Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural United States

Secondary inorganic aerosols play an important role in air pollution and climate change, and their formation modulates the atmospheric deposition of reactive nitrogen (including oxidized and reduced nitrogen), thus impacting the nitrogen cycle. Large-scale and long-term analyses of secondary inorganic aerosol formation based on model simulations have substantial uncertainties. Here we improve constraints on secondary inorganic aerosol formation using decade-long in situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the United States. We reveal a shift in the secondary inorganic aerosol formation regime in the rural United States between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of reactive nitrogen. This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, with ecosystem impacts warranting further investigation. Ammonia (NH3), a critical but not directly regulated precursor of fine particulate matter in the United States, has been increasingly scrutinized to improve air quality. Our findings, however, show that controlling NH3 became significantly less effective for mitigating fine particulate matter in the rural United States. We highlight the need for more collocated aerosol and precursor observations for better characterization of secondary inorganic aerosols formation in urban areas.

Secondary inorganic aerosols play an important role in air pollution and climate change, and their formation modulates the atmospheric deposition of reactive nitrogen (including oxidized and reduced nitrogen), thus impacting the nitrogen cycle.Large-scale and long-term analyses of secondary inorganic aerosol formation based on model simulations have substantial uncertainties.Here we improve constraints on secondary inorganic aerosol formation using decade-long in situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the United States.We reveal a shift in the secondary inorganic aerosol formation regime in the rural United States between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of reactive nitrogen.This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, with ecosystem impacts warranting further investigation.Ammonia (NH 3 ), a critical but not directly regulated precursor of fine particulate matter in the United States, has been increasingly scrutinized to improve air quality.Our findings, however, show that controlling NH 3 became significantly less effective for mitigating fine particulate matter in the rural United States.We highlight the need for more collocated aerosol and precursor observations for better characterization of secondary inorganic aerosols formation in urban areas.
Secondary inorganic aerosols (SIAs) are major components of fine particulate matter (PM 2.5 ), which has detrimental impacts on human health and regional visibility and substantially influences the radiative balance of the climate system [1][2][3][4] .SIAs are formed predominantly through the oxidation of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ), and subsequent reaction with ammonia (NH 3 ) 5 .These processes determine the physical and chemical properties of aerosols, including aerosol acidity, aerosol water uptake and growth, and potentially aerosol toxicity.SIA formation also influences the gas-particle partitioning of semivolatile inorganic reactive nitrogen (N r ) species, such as NH 3 , ammonium (NH 4 + ), nitric acid (HNO 3 ) and nitrate (NO 3 − ) 5 .Because gaseous NH 3 and HNO 3 species deposit much more quickly than N r compounds in PM 2.5 (refs.6,7), their phase partitioning modulates the spatial distribution of N r atmospheric deposition, which influences human exposure to PM 2.5 (and the associated health impacts), loss of biological diversity, soil and water acidification, and surface water eutrophication [8][9][10][11] .Therefore, a https://doi.org/10.1038/s41561-024-01455-9tration, in units of μg per m 3 of air) from −28 to 11% to −6 to 8% (Supplementary Table 3).The NMBs between CTM simulations and observations are much larger (−65 to 126%) because the built-in aerosol thermodynamic model is driven by inputs determined by emission, oxidation, transport and deposition processes [13][14][15][16][17][18][19][20]  )) to primary emissions, the large errors in CTMs could alter the SIA formation regime, and observations are needed to constrain these processes.Here, we first investigate regional precursor concentration responses to emission reductions by examining the relationship between precursor concentrations and their emissions.Then, with the improved constraints on SIA formation, we can better quantify the impacts of rapidly changing atmospheric composition on N r deposition, SIA properties and SIA sensitivities to precursor reductions.
