This article covers two essential compounds of nitrogen - ammonia and nitric acid - along with their preparation methods, properties and uses.

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Ammonia (NH3) is formed by decomposing nitrogenous organic matter, lượt thích urea. It is present in small quantities in air and soil, and it is also a common nitrogenous waste among aquatic organisms. Ammonia serves as a precursor to lớn 45% of the world’s food, thus contributing significantly khổng lồ the nutritional needs of terrestrial organisms. Nitric acid (HNO3) is an oxoacid of nitrogen. It is also known as aqua fortis in Latin, meaning ‘strong water’. It is a type of mineral acid.


Ammonia is a compound of nitrogen và hydrogen, i.e., a single nitrogen atom is covalently bonded khổng lồ 3 hydrogen atoms. It has the formula NH3. It is one of the most abundant hydrides found in our atmosphere. Its IUPAC name is Azane, & it has no colour & a distinct pungent smell.

At the apex, an ammonia molecule shows a trigonal pyramidal shape along with a nitrogen atom. It has one single electron pair & three bond pairs.

Preparation of ammonia

By the reaction of ammoniated salts with a strong base

NH4Cl + Na
OH → NH3 + Na
Cl + H2O

NH4Cl + KOH → NH3 + KCl + H2O

By hydrolysis of urea

NH2-CO-NH2 + H2O → 2NH3 + CO2

Haber’s process
The raw materials used here are nitrogen and hydrogen.Scrubbing removes impurities from the gases.The amalgamated gases are passed through a converter at a temperature of 450°C and 200atm pressure.Nitrogen reacts with hydrogen to form ammonia.Catalysts such as iron (Fe) & molybdenum (Mo) are used.

N2 + 3H2 → 2NH3

Physical properties of ammonia

Ammonia has a molar mass of 17.031 g/mol.Ammonia solutions appear clear and colourless.It is lighter than air.It is corrosive lớn metals và tissues.It has a sharp, pungent smell.It has a boiling point of -33.34 °C.Its melting point is −77.73 °C.Its mật độ trùng lặp từ khóa is 0.769 kg/m3 at Standard Temperature and Pressure (STP).It is trigonal pyramidal in shape.

Chemical properties of ammonia

Its aqueous solution is weakly basic due to lớn a lone pair of electrons.It works as the Lewis base.Reaction with halogens:Ammonia on reaction with excess chlorine forms nitrogen trichloride and hydrochloric acid.

NH3 + 3Cl2 (excess) → NCl3 + 3HCl

Excess ammonia on reaction with chlorine forms ammonia chloride & liberates nitrogen gas

8NH3 (excess) + 3Cl2 → 6NH4Cl + N2

When ammonia is heated and decomposed, it emits nitrogen oxides & toxic fumes.The p
H of the 1N aqueous solution of ammonia is 11.6.

Absorption, excretion & distribution of ammonia in the human body

Ammonia can be absorbed by inhalation. The inhaled ammonia gets collected in the upper respiratory tract và is subsequently exhaled out.The ammonia produced by the intestinal tract is also absorbed. It is converted lớn urea và glutamine by the liver.Excretion of urea is mainly through urinary urea, followed by faeces & exhalation.


Ammonia is used khổng lồ produce various nitrogenous fertilisers (ammonium nitrate, ammonium phosphate, urea).It is used in the manufacturing of inorganic nitrogen compounds like nitric acid.Liquid ammonia can be utilised as a refrigerant.

Nitric acid

Nitric acid (HNO3) is also known as aqua fortis, Latin for ‘strong water.’ It is bonded lớn one -OH group và two oxygen atoms. It is a mineral acid, & it is highly corrosive. It is pale yellow or reddish-brown in colour & has a suffocating odour. If the nitric acid concentration in an aqueous solution is more than 86%, then it is called red fuming nitric acid. If the concentration is above 95%, it is referred to lớn as white fuming acid.

Manufacturing of nitric acid

On a large scale, it is mainly prepared by Ostwald’s process. It is based on catalytic oxidation of NH3 by oxygen và involves the following steps:

Formation of NO in the presence of a catalyst

4NH3 + 5O2 → 4NO + 6H20

Formation of NO2

2NO + O2 → 2NO2

Dissolving in water to khung nitric acid

3NO2 + H2O → 2HNO3 + NO

Laboratory synthesis of nitric acid involves the thermal decomposition of copper (II) nitrate. Nitrogen dioxide và oxygen are formed, which in reaction with water, give nitric acid.

Another alternative method is reacting a nitrate salt (such as sodium nitrate) with sulphuric acid (H2SO4):

NO3 + H2SO4 → HNO3 + Na

Physical properties of nitric acid

The molar mass of nitric acid is 63.012 g/mol.It is colourless but turns yellow on decomposition.It is a very corrosive acid.It is toxic. Prolonged exposure may lead khổng lồ adverse health effects.Its boiling point is 355.6K.It has a freezing point of 231.4K.Its density is 1.51 g cm−3.It has a planar structure & shows resonance.

Chemical properties of nitric acid

On decomposition, nitric acid gives nitrogen dioxide.It is one of the best oxidising agents.It is monoprotic.Only concentrated nitric acid reacts with non-metals; dilute does not.

Oxidation of non-metals by nitric acid

C + HNO3 → CO2 + NO2S + HNO3 → H2SO4 + NO2P4 + HNO3 → H3PO4 + NO2

Oxidation of metals by nitric acid

Very dilute nitric acid

Out of all metals, only magnesium và manganese react with very dilute nitric acid. They liberate hydrogen gas & metal nitrates.

Mg + 2HNO3 → Mg(NO3)2 + H2

Mn + 2HNO3 → Mn(NO3)2 + H2

Cold dilute nitric acid

Metals like magnesium, zinc and iron react with cold dilute nitric acid khổng lồ liberate ammonium nitrate & metal nitrate.

