December 19, 2013
One-Third Of Atmosphere’s Aerosol Particles Comprised Of Pollen
redOrbit Staff & Wire Reports - Your Universe Online
Pollen comprises one-third of the aerosol particles present in the Earth’s atmosphere, and the powdery substance reflects more sunlight than experts had previously believed, according to new research appearing in the journal Atmospheric Environment.
The study, which was written by scientists at the Gwangju Institute of Science and Technology (GIST) in Korea and the Leibniz-Institute of Tropospheric Research (TROPOS), is reportedly the first project of its kind to analyze the optical properties of natural pollen using a laser operating at 532-nm emission wavelength.
Pollen, which is believed to be one of the primary causes of allergies, has attracted a lot of attention from scientists because its impact on human health could increase as a result of global warming. In particular, investigators are looking at how it spreads and becomes distributed throughout the atmosphere.
“In addition, pollen is a considerable natural source of pollution in the atmosphere where it scatters sunlight – so-called corona effects have been known about for a long time,” TROPOS explained in a statement Wednesday. However, because of “the extremely short measurement times in the near field of the laser,” the authors said that it was “difficult to measure close to the ground where pollen is usually found.”
Also, specific measurements needed to be made in order to separate pollen and other natural sources of pollution from those that are man-made, they said. Dr. Youngmin Noh of GIST and colleagues used sophisticated laser measurement techniques in order to measure different optical properties of pollen for the first time.
“Of specific interest is the light-extinction coefficient, as this parameter allows us to determine the so-called optical depth of the atmosphere. The optical depth determines how much sunlight from space arrives on the Earth's surface, and this measurement has a major influence on our planet's temperature,” the doctor said.
They research team used a tool known as a polarization lidar, which is used to observe particles in the atmosphere with a laser beam, to monitor the vertical distribution of pollen in Gwangju, South Korean in May 2009. The instrument allowed them to measure differences in time between the emission of a laser pulse and the reception of its reflection, helping them to compute the distance between the tool and the reflecting object.
By measuring changes in the polarization of the reflected laser light, researchers are able to determine the physical properties of the object that reflects them, the authors explained. The measurements illustrated that, due to air convection, pollen can be detected at the atmosphere’s highest point during noontime, while strong changes in polarization were reported up to 500 meters above the ground during the morning hours.
“In this study we report on the backscatter coefficient of pollen for the first time. We were able to determine this parameter by using the so-called lidar-depolarization technique,” Dr. Young Min Noh said. “For this, we separated the laser light that is reflected by pollen from the reflection signal of the total atmospheric aerosols.”
“The pattern of the backscatter signals shows that pollen particles from trees are distributed in the lower part of the atmosphere of cities. This is caused through small-scale convection, air updrafts and downdrafts, and, if there are good weather conditions, high temperatures and wind speed as well as low relative humidity,” the doctor added.
According to the researchers, May is the main pollen season in Korea, and while they were conducting laser measurements, they also used pollen traps to collect the powder at the ground in order to compare ground-level pollen data to atmospheric measurements. They discovered that pollen can be a significant natural source of aerosol particles during periods of particularly strong pollen emissions.
“We assume that the strong pollen emissions in densely wooded areas of the Northern Hemisphere, such as Alaska, Canada, Scandinavia and Siberia, may significantly influence the optical properties of the atmosphere, particularly at the start of the growing period in spring,” said Dr. Detlef Müller, leader of the lidar group at GIST and formerly a researcher at TROPOS. “Although our observations only represent a small portion of the overall large-scale effects, they nevertheless highlight the importance of pollen emissions.”
Image 2 (below): This is the Lidar in use at the TROPOS in Leipzig. With the portable mini-Raman lidar "Polly" a fully automatic system has been developed here, allowing the daily aerosol observations even without much infrastructure. "Polly" has also been used in field experiments in Perl River Delta in China or the Amazon in Brazil. Credit: Photo: Tilo Arnhold/TROPOS