Atmospheric Ammonia and the Air Quality of the North China Plain

by Kang Sun (CEE)

Figure 1. PM2.5 nonattainment US counties in 2006.

I stayed in Beijing and my hometown in Henan Province during the last three weeks in 2012. While I was amazed by the heavy aerosol loading in the winter, I did not expect that the Chinese air quality would draw the attention of the entire world one month later. In January-February 2013, the North China Plain experienced an epic period of atmospheric pollution, with PM2.5 (particles with diameters smaller than 2.5 µm) spiking up to 900 µg/cm-3. At that time, I already came back to the US and participated in a NASA field mission to measure atmospheric ammonia (NH3) in the San Joaquin Valley, California, which is also well known for PM2.5 pollution. I’d like to talk about this California case first, and then link it back to the North China Plain.

PM2.5 decreases visibility and is harmful when inhaled because it can penetrate deeply into the lungs. The US Environmental Protection Agency (EPA) sets a national standard of 24 h-average PM2.5 concentration of 35 µg/cm-3. Figure 1 illustrates the US counties that do not attain this standard in 2006 [1]. Apparently, the San Joaquin Valley (red circle in Fig. 1) in California is the largest nonattainment area.

We were driving our NH3 sensor for 2700 miles in the San Joaquin Valley, and took many pictures during the road trip [2]. As you can see in the left frame of Fig. 2, the air was hazy, and you can barely see the Sierra Nevada at the east edge of the valley (the California government is actually concerned about the impact of nitrogen aerosol deposition on the ecosystem health in the Sierra mountains).

These hazes generally consist of secondary aerosols, or aerosols formed in the air. In other words, they are not directly emitted, but generated from reactions of gas phase components, like SO2, NOx (NO+NO2), NH3, and volatile organic compounds (VOCs). The majorities of aerosols in the hazes are smaller than 2.5 µm in diameter, and thus counted as PM2.5. About two thirds of PM2.5 is ammonium salts, formed by the following chemical reactions:

Where H2SO4(g) is formed from SO2, which is mostly generated by fossil fuel combustion, and HNO3(g) is formed from NOx, which also comes from combustion processes. There are strong industrial SO2 and NOx sources in the San Joaquin Valley. Moreover, the valley also has very intensive agricultural activities, which contribute to substantial NH3 emissions. Dairy farms (the right frame of Fig. 2) are strong NH3 sources. You can certainly smell it when driving by.

Figure 2. Pictures taken during California road trip with NH3 sensor (shade in the right picture).

The North China Plain is comparable to the San Joaquin Valley in many aspects: dense human and automobile population, large gas and oil industry, intensive farming, and notorious air quality. There are even more emission sources, and less atmospheric observations/emission mitigations in the North China Plain, resulting in even higher pollution levels.

Figure 3 shows images taken by Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite [3]. The image from January 3 (bottom) shows snow blanketing much of Beijing and its environs. In the image from January 14 (top), Beijing and its surrounding plain was immersed in a milky haze. This past winter was abnormally cold, which favored the reactions (1) and (2) from the gas phase to the particulate phase. In other words, the NH3 is easier to be transformed to ammonium salts in the aerosols.

Figure 3. MODIS images of the North China Plain taken in January 2013.

The largest NH3 emission sources in the North China Plain are agricultural activities, including nitrogen fertilization and livestock emissions [4]. In the highly productive regions of China, two or three times more fertilizer is applied than Midwestern American or Europe. The applied nitrogen is often in excess of what the crop can take up, and then it is lost to the environment, mainly in the form of NH3. From the perspective of US media, the level of fertilization in the North China Plain is “astounding” [5]. Figure 4 shows the Chinese nitrogen fertilization rate in 1990 and 2008 [6]. In the most intensive farming land in the North China Plain, 40 g of nitrogen, which means > 100 g fertilizer was applied to every m2 land every year.

Figure 4. Chinese fertilization rate distributions in 1990 (left) and 2008 (right).

Figure 5 (a-b) further illustrate the increasing trend of Chinese NH3 sources, and a substantial portion of these sources concentrates in the North China Plain [7]. Both livestock units and nitrogen fertilization, the two dominant NH3 sources, have been increasing linearly (Fig. 5 (b)), leading to a linear increasing trend of NH3 emissions (Fig. 5 (a)).

Figure 5 (c) shows that the number of vehicles in China has been increasing exponentially, which has been used to elucidate the rapid enhancement of NOx emissions. However, auto vehicles are also significant sources of NH3. NH3 is generated as a byproduct when reducing NOx to N2 in the vehicle catalytic converters. Field studies in the US suggested that old vehicles (made before 1990s) have little NH3 emissions as they usually do not have effective catalytic converters, and that new vehicles show low NH3 emissions due to more strict emission standard [8]. However, on-road NH3 emissions from vehicles in China have been rarely investigated. Presumably these emissions could have been underestimated, as there is a wide spectrum of on-road and off-road vehicles in China, and they are often old models.

We were educated that China used 7% of the land to feed 22% of the world population. However, this is associated with intense fertilization and other agricultural activities, and concentrated/rapid growing industrial sectors. Pollutants like PM2.5, SO2 and NOx have drawn much attention, and active mitigations are being planned and applied. Nevertheless, NH3 emissions should also be emphasized, and it could be more beneficial to control NH3 emissions.

Figure 5. Trends in NH3 and NOx emissions and their main contributors between 1980 and 2010. a, NH3 and NOx emissions and ratios of NH3-N to NOx-N emission; b, number of domestic animals (expressed as livestock units) and N fertilizer consumption; c, number of motor vehicles and coal consumption.

Reference

    [1] http://www.epa.gov/airquality/particlepollution/designations/2006standards/documents/map2011.htm
    [2] http://www.fondriest.com/news/mobile-air-sensor-lab-takes-a-california-road-trip.htm
    [3] http://earthobservatory.nasa.gov/IOTD/view.php?id=80152
    [4] Zhang, Y., et al. "Agricultural ammonia emissions inventory and spatial distribution in the North China Plain." Environmental Pollution 158.2 (2010): 490-501.
    [5] http://www.scientificamerican.com/article.cfm?id=massive-nitrogen-pollution-accompanies-chinas-growth
    [6] Tian, Hanqin, et al. "Food benefit and climate warming potential of nitrogen fertilizer uses in China." Environmental Research Letters 7.4 (2012): 044020.
    [7] Liu, Xuejun, et al. "Enhanced nitrogen deposition over China." Nature (2013).
    [8] Bishop, Gary A., et al. "On-road emission measurements of reactive nitrogen compounds from three California cities." Environmental science & technology 44.9 (2010): 3616-3620.

 

 

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