WHAT IS THE CHEMICAL REACTION BETWEEN NH3 AND HNO3? WHAT ARE THEIR DIFFERENCES?

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.

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.

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.

Instrumentation

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:

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
Tech).

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). 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: