Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.3.5.2 Ammonium (NH <sub>4</sub> <sup>+</sup> ) and Nitrate Aerosols (NO <sub>3</sub> <sup>–</sup> ) ====

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Ammonium sulphate and ammonium nitrate aerosols are formed when NH <sub>3</sub> reacts with nitric acid (HNO <sub>3</sub> ) and sulphuric acid (H <sub>2</sub> SO <sub>4</sub> ), produced in the atmosphere by the oxidation of NO <sub>x</sub> and SO <sub>2</sub> respectively. Ammonium nitrate is formed only after H <sub>2</sub> SO <sub>4</sub> is fully neutralized. NH <sub>4</sub> <sup>+</sup> and NO <sub>3</sub> <sup>–</sup> aerosols produced via these gas-to-particle reactions are a major fraction of fine-mode particles (with diameter &lt;1µm) affecting air quality and climate. Coarse-mode nitrate, formed by the heterogeneous reaction of nitric acid with dust and sea salt, dominates the overall global nitrate burden, but has little radiative effect ( [[#Hauglustaine--2014|Hauglustaine et al., 2014]] ; [[#Bian--2017|Bian et al., 2017]] ). Trends in ammonium (NH <sub>4</sub> <sup>+</sup> ) and nitrate (NO <sub>3</sub> <sup>–</sup> ) were not assessed in AR5.

Global model present-day estimates of the global NH <sub>4</sub> <sup>+</sup> burden range from 0.1–0.6 TgN ( [[#Bian--2017|Bian et al., 2017]] ). Models generally simulate surface NH <sub>4</sub> <sup>+</sup> concentrations better than surface NH <sub>3</sub> concentrations ( [[#Bian--2017|Bian et al., 2017]] ), which reflects its thermodynamic control by SO <sub>4</sub> <sup>2–</sup> rather than NH <sub>3</sub> ( [[#Shi--2017|Shi et al., 2017]] ). The concomitant increases of NH <sub>3</sub> , SO <sub>2</sub> , and NO <sub>x</sub> emissions (see Section 6.2) have led to a factor of three to nine increase in the simulated NH <sub>4</sub> <sup>+</sup> burden from 1850–2000 ( [[#Hauglustaine--2014|Hauglustaine et al., 2014]] ; [[#Lund--2018a|Lund et al., 2018a]] ), driven primarily by ammonium sulphate (70–90%). The increases in the NH <sub>3</sub> and NH <sub>4</sub> <sup>+</sup> burdens are indirectly supported by the observed increase of NH <sub>4</sub> <sup>+</sup> concentration in ice cores in mid- to high latitudes (Kang et al. , 2002; Kekonen et al. , 2005; Lamarque et al. , 2013; Iizuka et al. , 2018) .