to e SO and e NO x reductions remained largely unchanged between and 2021 29 , and this period witnessed 90% and 65% reductions in e SO and e NO x , respectively 28 .However, the responses could change if SO and NO x emission reductions continue (Supplementary Text 1).In contrast, e NH 3 has not been directly regulated and remained approximately unchanged.c NH T 4 and e NH 3 are inversely correlated in the Southeastern United States and show no clear correlation in other regions (Extended Data Fig. 3).Regional Kendall tests show that these trends remain consistent with or without the sites established after 2015 (Supplementary Fig. 3 and Supplementary Table 4) 30 .More trend analyses and regression results are presented in Supplementary Figs.4- Influencing aerosol thermodynamic properties, aerosol acidity is a key indicator of potential changes in gas-particle partitioning and SIA formation caused by changes in aerosol composition 31 .Aerosol pH is difficult to measure directly, and is often estimated using aerosol thermodynamic simulations because of the challenges associated with collecting unperturbed samples 31 .Between 2011 and 2020, our simulations show that the annual mean aerosol pH increased by 0.2-0.6 units across the rural United States (Fig. 2a-e).The major contributor to the pH increase was a reduction in c SO 2− 4 (Extended Data Fig. 4) in all regions, and decreases in c NH T 4 ameliorated the extent of the pH increases in the Midwestern, Northeastern and Southeastern United States.Aerosol pH was primarily buffered by NH 3 in the Western, Central and Midwestern United States (Extended Data Fig. 5).Zheng and colleagues 32 have shown that this buffering regime suppresses the better understanding of SIA formation can facilitate policy-making in relation to many environmental challenges.
Aerosol thermodynamic analyses using measured gas concentrations and particle composition provide better constraints on SIA formation and the partitioning of semivolatile species than simulations with chemical transport models (CTMs) 12 .Compared to observations, regional and global CTM simulations vary substantially in terms of the simulated aerosol composition and phase partitioning of N r species in the United States [13][14][15][16] (Extended Data Table 1).This variability could result from uncertainties in emission inventories, transport, dry deposition, wet scavenging and/or heterogeneous chemical production 13,14,[17][18][19][20] .Directly modelling SIA formation with simultaneous measurements of gas concentrations and aerosol composition (that is, concentrations of NH 3 , HNO 3 , NH 4 , non-volatile cations (NVCs, including sodium, calcium, magnesium and potassium ions) and chloride ion (Cl − )) avoids the aforementioned uncertainties 12,21 .However, this is only available at a few sites or from a few intensive field campaigns with limited spatiotemporal coverage in the United States 12,22,23 .Moreover, past measurements are unlikely to reflect the current atmospheric composition due to rapid changes in the emissions of various precursors, impacts on gas-particle partitioning from climate change, and increases in the size and number of wildfires.
In this Article we overcome the above limitations of existing datasets and a lack of constraint on simulated SIA formation by using observations from multiple long-term air-quality-monitoring networks for aerosol thermodynamic analyses.Our results show that chemical regimes of SIA formation in the rural United States shifted from NH 3 -sensitive to NH 3 -insensitive between 2011 and 2020 and led to increases in N r deposition near NH 3 -emission hotspots.Although we focus on the rural United States because of the available observations, we demonstrate the benefits of collocated monitoring for aerosol composition and precursor concentrations, which should be considered for future monitoring network design in the United States and globally.