4Mg + 10HNO3 → 4Mg(NO3)2 + 3H2O + NH4NO3

4Zn + 10HNO3 → 4Zn(NO3)2 + 3H2O + NH4NO3

4Fe + 10HNO3 → 4Fe(NO3)2 + 3H2O + NH4NO3

Hot dilute nitric acid

When metals react with hot dilute nitric acid, nitrous oxide (N2O) is formed.

4Mg + 10HNO3 → 4Mg(NO3)2 + 5H2O + N2O

4Zn + 10HNO3 → 4Zn(NO3)2 + 5H2O + N2O

4Fe + 10HNO3 → 4Fe(NO3)2 + 5H2O + N2O

Concentrated nitric acid

In reaction with concentrated nitric acid, metals give off NO2 gas.

Zn + 4HNO3 → 4Zn(NO3)2 + 2H2O + NO2

Mg + 4HNO3 → 4Mg(NO3)2 + 2H2O + NO2


Nitric acid is essential for the manufacturing of fertilisers.Nitric acid is used for making various organic nitrogen compounds, lượt thích derivatives of aniline.It is also used in woodwork.It is commonly used for cleaning & dairy equipment.It helps pick stainless steel và clean silicon wafers in electronics.Aqua regia is formed when concentrated hydrochloric acid và nitric acid are mixed in the ratio of 3:1.


Ammonia và nitric acid are the two major compounds of nitrogen. On a large scale, ammonia is produced by Haber’s process. Ammonia acts as a Lewis base due khổng lồ the presence of a lone pair of electrons. Ostwald’s process is used lớn manufacture nitric acid. Nitric acid is an excellent oxidising agent, which is monoprotic and corrosive. It is toxic if inhaled.

New particle formation in the upper không tính tiền troposphere is a major global source of cloud condensation nuclei (CCN)1,2,3,4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid & ammonia size particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia & ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia và abundant nitric acid alone is sufficient khổng lồ drive rapid growth lớn CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable lớn desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3–H2SO4–NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.

Intense particle formation has been observed by airborne measurements as a persistent, global-scale band in the upper troposphere over tropical convective regions1,2,4. Upper tropospheric nucleation is thought to provide at least one-third of global CCN3. Increased aerosols since the industrial revolution, và their interactions with clouds, have masked a large fraction of the global radiative forcing by greenhouse gases. Projections of aerosol radiative forcing resulting from future reductions of air pollution are highly uncertain7. Present-day nucleation involves sulfuric acid (H2SO4) over almost all the troposphere8. However, binary nucleation of H2SO4–H2O is slow and, so, ternary or multicomponent nucleation with extra vapours such as ammonia (NH3)9 và organics10,11 is necessary to trương mục for observed new-particle-formation rates3,8,12.

Ammonia stabilizes acid–base nucleation and strongly enhances particle formation rates9. However, ammonia is thought lớn be extremely scarce in the upper troposphere because its solubility in water and reactivity with acids should lead lớn efficient removal in convective clouds. However, this assumption is not supported by observation. Ammonia vapour has been repeatedly detected in the Asian monsoon upper troposphere, with mixing ratios of up to 30 pptv (2.5 × 108 cm−3) for a three-month average5 and up khổng lồ 1.4 ppbv (1.2 × 1010 cm−3) in hotspots6. The release of dissolved ammonia from cloud droplets may occur during glaciation13. Once released in the upper troposphere, ammonia can size particles with nitric acid, which is abundantly produced by lightning14,15. These particles will live longer & travel farther than ammonia vapour, with the potential khổng lồ influence the entire upper troposphere and lower stratosphere of the Northern Hemisphere6.

Fundamental questions remain about the role & mechanisms of nitric acid và ammonia in upper tropospheric particle formation. Recent CLOUD (Cosmics Leaving Outdoor Droplets) experiments at CERN have shown that nitric acid và ammonia vapours below 278 K can condense onto newly formed particles as small as a few nanometres in diameter, driving rapid growth lớn CCN sizes16. At even lower temperatures (below 258 K), nitric acid và ammonia can directly nucleate to form ammonium nitrate particles, although pure HNO3–NH3 nucleation is too slow lớn compete with H2SO4–NH3 nucleation under comparable conditions. However, the results we present here show that, when all three vapours are present, a synergistic interaction drives nucleation rates orders of magnitude faster than those from any two of the three components. Once nucleated through this multi-acid–ammonia mechanism, the particles can grow rapidly by co-condensation of NH3 and HNO3 alone, both of which may be far more abundant than H2SO4 in the upper troposphere.

Here we report new-particle-formation experiments performed with mixtures of sulfuric acid, nitric acid và ammonia vapours in the CLOUD chamber9 at CERN between September và December 2019 (CLOUD 14; see Methods for experimental details). Lớn span ranges typical of the upper troposphere, we established quasi-steady-state vapour concentrations in the chamber of (0.26–4.6) × 106 cm−3 sulfuric acid (through photochemical oxidation of SO2), (0.23–4.0) × 109 cm−3 nitric acid (through either photochemical oxidation of NO2 or injection from an evaporator) & (0.95–6.5) × 108 cm−3 ammonia (through injection from a gas bottle). In an extreme experiment to lớn simulate hotspot conditions in the Asian monsoon anticyclone, we raised sulfuric acid, nitric acid and ammonia to lớn maximum concentrations of 6.2 × 107 cm−3, 3.8 × 109 cm−3 and 8.8 × 109 cm−3, respectively. The experiments were conducted at 223 K & 25% relative humidity, representative of upper tropospheric conditions.

