagues from the Gwangju Institute of Science and Technology (GIST) in Korea, we used a tool called a polarisation lidar to observe particles, including pollen particles, in the atmosphere."

"We found that pollen particles from trees are distributed in the lower part of the atmosphere in cities. This is caused by small air convections, air updrafts and downdrafts -- and good weather conditions, such as high temperatures and low relative humidity, also contribute to this."

Polarisation lidar (Light Detection and Ranging) is a relatively new research technology. Operating in a similar way to radar, it measures the difference in time between sending a laser pulse up into the sky and receiving the back-reflected laser pulse -- allowing the distance between the instrument and the reflecting object (pollution particles in this case) to be computed.

Using these sophisticated laser techniques, the team measured some different optical properties of airborne pollen for the first time -- including the light-extinction coefficient which enables the determination of the optical depth of the atmosphere. This optical depth determines the amount of sunlight falling onto Earth's surface, and hence directly influences Earth's temperature.

Dr Mueller continued: "Dust is known to act as carrier of bacteria and fungi as well as pollen. We observed an increase in the number of people who were hospitalised after large dust outbreaks over South Korea where we were taking the measurements. They suffered from breathing problems, eye inflammations and other strong allergic reactions. And we have noted similar conditions around the world, such as in the Sahel Zone in Africa, where cases of meningitis may have been caused by bacteria carried by dust."