Improving constraints on SIA formation
We identified locations where sites from the monitoring networks provide essential inputs to SIA formation simulations and are located within a spatial window of 50 km (Methods).Several national networks monitor trace-gas precursors and aerosol chemical composition, but observations from an individual network are insufficient for thermodynamic modelling.Integrating collocated observations provides the inputs needed as biweekly means (averaged every 2 weeks).There were 42 and 68 locations that had collocated observations for the periods of 2011-2015 and 2016-2020, respectively (Extended Data Fig. 1 and Supplementary Tables 1 and 2).Although these areas are located outside urban centres, many of them are still in the vicinity of high-population areas, especially in the Midwestern and Northeastern United States.The areas within 50 km of the locations account for 6.7% of the land surface areas, but 9.8%, 7.0%, 8.7% and 7.5% of the population, SO 2 emissions, NO x emissions and NH 3 emissions in the contiguous United States, respectively 24,25 .Moreover, because the aerosol composition and precursors observed at sites 50-100 km apart still show good agreement (Supplementary Fig. 1), our findings may apply to rural and suburban regions outside major urban centres more generally.
Using the ISORROPIA-II model 26 (a full thermodynamic model for inorganic aerosol formation) with the integrated dataset described above, we substaintially reduce uncertainties in simulating SIA formation (Extended Data Fig. 2).Although ISORROPIA-II and other aerosol thermodynamic models have been validated with hourly or daily observations 12,27 , they have not been validated with biweekly observations made with different sampling methods.We conducted sensitivity tests and uncertainty analyses to develop the necessary preprocessing steps to integrate collocated observations (Methods and Supplementary Table 3), reducing the normalized mean biases (NMBs) between simulated and observed c NH 3 , c NH + Article https://doi.org/10.1038/s41561-024-01455-9influence of compositional differences on aerosol pH and makes aerosol water content (AWC) and temperature (T) the primary determinants of aerosol pH, leading to larger seasonal variations in aerosol pH in those three regions.The changes in aerosol acidity and its seasonal variations could have implications for aerosol toxicity and the oxidation rates of SO 2 and NO x , which requires further investigation.For example, the effectiveness of controlling SO 2 emissions on reducing c SO 2− 4 could decrease due to enhanced SO 2 oxidation as aerosol pH increases 17,31,33 .

Regime changes in SIA formation and N r deposition
Increases in aerosol pH led to decreases of −2 to −4% per year in the molar fraction of NH 4 + in NH 4 T (ε NH +

4
) in all regions (Fig. 2f-j provides a time series and Supplementary Table 4 shows the trends), implying that more NH 4 T remained as NH 3 in the atmosphere in 2020 than in 2011.Thus, a greater fraction of NH 4 T could deposit near emission sources as NH 3 , because gas-phase NH 3 deposits more rapidly than PM 2.5 (ref.By dividing the contiguous United States into four zones according to their distances to the nearest NH 3 -emission hotspot (<50 km, 50-150 km, 150-300 km and >300 km), we analysed the trend of annual N r total deposition from the 'Total Deposition Estimates Using the Measurement Model Fusion' (TDep MMF) 34 model between 2010 and 2019 (Fig. 3a and Supplementary Text 2).N r total deposition showed statistically significant increasing trends in areas within 150 km of an NH 3 -emission hotspot (Fig. 3b) and insignificant trends at >150 km from these hotspots, despite reductions in NO 3 T deposition (Fig. 3c).NH 4  T deposition increased more quickly than NH 3 emissions in the corresponding zones (Fig. 3d).These results are indicative of increased NH 4 T near the source and probably the results of decreased ε NH + 4 and higher dry deposition rates of NH 3 relative to NH 4 + .There are large discrepancies between the hotspots defined by NH 3 emissions and those identified by satellite observations [35][36][37] (Extended Data Fig. 6 and Supplementary Figs. 7 and 8), highlighting the need for more NH 3 observations.
As the aerosol composition changed and NH 4 T partitioned less into aerosols, the SIA formation regime became less sensitive to c NH T 4 in the rural United States (Fig. 2k-o).Although NH 4 NO 3 -containing SIA always responds to c NH T 4 changes to some degree, a boundary is needed to distinguish NH 3 -sensitive and NH 3 -insensitive regimes to facilitate decision-making for air-quality and nitrogen-deposition purposes.Here, we define the boundary using both comparative and aerosol property-based approaches.In the comparative approach, we simulate c SIA changes (Δc SIA ) caused by 10%, 40% and 70% reductions in each precursor (Δc p , p = NH .Figure 2k-o shows Δc SIA /Δc p with 40% reduction in each precursor, and Extended Data Fig. 7 shows Δc SIA /Δc p with 10% and 70% reductions.With a 40% reduction, annual Δc SIA /Δc NH T 4 decreased by 2-5% per year in all regions between 2011 and 2020 (Supplementary Table 4).As a result, by 2020, SIA formation became NH 3 -insensitive in all regions except the Northeastern United States using the comparative approach.Seasonally, SIA formation was still NH 3 -sensitive in 2020 in the winter in the Midwestern, Northeastern and Southeastern United States (Extended Data Fig. 7).We found a similar regime shift trend using the aerosol property-based approach developed by Nenes and colleagues 38 (Supplementary Text 3 and Extended Data Fig. 8) 38 .
The rapid decrease in Δc SIA /Δc NH T highlights the importance of the SIA formation regime change between 2011 and 2020 and indicates that NH 3 controls will be less effective for PM 2.5 reduction in 2020 than in 2011.