Figure 1 shows the evolution of a representative new-particle-formation experiment in the presence of around 6.5 × 108 cm−3 ammonia. The đứng đầu three panels show particle number concentrations above 1.7 nm & above 2.5 nm (Fig. 1a), particle formation rate at 1.7 nm (J1.7) (Fig. 1b) và particle size distribution (Fig. 1c). The bottom panel shows HNO3 & H2SO4 vapour concentrations (Fig. 1d). We switched on the ultraviolet (UV) lights at t = 0 min to oxidize SO2 with OH radicals & form H2SO4. Sulfuric acid started to lớn appear shortly thereafter and built up to a steady state of 2.3 × 106 cm−3 over the wall-loss timescale of about 10 min. Under these conditions, the data show a modest formation rate of 1.7-nm particles from H2SO4–NH3 nucleation, consistent with previous CLOUD measurements8. These particles grew only slowly (about 0.5 nm h−1 at this H2SO4 & particle size17). No particles reached 2.5 nm within 2 h, owing lớn their slow growth rate và low survival probability against wall loss.

Fig. 1: Example experiment showing nitric acid enhancement of H2SO4–NH3 particle formation.


a, Particle number concentrations versus time at mobility diameters >1.7 nm (magenta) & >2.5 nm (green). The solid magenta trace is measured by a PSM1.7 và the solid green trace is measured by a CPC2.5. The fixed experimental conditions are about 6.5 × 108 cm−3 NH3, 223 K and 25% relative humidity. A microphysical mã sản phẩm reproduces the main features of the observed particle formation (dashed lines; see text for details). b, Particle formation rate versus time at 1.7 nm (J1.7), measured by a PSM. c, Particle kích thước distribution versus time, measured by an SMPS. d, Gas-phase nitric acid & sulfuric acid versus time, measured by an I− CIMS và a NO3− CIMS, respectively. Sulfuric acid through SO2 oxidation started to appear soon after switching on the UV lights at time = 0 min, building up lớn a steady state of 2.3 × 106 cm−3 after a wall-loss-rate timescale of around 10 min. The subsequent H2SO4–NH3 nucleation led khổng lồ a relatively slow formation rate of 1.7-nm particles. The particles did not grow above 2.5 nm because of their slow growth rate & corresponding low survival probability against wall loss. Following injection of 2.0 × 109 cm−3 nitric acid into the chamber after 115 min, while leaving the production rate of sulfuric acid & the injection rate of ammonia unchanged, we observed a sharp increase in particle formation rate (panel b), together with rapid particle growth of 40 nm h−1 (panel c). The overall systematic scale uncertainties of ±30% on particle formation rate, −33%/+50% on sulfuric acid concentration and ±25% on nitric acid concentration are not shown.

Source data

At t = 115 min, we raised the nitric acid concentration to lớn 2.0 × 109 cm−3, through direct injection instead of photochemical production, so that we could independently control the nitric acid & sulfuric acid concentrations. The particle number increased 30-fold and 1,300-fold for particles larger than 1.7 nm and 2.5 nm, respectively. In addition, these newly formed particles grew much more rapidly (40 nm h−1), reaching 20 nm within 30 min. This experiment shows that nitric acid can substantially enhance particle formation và growth rates for fixed levels of sulfuric acid and ammonia.

We also conducted mã sản phẩm calculations on the basis of known thermodynamics và microphysics (Methods). Our mã sản phẩm results (dashed traces in Fig. 1a) consistently & quantitatively confirm the experimental data: sulfuric acid và ammonia nucleation produces only 1.7-nm particles, whereas addition of nitric acid strongly enhances the formation rates of both 1.7-nm & 2.5-nm particles.

We conducted two further experiments under conditions similar lớn Fig. 1 but holding the concentrations of a different pair of vapours constant while varying the third. For the experiment shown in Extended Data Fig. 1, we started by oxidizing NO2 to produce 1.6 × 109 cm−3 HNO3 in the presence of about 6.5 × 108 cm−3 NH3 and then increased H2SO4 from 0 to 4.9 × 106 cm−3 by oxidizing progressively more injected SO2. For the experiment shown in Extended Data Fig. 2, we first established 4.6 × 106 cm−3 H2SO4 and 4.0 × 109 cm−3 HNO3, and then increased NH3 from 0 to lớn about 6.5 × 108 cm−3. We consistently observed relatively slow nucleation when only two of the three vapours are present, whereas addition of the third vapour increased nucleation rates by several orders of magnitude.

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Figure 2 shows particle formation rates measured by CLOUD at 1.7-nm mobility diameter (J1.7) versus ammonia concentration, at 223 K. The J1.7 data were all measured in the presence of ions from galactic cosmic rays (GCR) và — so — represent the sum of neutral and ion-induced channels. The black diamond shows the measured J1.7 of 0.3 cm−3 s−1 for HNO3–NH3 nucleation with 1.5 × 109 cm−3 nitric acid, about 6.5 × 108 cm−3 ammonia & sulfuric acid below the detection limit of 5 × 104 cm−3 (this is the event shown in Extended Data Fig. 1). At this same ammonia concentration, we measured J1.7 = 6.1 cm−3 s−1 at 2.3 × 106 cm−3 H2SO4, demonstrating the much faster rate of H2SO4–NH3 nucleation (not shown). This measurement is consistent with models on the basis of previous CLOUD studies of H2SO4–NH3 nucleation18,19, as illustrated by the model simulations for 4.0 × 106 cm−3 sulfuric acid (red solid curve). The xanh circles show our measurements of J1.7 for HNO3–H2SO4–NH3 nucleation at 4.0 × 106 cm−3 sulfuric acid & (1.6–6.5) × 108 cm−3 ammonia, in the presence of 1.5 × 109 cm−3 nitric acid (the sự kiện shown in Extended Data Fig. 2). The xanh dashed curve is a nguồn law fit lớn the measurements, indicating a strong sensitivity to lớn ammonia concentration ((,J_1.7=k< mNH_3>^3.7)).

Fig. 2: Particle formation rates at 1.7 nm (J1.7) versus ammonia concentration at 223 K and 25% relative humidity.


