A central aim of the ICASIF project is to understand atmospheric aerosols (pollution/haze) and their effect on the solar radiation reaching earth's surface. During a trip to India in January 2015, I took the opportunity to observe the sun and the sky in India, and think about how the effect of aerosols attenuating the incoming solar radiation is visible to the human eye.
During the trip, we visited Delhi and TERI for the kick-off meeting of the project, and Mukteshwar, a measurement station in the Himalayan foothills some 300 km North East of Delhi, which is jointly run by TERI and FMI. In ICASIF, we will perform dedicated solar radiation measurements at the station.
Can you believe the drive to Mukteshwar took us roughly 10 hours? This came as some surprise to me, being in India for the first time of my life. The roads and the traffic are rather different from Finland, and the same goes for the solar radiation and aerosol conditions!
The picture below shows the sun and the sky in Delhi, outside TERI's main office on the day of our kick-off meeting. The sun is visible, although partly obscured by a hazy sky, indicating a heavy aerosol load attenuating the incoming solar radiation. Based on the looks of the sky, I would estimate that the aerosol optical depth (AOD), a measure of the amount of aerosols in the atmosphere, is around 0.6 or so. This is much higher than the typical values of AOD below 0.15 in the clean atmosphere of Finland.
While driving to Mukteshwar, roughly half way, we stopped for a break at a place where I very much enjoyed the view of the spacious and scenic restroom shown in the picture below. The sky here was similar to that in Delhi, a blurry haze indicating heavy aerosol load again. Note that the sun is still bright enough to cast shadows.
Finally, at Mukteshwar station in the Himalayan foothills, at an altitude of around 2 km above sea level, we found crispy, deep blue skies indicating a clean atmosphere! The sun was bright because it was not attenuated to any great extent by atmospheric aerosols. In Mukteshwar, conditions vary throughout the year and more pollution is expected before the moonsoon period. It will be interesting to measure and think about how the aerosol load changes over time and how that is reflected in the incoming solar radiation available for solar energy production. In the picture below, the PI of our project, Dr. Heikki Lihavainen, poses outside the Mukteshwar measurement hut next to a portable weather station that will be used in an expedition to a Himalayan glacier later this year. The Himalayas can be seen in the background. Note the solar panel providing electricity for the instruments making the measurements. A small battery is included in the package to provide electricity during the dark hours.
I find that by viewing the sky, we can learn a lot about atmospheric aerosols and how they relate to solar radiation. However, it seems I need to practice more to make accurate estimates of the AOD. MODIS satellite data for Delhi on the day of our kick-off meeting show values around 1, which is clearly higher than my estimate of 0.6.
Text: Heikki Lihavainen