Air quality and N r deposition implications
Past studies have identified NH 3 controls as potentially effective PM 2.5 mitigation measures in the United States, because emissions have not been directly controlled and the marginal cost for low-level reductions from agricultural sources is relatively low 39,40 .Gu and colleagues 39 argued that the US abatement cost of NH 3 emissions is one-tenth the cost of NO x controls, while bringing similar welfare benefits in preventing mortality by reducing PM 2.5 levels 39 .More broadly, of 17 studies (from 2007 to 2021) that compared the effectiveness of SO 2 , NO x and NH 3 emission controls in the United States, eight found that controlling NH 3 emissions is the most effective way to reduce PM 2.5 concentrations [39][40][41] (Supplementary Table 5 provides a full list of the studies reviewed).Because of these studies and legal action by environmental organizations, in 2016 the US Environmental Protection Agency (EPA) asked state and regional air-quality regulators to evaluate potential control measures for NH 3 when designing State Implementation Plans (SIPs) for PM 2.5 National Ambient Air Quality Standards (NAAQS) 42 .Despite the updated requirements, most relevant regulatory agencies found additional NH 3 -emission controls unnecessary, and only one PM 2.5 NAAQS nonattainment area (Imperial County, California) included a new rule to control NH 3 emissions 43 .For the Regional Haze Rule, which aims to restore visibility in national parks and wilderness areas in the United States, the US EPA recommends that states ignore NH 3 in their SIPs 44 .
Our results show that the United States has missed an opportunity to more efficiently improve air quality in rural regions by controlling NH 3 emissions, especially from agricultural sources, as SIA formation transitioned from more NH in winter, when c SIA loadings are high, was still an effective complementary measure to SO 2 -and NO x -emission controls for PM 2.5 reductions in 2020 in the rural Midwestern, Northeastern and Southeastern United States (Extended Data Fig. 7 and 8).However, wintertime NH 3 emissions were low in these regions, especially from agricultural sources (Supplementary Table 6), and NH 3 -emission reductions from vehicular and industrial sources might be needed to achieve the required reductions.Recent studies have shown that NH 3 emissions from mobile and industrial sources are significantly underestimated 45 .Finally, the shift towards an NH 4 T -insensitive regime and the lack of incentive for NH 3 controls for air-quality purposes in the rural United States (for example, the Regional Haze Rule) are likely to continue in rural areas as climate policies increase renewable power generation and electrify transportation.SO 2 and NO x emissions from fuel combustion are expected to decrease further 46 .
More importantly, our analyses also show that the inorganic N r deposition regime shifted due to SO 2 -and NO x -emission reductions.As NO x emissions decreased, reduced-form N r deposition became the dominant component of N r deposition and a major concern in many sensitive ecosystems 47 .Our results further illustrate that deposition patterns could change as more gaseous NH 3 deposits near sources  rather than being converted into SIAs and being transported away, shortening the effective atmospheric lifetime of reduced forms of N r .
On the one hand, NH 3 mitigation will be needed to protect sensitive ecosystems and reduce coastal eutrophication caused by increased N r deposition near hotspots.Pan and colleagues found that 26 national parks in the United States are within 200 km of an NH 3 hotspot (identified by satellite observations) 35 .On the other hand, increased N r deposition, together with CO 2 fertilization, has enhanced terrestrial carbon uptake, and it is unclear how the terrestrial ecosystems will respond to N r deposition pattern and composition changes 48 .More flux and ecological observations are needed to investigate the multifaceted impacts of increasingly inhomogeneous N r deposition.
Our method can be applied to routine monitoring for faster environmental policy evaluation and provides a rationale for new integrated monitoring networks in urban areas and regions impacted by enhanced wildfire and dust emissions.The integrated data and thermodynamic analysis with uncertainty estimates can also be used to improve CTMs.Our conclusions are limited to the rural United States, and urban conditions might be different.However, the approach demonstrated in this work can be used to characterize the SIA response to precursor reductions in urban regions in the United States if simultaneous observations of gaseous NH 3 and HNO 3 , aerosol composition and meteorological conditions become available.As wildfires increase and US EPA lowers the current NAAQS for PM 2.5 to 9 μg m −3 (ref.49), the impacts on SIA formation of NVCs from dust and organic compounds from wildfires will probably become important for air-quality management in rural regions and warrants further investigation.For example, OAs are not considered in the inorganic aerosol model used in this study.Although organic acids could influence SIA formation, we do not find significant impacts of OAs on model performance, except for wildfire episodes with extremely high c OA (Extended Data Fig. 9).During those events, the model underestimates both ε NH + 4 and ε NO − 3 , which needs more examination with speciated OA observations.Finally, the benefits of collocated monitoring for aerosol composition and precursor concentrations demonstrated here should be considered in countries developing their own aerosol-monitoring networks.