The chemical systems are HNO3–NH3 (black), H2SO4–NH3 (red) & HNO3–H2SO4–NH3 (blue). The black diamond shows the CLOUD measurement of HNO3–NH3 nucleation at 1.5 × 109 cm−3 HNO3, 6.5 × 108 cm−3 NH3 and with H2SO4 below the detection limit of 5 × 104 cm−3. The red solid curve is J1.7 versus ammonia concentration at 4.0 × 106 cm−3 sulfuric acid from a H2SO4–NH3 nucleation parameterization on the basis of previous CLOUD measurements18,19. The xanh circles show the CLOUD measurements of HNO3–H2SO4–NH3 nucleation at 4.0 × 106 cm−3 H2SO4, 1.5 × 109 cm−3 HNO3 & (1.6–6.5) × 108 cm−3 NH3. The data are fitted by a power law, J1.7 = k3.7 (blue dashed curve). The vertical grey dotted line separates ammonia concentrations measured in different regions in the upper troposphere5; the region to lớn the right indicates the Asian monsoon conditions. The horizontal grey solid lines show J1.7 upper limits for ion-induced nucleation resulting from the GCR ionization rate of around 2 ion pairs cm−3 s−1 at ground level và 35 ion pairs cm−3 s−1 in the upper troposphere. Among the three nucleation mechanisms, H2SO4–NH3 nucleation dominates in regions with low ammonia (below around 1.0 × 108 cm−3, or 12 pptv), whereas HNO3–H2SO4–NH3 nucleation dominates at higher ammonia levels characteristic of the Asian monsoon upper troposphere. The bars indicate 30% estimated total error on the particle formation rates. The overall systematic scale uncertainties are −33%/+50% for sulfuric acid and ±25% for nitric acid concentrations.

Source data

The vertical grey dotted line in Fig. 2 separates ammonia concentrations measured in different regions in the upper troposphere5; Asian monsoon conditions are to the right of this vertical line. Our results indicate that H2SO4–NH3 nucleation is probably responsible for new particle formation in regions with ammonia concentrations below around 108 cm−3 (12 pptv), but that HNO3–H2SO4–NH3 nucleation probably dominates at higher ammonia levels in the Asian monsoon upper troposphere. Our nucleation rate measurements confirm that the stronger sulfuric acid is favoured by ammonia in the ammonia-limited regime, so nitric acid will evaporate from the clusters, as it may be displaced by sulfuric acid. However, as ammonia increases from 1.6 lớn 6.5 × 108 cm−3, we observe sharp increases in J1.7 for HNO3–H2SO4–NH3 nucleation from 10 to 400 cm−3 s−1 and in the ratio of particle formation rates (HNO3–H2SO4–NH3:H2SO4–NH3) from 4 khổng lồ 30. Our nucleation model (as in Fig. 1) yields slightly higher J1.7 than that observed, as shown in Extended Data Fig. 3, but the formation rate variation with ammonia, nonetheless, shows a similar slope.

CLOUD has previously shown that ions enhance nucleation for all but the strongest acid–base clusters; HNO3–H2SO4–NH3 is probably not an exception. However, the ion enhancement is limited by the GCR ion-pair production rate. We show with the horizontal grey solid lines in Fig. 2 the upper limits on J1.7 for ion-induced nucleation of about 2 cm−3 s−1 at ground level và 35 cm−3 s−1 in the upper troposphere. Our experimental nucleation rates for HNO3–H2SO4–NH3 are mostly above upper tropospheric GCR ion production rates. This is confirmed by similar J1.7 measured during a neutral nucleation experiment, in which an electric field was used lớn rapidly sweep ions from the chamber. Thus, for this nucleation scheme, the neutral channel will often prevail over the ion-induced channel in the Asian monsoon upper troposphere. However, when ammonia is diluted away outside the Asian monsoon anticyclone, ions may enhance the nucleation rate up to lớn the GCR limit near 35 cm−3 s−1.

In a formal sense, the new-particle-formation mechanism could be one of two types: formation of stable H2SO4–NH3 clusters, followed by nano-Köhler-type activation by nitric acid and ammonia16; or else true synergistic nucleation of nitric acid, sulfuric acid & ammonia9. In a practical sense, it makes little difference because coagulation loss is a major sink for all small clusters in the atmosphere20, so appearance of 1.7-nm particles by means of any mechanism constitutes new particle formation. Regardless, we can distinguish between these two possibilities from our measurements of the molecular composition of negatively charged clusters using an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer. In Fig. 3, we show cluster mass defect plots during H2SO4–NH3 & HNO3–H2SO4–NH3 nucleation events at 223 K. The marked difference between Fig. 3a, b indicates that nitric acid changes the composition of the nucleating clusters down lớn the smallest sizes; thus, the mechanism is almost certainly synergistic HNO3–H2SO4–NH3 nucleation.

Fig. 3: Molecular composition of negatively charged clusters during H2SO4–NH3 and HNO3–H2SO4–NH3 nucleation events at 223 K & 25% relative humidity.


