Ammonia: An Overlooked Byproduct from Energy Generation
|08/19/2012 by Kang Sun|
It has been four years since the Beijing Olympic. At that time I was in a big research team to monitor the air quality of Beijing during the Olympic games. However, my work was not that exciting as it sounds. I had to stay in a small windowless room on top of the science building in Peking University, taking liquid samples exactly every 10 minutes for at least 4 hours per day. It took me even more time to analyze these samples afterwards. We were trying to measure ammonia (NH3), a very important atmospheric component, but we only had this tedious way to do it.
Ammonia plays a critical role in the atmosphere because it contributes to the formation of aerosols, or particulate matter. Aerosols affect the earth’s radiation budget and climate through their effects on cloud formation and precipitation. Moreover, smaller aerosols have been associated with adverse health effects. I have heard many friends complaining that they are more susceptible to colds and flu when staying in the northern China due to polluted air. If you take a look at the satellite image (a MODIS image on October 18, 2011 is shown here), you can find that the huge aerosol plume covers the entire north China plains and extend to the Pacific Ocean. When you land on the Beijing Capital International Airport, often times you can see the whole city is immersed in a milky, sometimes bluish haze. These aerosols can stay in the atmosphere for more than a week, enough time for them to travel across the Pacific when the atmospheric condition is favorable for transport.
These hazes or plumes generally consist of secondary aerosols, or aerosols formed in the air. In other words, they are not directly emitted, but generated by reactions of gas phase components, like SO2, NOx (NO, and NO2), ammonia, and VOCs (volatile organic compounds). The composition of these aerosols is well known: about two thirds are ammonium salts, such as ammonium sulfate, ammonium nitrate, etc. Nitrate and sulfate in the aerosols come from some notorious pollutants: SO2, NO, and NO2. These pollutants are now routinely measured in most air quality monitoring stations and the regulation standards have been well established in both China and the US. However, it is very difficult to get aerosol formation if only SO2 and NOx are present. Ammonia is the key step to neutralize SO2 and NOx to aerosols, because SO2 and NOx are acidic gases and ammonia is the only alkaline readily available in the atmosphere.
Considering its importance in atmospheric chemistry, our knowledge on atmospheric ammonia is extremely limited. There are not many efficient and cheap ways to measure it. Otherwise, we would not sacrifice our time during the Olympic to do the boring sampling. In the US and Europe, some universities and research institutes use very expensive instrument to monitor ammonia, but there are very few measurements in China. Given the heavy aerosol load, we would expect very high ammonia emissions in north China plain. Power plants and auto vehicles can generate SO2 and NOx, but serious aerosol pollution can only happen when enough ammonia is provided. Comparing to SO2 and NOx, the regulations on ammonia emissions are very weak even in the US. When you go to the emission test of your car, generally CO, SO2 and NOx are all tested, but not ammonia. Nevertheless car emission is a significant ammonia source. I have tested the ammonia emission from my own car using sensors in my lab, and I did see a lot.
The diversity and variety of ammonia sources also complicates the problem. Livestock and fertilizer are among the largest sources. A dairy farm is to ammonia as a power plant is to SO2. Modern power plants have scrubbers to eliminate SO2 emission, but dairy farms have little control on their ammonia emission. Just go to a dairy farm and smell it! On-road vehicles are another significant ammonia source, especially in China, where more vehicles are sold per month than in the US.
The environment and climate concerns are always associated with energy issues. SO2 and NOx emitted in energy generation have been emphasized for a long time, but ammonia is rarely discussed equally. Controlling ammonia actually can be a more efficient way to mitigate the environment impact from energy generation.
Copyright 2012 by Princeton University China Energy Group. All rights reserved.