Online content
Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41561-024-01455-9.with different spatial windows.However, T and RH from CASTNET and ISD significantly differ when a spatial window of 100 km is used.Therefore, a spatial window of 50 km was selected for observation integration.With this spatial window, we found 68 AMoN sites with at least CASTNET and ISD sites located within 50 km.Combining observations from these three networks provided all the inputs needed for aerosol thermodynamic modelling.All observations were averaged biweekly to match the start and end dates of AMoN observations, as it has the lowest sampling frequency.
Sites with integrated observations are shown in Fig. 1a.The black and red crosses in Fig. 1a are sites established before and after 2015, respectively.The sites are grouped according to the five US Regional Planning Organizations (RPOs): the Western Regional Air Partnership (WRAP), the Central States Air Resource Agencies (CENSARA), the Lake Michigan Air Directors Consortium (LADCO), the Mid-Atlantic/Northeast Visibility Union (MANE-VU) and the Southeastern Air Pollution Control Agencies (SESARM).These RPOs help state and county agencies develop regional strategies to achieve their air-quality goals.Here, these RPOs are referred to as the Western (WRAP), Central (CENSARA), Midwestern (LADCO), Northeastern (MANE-VU) and Southeastern (SESARM) United States, respectively.Observations from each site are shown in Supplementary Figs.13-17.Annual numbers of biweekly observations are listed in Supplementary Table 2.Only sites with more than 70% seasonal coverage since establishment are included in the following analyses.Excluding the sites established after 2015 does not change our trend analyses (Extended Data Fig. 3, Supplementary Fig. 3 and Supplementary Table 4) and therefore the simulation results or conclusions.The regional Mann-Kendall test was used to derive consistent regional trends 30 , and only statistically significant trends (P < 0.05) are reported (Supplementary Table 4).
Although the sites are considered rural, they are generally representative of regional population density and emissions, especially in the Midwestern and Northeastern United States (Supplementary Table 2).Sites in the Western and Central United States are slightly more remote, with lower-than-average population densities and SO 2 and NO x emissions.Although some AMoN sites have been reported to be impacted by nearby agricultural emission sources 37,59 , they are not collocated with CASTNET sites.About 50% of SO 2 emissions in the United States came from power plants and were mostly located in rural regions in 2017 25 .Highway vehicle emissions accounted for one-third of NO x emissions in 2017, which were spread across the United States.In 2017, 10% and 5% of NO x emissions were related to power generation and oil and gas production outside urban areas 25 .Therefore, the majority of the rural sites discussed in this study are representative of regional conditions.