Mass defect (difference from integer mass) versus mass/charge (m/z) of negatively charged clusters measured with an APi-TOF mass spectrometer for 1.7 × 106 cm−3 sulfuric acid và 6.5 × 108 cm−3 ammonia (a) and 2.0 × 107 cm−3 sulfuric acid, 3.2 × 109 cm−3 nitric acid và 7.9 × 109 cm−3 ammonia (b). The symbol colours indicate the molecular composition as shown. The symbol area is proportional khổng lồ the logarithm of signal rate (counts per second). The labels (m:n) near the symbols indicate the number of sulfuric acid (H2SO4)m và ammonia (NH3)n molecules in the clusters, including both neutral & charged species. The grey dashed lines follow clusters that contain pure H2SO4 molecules with an HSO4− ion (or SO4 instead of H2SO4 and/or SO4− instead of HSO4− for pure H2SO4 clusters falling below this line in b). The grey solid lines follow the 1:1 H2SO4–NH3 addition starting at (H2SO4)4–(NH3)0. Nearly all clusters in panel a lie above this line, whereas nearly all clusters in panel b fall below it. Most clusters containing HNO3 lack NH3 by the time they are measured (they fall near the (m:0) grey dashed line), but the marked difference between ab indicates that the nucleating clusters had distinctly different compositions, probably including relatively weakly bound HNO3–NH3 pairs in b. It is probable that nucleating clusters in the CLOUD chamber at 223 K contain HNO3–H2SO4–NH3 with a roughly 1:1 acid–base ratio. However, during the transmission from the chamber khổng lồ the warm APi-TOF mass spectrometer at 293 K, the clusters lose HNO3 and NH3, leaving a less volatile bộ vi xử lý core of H2SO4 with depleted NH3. The evaporation of a single NH3 or HNO3 molecule from a cluster displaces it on the mass defect plot by a vector distance indicated by the black arrows in b.

Source data

In Fig. 3a, the predominant ions are one of several deprotonated sulfuric acid species, including HSO4−, SO4−, HSO5−, SO5− and so on, resulting in a group of points for clusters with similar molecular composition but different mass & mass defect. In the figure, we use the labels (m:n) to indicate the number of sulfuric acid và ammonia molecules in the (H2SO4)m–(NH3)n clusters, including both neutral and charged species. The mass defect plot closely resembles those previously measured for H2SO4–NH3 nucleation21. Negative-ion-induced nucleation proceeds with the known acid–base stabilization mechanism, in which sulfuric acid dimers form as a first step (with HSO4− serving as a conjugate base for the first H2SO4) và then clusters subsequently grow by 1:1 H2SO4–NH3 addition (that is, as ammonium bisulfate)9. We use a grey line lớn illustrate the 1:1 addition path, beginning at (H2SO4)4–(NH3)0. Clusters larger than the sulfuric acid tetramers mostly contain several ammonia molecules and, so nearly all clusters in Fig. 3a lie above the grey line.

Figure 3b shows a pronounced change in the cluster APi-TOF signal during HNO3–H2SO4–NH3 nucleation. In addition lớn pure (H2SO4)m–(NH3)n clusters, we observe clusters with one extra HNO3 molecule (or NO3− ion), that is, (HNO3)1–(H2SO4)m–(NH3)n, and the pure nitric acid monomer và dimer. In sharp contrast with Fig. 3a, all these clusters are deficient in NH3, falling below the same grey line as in Fig. 3a. The most deficient contain up to lớn nine bare acids, that is, (H2SO4)9 or (H2SO4)8–(HNO3)1. Figure 3b almost certainly does not represent the true cluster composition in the chamber because binary nucleation of H2SO4 does not proceed under these exact conditions of H2SO4, NH3, temperature & relative humidity (as demonstrated by Fig. 3a). We can interpret Fig. 3b as follows. It is probable that clusters in the CLOUD chamber (223 K) contain HNO3–H2SO4–NH3 with a roughly 1:1 acid–base ratio, representing partial neutralization. However, during the transmission from the cold chamber to the warm APi-TOF mass spectrometer (about 293 K), the clusters thua relatively weakly bound HNO3 and NH3 molecules but not the lower-volatility H2SO4 molecules. Regardless of the interpretation, however, the notable difference between Fig. 3a, b indicates that the sampled clusters had very different compositions & that nitric acid participated in the formation of clusters as small as a few molecules.

Nitric acid & ammonia not only enhance the formation rate of new particles but also drive their rapid growth khổng lồ sizes at which they may act as CCN or ice nucleating particles (INP), above around 50 nm. Lớn assess their effect on cirrus clouds, we measured the ice nucleation ability of particles formed from HNO3–H2SO4–NH3 nucleation in the CLOUD chamber. Simulating ‘hotspot’ conditions, we first formed pure ammonium nitrate particles by means of HNO3–NH3 nucleation and then increased the H2SO4 fraction in the particles by oxidizing progressively more SO2. We measured their ice nucleation ability using the online continuous flow diffusion instrument, m
INKA (Methods và Extended Data Fig. 4). As shown in Fig. 4a, pure ammonium nitrate particles (purple data points) nucleate ice only at high ice saturation ratios (Sice), characteristic of homogeneous nucleation (shown by a steep increase of ice activation above Sice = 1.60 at 215 K). This indicates that pure ammonium nitrate particles, formed by means of HNO3–NH3 nucleation, are probably in a liquid state initially, albeit at a relative humidity below the deliquescence point22. However, addition of sulfate, with a particulate sulfate-to-nitrate molar ratio as small as 10−4, triggers crystallization of ammonium nitrate. For these particles, we observed a small heterogeneous ice nucleation mode at Sice of 1.54 (blue data points), with other conditions & the particle kích cỡ distribution held almost constant. Moreover, as the sulfate molar fraction progressively rises lớn just 0.017 (still almost pure but now solid ammonium nitrate), an active surface site density (ns) of 1010 m−2 is reached at Sice as low as 1.26. This is consistent with previous findings, in which particles were generated through nebulization, with a much larger particle diameter & a much higher sulfate-to-nitrate ratio23. Our measurements show that HNO3–H2SO4–NH3 nucleation followed by rapid growth from nitric acid and ammonia condensation — which results in low sulfate-to-nitrate ratio — could provide an important source of INP that are comparable with typical desert dust particles at nucleating ice24.