Ammonium nitrate is semi-volatile, which results in complex spatial and temporal patterns in its concentrations ( [[#Putaud--2010|Putaud et al., 2010]] ; [[#Hand--2012|Hand et al., 2012]] a; H. [[#Zhang--2012|]] [[#Zhang--2012|Zhang et al., 2012]] ), reflecting variations in its precursors, NH <sub>3</sub> and HNO <sub>3</sub> , as well as SO <sub>4</sub> <sup>2–</sup> , non-volatile cations, temperature and relative humidity (Nenes et al. , 2020) . High relative humidity and low temperature as well as elevated fine particulate matter loading (Huang et al. , 2014; Petit et al. , 2015; H. Li et al. , 2016; Sandrini et al. , 2016) favour nitrate production. Measurements reveal the high contribution of NO <sub>3</sub> <sup>–</sup> to surface PM <sub>2.5</sub> (&gt;30%) in regions with elevated regional NO <sub>x</sub> and NH <sub>3</sub> emissions, such as the Paris area ( [[#Beekmann--2015|Beekmann et al., 2015]] ; [[#Zhang--2019|Zhang et al., 2019]] ), northern Italy ( [[#Masiol--2015|Masiol et al., 2015]] ; [[#Ricciardelli--2017|Ricciardelli et al., 2017]] ), Salt Lake City ( [[#Kuprov--2014|Kuprov et al., 2014]] ; [[#Franchin--2018|Franchin et al., 2018]] ), the North China Plains ( [[#Guo--2014|Guo et al., 2014]] ; [[#Chen--2016|Chen et al., 2016]] ) and New Delhi ( [[#Pant--2015|Pant et al., 2015]] ). Recent observations also show that ammonium nitrate contributes to the Asian tropopause aerosol layer ( [[#Vernier--2018|Vernier et al., 2018]] ; [[#Höpfner--2019|Höpfner et al., 2019]] ). Model diversity in simulating the present-day global fine-mode NO <sub>3</sub> <sup>–</sup> burden is large with two multi-model intercomparison studies reporting estimates in the range of 0.14–1.88 Tg and 0.08–0.93 Tg respectively ( [[#Bian--2017|Bian et al., 2017]] ; [[#Gliß--2021|Gliß et al., 2021]] ). Models differ in their estimates of the global tropospheric nitrate burden by up to a factor of 13 with differences remaining nearly the same across the CMIP5 and CMIP6 generation of models ( [[#Bian--2017|Bian et al., 2017]] ; [[#Gliß--2021|Gliß et al., 2021]] ). While regional patterns in the concentration of fine-mode NO <sub>3</sub> <sup>–</sup> are qualitatively captured by models, the simulation of fine-mode NO <sub>3</sub> <sup>–</sup> is generally worse than that of NH <sub>4</sub> <sup>+</sup> or SO <sub>4</sub> <sup>2–</sup> ( [[#Bian--2017|Bian et al., 2017]] ). This can be partly attributed to the semi-volatile nature of ammonium nitrate and biases in the simulation of its precursors ( [[#Heald--2014|Heald et al., 2014]] ; [[#Paulot--2016|Paulot et al., 2016]] ), including the sub-grid scale heterogeneity in NO <sub>x</sub> and NH <sub>3</sub> emissions ( [[#Zakoura--2018|Zakoura and Pandis, 2018]] ).

Models indicate that the burden of fine-mode NO <sub>3</sub> <sup>–</sup> has increased by a factor of two to five from 1850–2000 ( [[#Xu--2012|Xu and Penner, 2012]] ; [[#Hauglustaine--2014|Hauglustaine et al., 2014]] ; [[#Lund--2018a|Lund et al., 2018a]] ), an increase that has accelerated between 2001 and 2015 ( [[#Lund--2018a|Lund et al., 2018a]] ; [[#Paulot--2018b|Paulot et al., 2018b]] ). The sensitivity of NO <sub>3</sub> <sup>–</sup> to changes in NH <sub>3</sub> , SO <sub>4</sub> <sup>2–</sup> , and HNO <sub>3</sub> is determined primarily by aerosol pH, temperature, and aerosol liquid water ( [[#Guo--2016|Guo et al., 2016]] , 2018; [[#Weber--2016|Weber et al., 2016]] ; [[#Nenes--2020|Nenes et al., 2020]] ). In regions where aerosol pH is high, changes in NO <sub>3</sub> <sup>–</sup> follow changes in NO <sub>x</sub> emissions, consistent with the observed increase of ammonium nitrate in northern China from 2000–2015 ( [[#Wen--2018|Wen et al., 2018]] ) and its decrease in the US Central Valley ( [[#Pusede--2016|Pusede et al., 2016]] ). In contrast, the decrease in SO <sub>2</sub> emissions in the south-east USA has caused little change in NO <sub>3</sub> <sup>–</sup> <sub></sub> from 1998–2014 as nitric acid largely remains in the gas phase due to highly acidic aerosols ( [[#Weber--2016|Weber et al., 2016]] ; [[#Guo--2018|Guo et al., 2018]] ).

In summary, there is ''high confidence'' that the NH <sub>4</sub> <sup>+</sup> and NO <sub>3</sub> <sup>–</sup> burdens have increased from the pre-industrial period to the present day, although the magnitude of the increase is uncertain especially for NO <sub>3</sub> <sup>–</sup> . The sensitivity of NH <sub>4</sub> <sup>+</sup> and NO <sub>3</sub> <sup>–</sup> to changes in NH <sub>3</sub> , H <sub>2</sub> SO <sub>4</sub> and HNO <sub>3</sub> is well understood theoretically. However, it remains challenging to represent in models, in part because of uncertainties in the simulation of aerosol pH, and only a minority of ESMs consider nitrate aerosols in CMIP6.

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