Aerosol thermodynamic modelling
We use ISORROPIA-II, a full thermodynamic model for inorganic aerosol formation, to simulate the aerosol properties and sensitivities of SIA formation to precursors.
, T and RH from the integrated dataset are used as inputs to ISORROPIA-II.The model is run in the 'forward mode' to simulate gas-particle partitionings of NH 4 T and NO 3 T .Although ISORROPIA-II has been validated with observations from intensive field campaigns, using it with biweekly averaged observations from monitoring networks has not been tested before and requires careful evaluation.We conducted nine case studies to investigate the impacts of measurement biases and low temporal resolutions (Supplementary ) in an aerosol multiphase buffer system is where μ H + and μ OH − are the molar masses of H + and OH − , c i is the total concentration of the buffering agent in μmol per m 3  ) , for volatile acid HA (5)   where K a, BOH and K a, HA are the liquid-phase acid dissociation constant for BOH and HA expressed in molality 32 , H i is the Henry's law constant for BOH or HA in molality (mol kg −1 atm −1 ) 32 , and the contributions of HSO 4 − /SO 4 2− , HNO 3 /NO 3 − and NH 4 + /NH 3 acid-base conjugate pairs to the total buffering capacity can be expressed as

Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this Article.

Randomization
We grouped our observations into five regions based on the boundaries of Regional Planning Organizations.Randomization is irrelevant to this grouping because its purpose is to illustrate regional differences.

Blinding
Blinding was irrelevant to this study because the unique properties of each sample (i.e., observations from certain site at a given time) is critical to our analysis.

Did the study involve field work?
Yes No Reporting for specific materials, systems and methods We require information from authors about some types of materials, experimental systems and methods used in many studies.Here, indicate whether each material, system or method listed is relevant to your study.If you are not sure if a list item applies to your research, read the appropriate section before selecting a response.

6 and Supplementary Text 1 . 4 − 3 and c SO 2
The inverse correlations and less clear c NH T e NH 3 relationship reflect large uncertainties in NH 3 emissions and/or increased NH 4 T removal associated with c NO T changes in e NH 3 .
7).The decrease in the atmospheric lifetime of NH 4 T could reduce the NH 4 T transported from NH 3 sources in the Western, Central and Midwestern United States to the Northeastern and Southeastern United States, explaining the decreasing trends of c NH T 4 in the Northeastern and Southeastern United States without significant e NH 3 changes.

4 Fig. 1 | 2 and c SO 2− 4 ( 3 (
Fig. 1 | Site locations and relationships between emissions of SO 2 , NO x and NH 3 and concentrations of SO 4 2− , NO 3 T and NH 4 T .a, Site map.Black and red crosses represent measurement sites established before and after 2015, respectively, in the five regions indicated by specific colours.Corresponding site numbers are listed in the legends.The base map was obtained from Natural Earth.The five regions are defined according to the Regional Planning Organizations (Methods).The numbers of samples for these regions for each year are listed in Supplementary Table 2. b-d, Annual SO 2 (b), NO x (c) and NH 3 (d) emissions (e SO 2 , e NO x and e NH 3 ) in the five regions.e-g, Annual mean concentrations of SO 4 2− (e), NO 3 T (f) and NH 4 T (g).h-j, Orthogonal distance regressions of annual mean e SO 2 and c SO 2− 4 (h), e NO x and c NO T 3 (i) and e NH 3 and c NH T