Fig. 4: Ice nucleation properties & modelled regional contribution of upper tropospheric particles formed from HNO3–H2SO4–NH3 nucleation.


a, Active surface site density versus ice saturation ratio, measured by the m
INKA instrument at CLOUD, at 233 K & 25% relative humidity. Pure ammonium nitrate particles (purple points) show homogeneous freezing. However, addition of only small amounts of sulfate creates highly ice-nucleation-active particles. At around 1.7% sulfate fraction (red points), the ice nucleating efficiency is comparable with desert dust particles24. b, Simulation of particle formation in a global model (EMAC) with efficient vertical transport of ammonia into the upper troposphere during the Asian monsoon. Including multi-acid HNO3–H2SO4–NH3 nucleation (on the basis of the xanh dashed curve in Fig. 2) enhances particle number concentrations (nucleation mode) over the Asian monsoon region by a factor of 3–5 compared with the same mã sản phẩm with only H2SO4–NH3 nucleation (from Dunne et al.8, similar khổng lồ the red solid curve in Fig. 2).

Source data

Our findings suggest that HNO3–H2SO4–NH3 nucleation may dominate new particle formation in the Asian monsoon region of the upper troposphere, with a ‘flame’ of new particles in the outflow of convective clouds, in which up to lớn 1010 cm−3 ammonia6 mixes with low (background) levels of sulfuric acid và nitric acid. Without this mechanism, particle formation through the traditional ternary H2SO4–NH3 nucleation would be much slower và most probably rate-limited by the scarce sulfuric acid. Furthermore, by co-condensing with nitric acid, the convected ammonia also drives the growth of the newly formed particles. Given typical acid-excess conditions in the upper troposphere, condensational growth is governed by the availability of ammonia. Consequently, particles will steadily (and rapidly) grow until ammonia is depleted after several e-folding times mix by the particle condensation sink. On the basis of condensation sinks generally observed in the tropical upper troposphere4, this timescale will be several hours. Within this time interval, given the observed ammonia levels, newly formed particles will be able to lớn grow to CCN sizes & even small admixtures of sulfuric acid will render these particles efficient INP.

Our laboratory measurements provide a mechanism that can trương mục for recent observations of abundant ammonium nitrate particles in the Asian monsoon upper troposphere6. To lớn evaluate its importance on a global scale, we first parameterized our experimentally measured J1.7 for HNO3–H2SO4–NH3 nucleation as a function of sulfuric acid, nitric acid và ammonia concentrations (Methods). The parameterization is obtained using a power-law dependency for each vapour (Extended Data Fig. 5), given that the critical cluster composition is associated with the exponents according to lớn the first nucleation theorem25. Then we implemented this parameterization in a global aerosol mã sản phẩm (EMAC, see Methods for modelling details). The EMAC model predicts that HNO3–H2SO4–NH3 nucleation at 250 h
Pa (11 km, approximately 223 K) produces an annual average exceeding 1,000 cm−3 new particles over an extensive area (Extended Data Fig. 6). This corresponds to an increase in particle number concentration (Fig. 4b) up lớn a factor of five higher than in a control simulation with only ternary H2SO4–NH3 nucleation8. The strongest increase occurs mostly over Asia, in which ammonia is ample because of deep convection from ground sources.

However, another global mã sản phẩm (TOMCAT, see Methods) shows much lower ammonia mixing ratios in the upper troposphere than EMAC (7a, b). This large variability of upper tropospheric ammonia is also indicated by recent field measurements on local6,26 và global5,27 scales. In view of its importance for both H2SO4–NH3 and HNO3–H2SO4–NH3 nucleation, there is an urgent need to lớn improve upper tropospheric measurements of ammonia, as well as improve knowledge of its sources, transport và sinks.

We thus turned to lớn a cloud-resolving mã sản phẩm to estimate the ammonia vapour fraction remaining after deep convection (see Methods). We show in Extended Data Fig. 8 that around 10% of the boundary layer ammonia can be transported into the upper troposphere and released as vapour by a base-case convective cloud. The sensitivity tests further illustrate that the key factor governing the fraction of ammonia remaining in the cloud outflow is the retention of ammonia molecules by ice particles (Extended Data Fig. 8e), whereas cloud water p
H (Extended Data Fig. 8c) and cloud water content (Extended Data Fig. 8d) only play minor roles once glaciation occurs. Given that more than 10 ppbv of ammonia is often observed in the Asian boundary layer28, it is plausible that the observed 1.4 ppbv (1010 cm−3) ammonia in the upper troposphere6 is indeed efficiently transported by the convective systems.

Although the ammonium–nitrate–sulfate particles are formed locally, they can travel from Asia lớn North America in just three days by means of the subtropical jet stream, as the typical residence time of Aitken mode particles ranges from one week khổng lồ one month in the upper troposphere29. As a result, these particles can persist as an intercontinental band, covering more than half of the mid-latitude surface area of the Northern Hemisphere (Extended Data Fig. 6). In summary, synergistic nucleation of nitric acid, sulfuric acid & ammonia could provide an important source of new CCN & ice nuclei in the upper troposphere, especially over the Asian monsoon region, & is closely linked with anthropogenic ammonia emissions27.

The CLOUD facility

We conducted our measurements at the CERN CLOUD facility, a 26.1-m3, electropolished, stainless-steel CLOUD chamber that allows new-particle-formation experiments under the full range of tropospheric conditions with scrupulous cleanliness & minimal contamination9,30. The CLOUD chamber is mounted in a thermal housing, capable of keeping the temperature constant in the range 208 K và 373 K with a precision of ±0.1 K (ref. 31). Photochemical processes are initiated by homogeneous illumination with a built-in UV fibre-optic system, including four 200-W Hamamatsu Hg-Xe lamps at wavelengths between 250 and 450 nm and a 4-W Kr
F excimer UV laser at 248 nm with adjustable power. New particle formation under different ionization levels is simulated with & without the electric fields (±30 k
V), which can artificially scavenge or preserve small ions produced from ground-level GCR. Uniform spatial mixing is achieved with magnetically coupled stainless-steel fans mounted at the top & bottom of the chamber. The characteristic gas mixing time in the chamber during experiments is a few minutes. The loss rate of condensable vapours và particles onto the chamber walls is comparable with the ambient condensation sink. Khổng lồ avoid contamination, the chamber is periodically cleaned by rinsing the walls with ultra-pure water và heating to 373 K for at least 24 h, ensuring extremely low contaminant levels of sulfuric acid 4 cm−3 & total organics 32,33). The CLOUD gas system is also built khổng lồ the highest technical standards of cleanliness & performance. The dry air supply for the chamber is provided by boil-off oxygen (Messer, 99.999%) và boil-off nitrogen (Messer, 99.999%) mixed at the atmospheric ratio of 79:21. Highly pure water vapour, ozone and other trace gases such as nitric acid and ammonia can be precisely added at the pptv cấp độ from ultra-pure sources.


