Rapid SO<sub>2</sub> emission reductions significantly increase tropospheric ammonia concentrations over the North China Plain

Abstract

Abstract. The North China Plain has been identified as a significant hotspot of ammonia (NH3) due to extensive agricultural activities. Satellite observations suggest a significant increase of about 30 % in tropospheric gas-phase NH3 concentrations in this area during 2008–2016. However, the estimated NH3 emissions decreased slightly by 7 % because of changes in Chinese agricultural practices, i.e., the transition in fertilizer types from ammonium carbonate fertilizer to urea, and in the livestock rearing system from free-range to intensive farming. We note that the emissions of sulfur dioxide (SO2) have rapidly declined by about 60 % over the recent few years. By integrating measurements from ground and satellite, a long-term anthropogenic NH3 emission inventory, and chemical transport model simulations, we find that this large SO2 emission reduction is responsible for the NH3 increase over the North China Plain. The simulations for the period 2008–2016 demonstrate that the annual average sulfate concentrations decreased by about 50 %, which significantly weakens the formation of ammonium sulfate and increases the average proportions of gas-phase NH3 within the total NH3 column concentrations from 26 % (2008) to 37 % (2016). By fixing SO2 emissions of 2008 in those multi-year simulations, the increasing trend of the tropospheric NH3 concentrations is not observed. Both the decreases in sulfate and increases in NH3 concentrations show highest values in summer, possibly because the formation of sulfate aerosols is more sensitive to SO2 emission reductions in summer than in other seasons. Besides, the changes in NOx emissions and meteorological conditions both decreased the NH3 column concentrations by about 3 % in the study period. Our simulations suggest that the moderate reduction in NOx emissions (16 %) favors the formation of particulate nitrate by elevating ozone concentrations in the lower troposphere.

Trends of Precipitation Acidification and Determining Factors in China During 2006–2015

“…The concentrations of modeled ammonium ion experienced a mean decrease of 12.7 μeq/L over northern China during 2006–2015, which were the combined results from the increased gaseous NH 3 wet scavenging (+11.4 μeq/L), decreased particulate ammonium wet scavenging (−22.4 μeq/L), and an equilibrium shift (from NH 4 + to NH 3 H 2 O) due to the decreased precipitation acidity. Interestingly, because of the reduced formation of sulfate aerosols by SO 2 emission control, the phase equilibrium of ammonia shifted from particle (ammonium sulfate) to gas phase (i.e., free ammonia in the atmosphere) (also shown in our recent study, Liu et al, 2018), which would further weaken precipitation acidification due to the increased scavenging of gaseous NH 3 levels in our simulations. On the other hand, when comparing the 2006 and 2015_06S cases (using the same SO 2 emissions with 2006), we find that the ammonium ion concentrations decreased on average by about 40 μeq/L in northern China, contributed by both the reductions in wet scavenged particle and gas ammonia.…”

“…It is known that nitrate formation is determined by the non-linear NO x -VOCs (Volatile Organic Compounds)-O 3 chemistry and mainly via NO 2 + OH and NO 3 + NO 2 pathways (Seinfeld & Pandis, 2016). For instance, Liu et al (2018) suggest that the moderate reduction in NO x emissions (~20%) in China during recent years might favor the formation of particulate nitrate by elevating ozone concentrations in the lower troposphere. On the other hand, the improvement in the meteorological conditions in parts of China between 2013 and 2017 was conducive to PM 2.5 reductions (Zhang, Xu, et al, 2019) and may also decrease particulate nitrate (Zhang, Zheng, et al, 2019).…”

Paradigm shift in aerosol chemical composition over regions downwind of China

“…The change in NH 3 concentration between CNTL and S04N08 is of great interest, and this result is shown in Supplementary Figure S5. The NH 3 concentration in the S04N08 experiment was more than double that over CEC (i.e., increase in free NH 3 ) 28 , and NH 3 concentration increases were simulated over western Japan, including Fukue Island. The changes in NH 3 concentration over CEC were also supported by Infrared Atmospheric Sounding Interferometer (IASI) satellite observations 29 .…”

Role of Ammonia on the Feedback Between AWC and Inorganic Aerosol Formation During Heavy Pollution in the North China Plain

“…Fu et al (2017) also found an obvious increase of NH 3 vertical column density in recent years (from 2011 to 2014) over magecity clusters of the North China Plain (NCP), the Yangtze River Delta, and the Pearl River Delta. The increase in atmospheric NH 3 is believed to the benefits of the reduction in SO 2 and NO x emissions (Lachatre et al, 2019;Liu et al, 2018), especially in East China (Fu et al, 2017). Several studies revealed the importance of NH 3 to the formation of secondary inorganic aerosols (SIAs; Cao et al, 2009;Meng et al, 2018) and organic aerosols as well (Bin Babar et al, 2017;Na et al, 2007).…”