4 T- 4 ( 40 -
sensitive to less NH 4 T -sensitive between 2011 and 2020.In the early 2010s, reducing c NH T 4 could bring significant reductions in c SIA in all regions except the Western United States.In 2020, however, deep c NH T 70%) reductions would be needed to achieve reductions in annual c SIA similar to those resulting from 10-40% reductions in c SO 2− 4 and c NO T 3 in all regions except the Northeastern United States.Reducing c NH T 4

Fig. 3 |
Fig. 3 | Spatial distribution and trends of total reactive nitrogen and NH 4 T deposition.a, The average annual total reactive nitrogen (N r ) deposition (dep) in the United States between 2010 and 2019.Solid, dashed and dotted lines show the boundaries of the areas within 50 km, 150 km and 300 km of an NH 3 -emission hotspot (Supplementary Text 2).The base map was obtained from Natural Earth.b-d, The 2010-2019 trends of annual total N r deposition (b), NO 3 T deposition (c) and NH 4 T deposition normalized by NH 3 emission (emis) (d) trends relative to the

Extended Data Fig. 5 |Extended Data Fig. 6 |Extended Data Fig. 8 | 4 T. 4 = 0 .
Regional means of temperature, aerosol composition, and pH buffering capacity composition.Regional means of temperature, aerosol composition (calculated using ion-equivalent concentrations to reflect aerosol charge balance), and pH buffering capacity composition from 2011 to 2020.Blue, green, and orange areas in panels (f-o) show contributions (cntr) of SO composition or aerosol pH buffering capacity.Grey and purple areas show contributions of non-volatile cations (NVC) and chloride ion (Cl − ) to aerosol composition.Differences in ammonia emission hotspots and satellite observed ammonia hotspots.Differences in NH 3 hotspots defined based on the 2017 emission inventory and satellite observations.(a) NH 3 emissions and (b) NH 3 column densities from the Infrared Atmospheric Sounding Interferometer (IASI).IASI NH 3 column densities are derived from observations between 2008-2017 (IASI v2.2R) 37 .Solid, dashed, and dotted lines show the boundaries of the areas <50 km, 50-150 km, 150-300 km within an NH 3 emission hotspot in panel (a) or an NH 3 hotspot in panel (b).NH 3 emission hotspots in panel (a) are the areas of the areas of the 95 th or high NH 3 emission rates in 2017 in the Contiguous US.NH 3 hotspots in panel (b) are the areas of the 95 th percentile NH 3 column density in the Contiguous US.The base map is obtained from Natural Earth.Chemical regimes for SIA formation in summer and winter.Panels (a)-(e) show the summer chemical regimes of (NH 4 ) 2 SO 4 formation.The lines in panels (a-e) indicate the condition that NH 4 T explicitly balances SO 4 2− to form (NH 4 ) 2 SO 4 (1:2 line) for a system that is only consist of NH Above the line, reducing NH 4 T only removes NH 3g from the system, such thatΔc SIA /Δc NH T Panels (f)-(o) show NH 4 NO 3 evaporation plays a major role.The framework of using aerosol pH and aerosol water content to determine NH 4 NO 3 regime developed by Nenes et al. 38 is used here (Text S3) 20,21 .The four regimes shown in panel (f)-(o) are relatively consistent when aerosol composition changes.The colour of the markers and the lines in panels (f)-(o) indicate the year.nature portfolio | reporting summary April 2023

Table 3 )
12Following refs.21and12,weevaluated the model performance by comparing simulated and observed partitionings of NH 4T and NO 3 T .The simulation results shown in this study include preprocessing of the integrated observations from the monitoring networks (case 1, Extended Data Fig.2), because running ISORROPIA-II with raw CASTNET inputs and a time step of two weeks (case 3, Supplementary Fig.18) leads to large errors in both ε NH + 4 and ε NO − 3 .orbase added to the system (n acid or n base in moles per kg solution) and the associated pH change.The analytical expression for the buffering capacity (β =