Gas-phase sulfuric acid was measured using a nitrate chemical ionization APi-TOF (nitrate-CI-APi-TOF) mass spectrometer34,35 & an iodide chemical ionization time-of-flight mass spectrometer equipped with a Filter Inlet for Gases and Aerosols (I-FIGAERO-CIMS)36,37. The nitrate-CI-APi-TOF mass spectrometer is equipped with an electrostatic filter in front of the inlet to lớn remove ions & charged clusters formed in the chamber. A corona charger is used lớn ionize the reagent nitric acid vapour in a nitrogen flow38. Nitrate ions are then guided in an atmospheric pressure drift tube by an electric field khổng lồ react with the analyte molecules in the sample flow. Sulfuric acid is quantified for the nitrate-CI-APi-TOF with a detection limit of about 5 × 104 cm−3, following the same calibration & loss correction procedures described previously9,32,39. FIGAERO is a manifold inlet for a CIMS with two operating modes. In the sampling mode, a coaxial bộ vi xử lý core sampling is used to lớn minimize the vapour wall loss in the sampling line. The total flow is maintained at 18.0 slpm và the core flow at 4.5 slpm; the CIMS samples at the centre of the core flow with a flow rate of 1.6 slpm. Analyte molecules are introduced into a 150-mbar ion-molecule reactor, chemically ionized by iodide ions that are formed in a Po-210 radioactive source và extracted into the mass spectrometer. The sulfuric acid calibration coefficient for the I-FIGAERO-CIMS is derived using the absolute sulfuric acid concentrations measured with the pre-calibrated nitrate-CI-APi-TOF.

Gas-phase nitric acid was also measured using the I-FIGAERO-CIMS. Nitric acid concentration was quantified by measuring HNO3/N2 mixtures with known nitric acid concentrations, following similar procedures described previously16. The HNO3/N2 mixture was sourced from flowing 2 slpm ultra-pure nitrogen through a portable nitric acid permeation tube, at constant 40 °C. The permeation rate of nitric acid was determined by passing the outflow of the permeation tube through an impinger containing deionized water and analysing the resulting nitric acid solution through spectrophotometry.

Gas-phase ammonia was either measured or calculated. We measured ammonia using a proton transfer reaction time-of-flight mass spectrometer (PTR3-TOF-MS, or PTR3 for short)40. As a carrier gas for the primary ions, we used argon (ultra-high purity 5.0) to ensure that ammonium ions could not be artificially formed in the region of the corona discharge. Although the theoretical detection limit from peak height và width would be even smaller, the lowest concentration we were able to measure during the first fully ammonia-free runs of the beginning of the chiến dịch was 109 cm−3. An explanation for this is that, when concentrations of ammonia are low, effects of wall interaction of the highly soluble ammonia become important & the decay of ammonia in the inlet line becomes very slow. To reduce inlet wall contacts, we used a core-sampling technique directly in front of the instrument to lớn sample only the centre 2 slpm of the 10 slpm inlet flow, but owing to lớn frequent necessary on-site calibrations of volatile organic compounds, a Teflon ball valve was placed within the sample line that probably influenced measurements during times of low ammonia concentrations. At concentrations above about 2 × 109 cm−3 ammonia, however, the response of the instrument was very fast, so that, for example, changes in the chamber ammonia flow rate were easily detectable. Off-site calibrations showed a humidity-independent calibration factor of 0.0017 ncps/ppb. Calibrated data from the PTR3 agree very well with the Picarro above 1010 cm−3 (detection limit of the Picarro). The PTR3 also provides information about the overall cleanliness of the volatile organic compounds in the chamber. The technique was extensively described previously40.

For ammonia concentrations below 109 cm−3, we calculated concentration using the calibrated ammonia injection flow and an estimated first-order wall-loss rate. The wall-loss rate (kwall) for ammonia inside the CLOUD chamber is confirmed khổng lồ be faster than for sulfuric acid41, và can be determined from the following expression42:

$$k_ mwall=fracAV,frac2 mpi ,sqrtk_ me,D_i=C_ mwall,sqrtD_i$$

in which A/V is the surface-to-volume ratio of the chamber, ke is the eddy diffusion constant (determined by the turbulent mixing intensity, not the transport properties of the gases) and Di is the diffusion coefficient for each gas. Cwall is thus referred lớn as an empirical parameter of experiment conditions in the chamber. Here we first determine the kwall for sulfuric acid và nitric acid lớn be 1.7 × 10−3 and 1.9 × 10−3 s−3, respectively, by measuring their passive decay rates & subtracting the loss rate of chamber dilution for both (1.2 × 10−3 s−1), as well as the loss rate of dimer formation for sulfuric acid (around 1.6 × 10−3 s−1 for 5 × 106 cm−3 H2SO4). The kwall for sulfuric acid agrees with our measurements from previous campaigns43. We then derive the Cwall for sulfuric acid và nitric acid both to be 2.0 × 10−4 torr−0.5 cm−1 s−0.5, with (D_ mH_2 mSO_4) of 74 torr cm2 s−1 and (D_ mHNO_3) of 87 torr cm2 s−1 (ref. 44). Finally, we calculate the kwall for ammonia lớn be 2.7 × 10−3 s−1, with (D_ mNH_3) of 176 torr cm2 s−1 (ref. 44). Ammonia desorption from the chamber surface is a strong function of the temperature & is believed to be negligible at low temperatures30. Even after a long time exposure, ammonia desorption should be less than 1.6 × 106 cm−3, according to previous parameterization of ammonia background contamination in the CLOUD chamber41.

The composition of negatively charged ions and clusters were determined using an APi-TOF mass spectrometer45. The APi-TOF mass spectrometer is connected khổng lồ the CLOUD chamber by means of a 1-inch (21.7-mm inner diameter) sampling probe, with coaxial vi xử lý core sampling khổng lồ minimize the wall losses in the sampling line. The total sample flow is maintained at 20 slpm & the vi xử lý core sample flow for the APi-TOF mass spectrometer at 0.8 slpm. Because this instrument only measures charged clusters, the measurements were made during GCR conditions. Owing lớn a large temperature difference between the cold chamber (223 K) và the warm APi-TOF mass spectrometer (around 293 K), HNO3–H2SO4–NH3 clusters probably thua trận relatively weakly bonded HNO3 and NH3 molecules. This resembles the chemical ionization process of detecting ammonia with the nitrate-CI-APi-TOF, in which HNO3 and NH3 molecules rapidly evaporate from the resulting ammonia nitrate cluster in the CI-APi-TOF vacuum regions46.

Gas monitors were used to lớn measure ozone (O3, Thermo Environmental Instruments TEI 49C), sulfur dioxide (SO2, Thermo Fisher Scientific Inc. 42i-TLE) and nitric oxide (NO, ECO Physics, CLD 780TR). Nitrogen dioxide (NO2) was measured by a cavity attenuated phase shift nitrogen dioxide monitor (CAPS NO2, Aerodyne Research Inc.) và a home-made cavity enhanced differential optical absorption spectroscopy (CE-DOAS) instrument. The relative humidity of the chamber was determined by dew point mirrors (Edge

Particle number concentrations were monitored by condensation particle counters (CPCs), including an Airmodus A11 nano Condensation Nucleus Counter (n
CNC), consisting of a particle size magnifier (PSM) and a laminar-flow butanol-based CPC47, as well as a butanol TSI 3776 CPC. Particle form size distributions between 1.8 nm và 500 nm were measured by a nano-scanning electrical mobility spectrometer (n
SEMS), a nano-scanning mobility particle sizer (nano-SMPS) & a long-SMPS. The n
SEMS used a new, radial opposed migration ion và aerosol classifier (ROMIAC), which is less sensitive to lớn diffusional resolution degradation than the DMAs48, & a soft X-ray charge conditioner. After leaving the classifier, particles were first activated in a fast-mixing diethylene glycol stage49 and then counted with a butanol-based CPC. The n
SEMS transfer function that was used to invert the data khổng lồ obtain the particle size distribution was derived using 3 chiều finite element modelling of the flows, electric field và particle trajectories50,51. The two commercial mobility particle size spectrometers, nano-SMPS and long-SMPS, have been fully characterized, calibrated and validated in several previous studies52,53,54.

Particle-phase chemical composition was quantified using a high-resolution time-of-flight aerosol mass spectrometer (HR-To
F-AMS, Aerodyne Research). The working principles of the HR-To
F-AMS have been explained in detail previously55,56. In brief, particles are focused by an aerodynamic lens and flash-vaporized by impact onto a hot surface at 600 °C under a high vacuum. The vapours are then ionized by 70-e
V electrons and the ions are detected with a To
F mass spectrometer. Ionization efficiency calibrations were conducted before and after the campaign and the variation is within 30%. The particle collection efficiency was considered constant during the experiments because temperature và relative humidity in the chamber were fixed và the particle composition was dominated by ammonium nitrate.

INP were measured in real time at 215 K, as a function of ice saturation ratio (Sice), by the mobile ice nucleation instrument of the Karlsruhe Institute of technology (m
INKA is a continuous flow diffusion chamber with vertical cylindrical geometry57, on the basis of the thiết kế of INKA58,59. A detailed mô tả tìm kiếm of the continuous flow diffusion chamber working principle is presented elsewhere57. Here, predefined scans of the water vapour saturation ratios were performed in the diffusion chamber every 30 min. For each scan, Sice steadily increased from 1.2 khổng lồ 1.8 while the temperature was kept constant. The errors associated to temperature and Sice inside the diffusion chamber were derived from the uncertainty of the thermocouples attached to the instrument walls (±0.5 K)59.

Determination of particle formation rate

The particle formation rate, J1.7, is determined at 1.7-nm mobility diameter (1.4-nm physical diameter), here using a PSM. At 1.7 nm, a particle is normally considered lớn be above its critical form size and, therefore, thermodynamically stable. J1.7 is calculated using the flux of the total concentration of particles growing past a specific diameter (here at 1.7 nm), as well as correction terms accounting for aerosol losses owing to lớn dilution in the chamber, wall losses and coagulation. Details were described previously47.

Nucleation model

The nucleation model is on the basis of the thermodynamic mã sản phẩm for H2SO4–NH3 nucleation described in detail previously18,19. It is developed from the general dynamic equations60, khổng lồ calculate the production and losses for each cluster/particle size to determine the formation rates of the acid–base clusters. For HNO3–H2SO4–NH3 nucleation, we simplify the mã sản phẩm simulations by extrapolating nano-Köhler-type activation by nitric acid và ammonia to lớn clusters down khổng lồ sulfuric acid trimers. Eighty size bins, ranging from one ammonium sulfate cluster lớn 300 nm, are used khổng lồ capture the evolution of the kích thước and composition of polydisperse particles.

In brief, we calculate the equimolar condensation flux of nitric acid and ammonia on the basis of the supersaturation of gas-phase nitric acid and ammonia over particle-phase ammonium nitrate39,60: