NH3 Mixing Ratios Research Articles

Validation of TES ammonia observations at the single pixel scale in the San Joaquin Valley during DISCOVER‐AQ

AbstractAmmonia measurements from a vehicle‐based, mobile open‐path sensor and those from aircraft were compared with Tropospheric Emission Spectrometer (TES) NH3 columns at the pixel scale during the NASA Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality field experiment. Spatial and temporal mismatches were reduced by having the mobile laboratory sample in the same areas as the TES footprints. To examine how large heterogeneities in the NH3 surface mixing ratios may affect validation, a detailed spatial survey was performed within a single TES footprint around the overpass time. The TES total NH3 column above a single footprint showed excellent agreement with the in situ total column constructed from surface measurements with a difference of 2% (within the combined measurement uncertainties). The comparison was then extended to a TES transect of nine footprints where aircraft data (5–80 ppbv) were available in a narrow spatiotemporal window (<10 km, <1 h). The TES total NH3 columns above the nine footprints agreed to within 6% of the in situ total columns derived from the aircraft‐based measurements. Finally, to examine how TES captures surface spatial gradients at the interpixel scale, ground‐based, mobile measurements were performed directly underneath a TES transect, covering nine footprints within ±1.5 h of the overpass. The TES total columns were strongly correlated (R2 = 0.82) with the median NH3 mixing ratios measured at the surface. These results provide the first in situ validation of the TES total NH3 column product, and the methodology is applicable to other satellite observations of short‐lived species at the pixel scale.

Low-power, open-path mobile sensing platform for high-resolution measurements of greenhouse gases and air pollutants

A low-power mobile sensing platform has been developed with multiple open-path gas sensors to measure the ambient concentrations of greenhouse gases and air pollutants with high temporal and spatial resolutions over extensive spatial domains. The sensing system consists of four trace gas sensors including two custom quantum cascade laser-based open-path sensors and two LICOR open-path sensors to measure CO2, CO, CH4, N2O, NH3, and H2O mixing ratios simultaneously at 10 Hz. In addition, sensors for meteorological and geolocation data are incorporated into the system. The system is powered by car batteries with a low total power consumption (~200 W) and is easily transportable due to its low total mass (35 kg). Multiple measures have been taken to ensure robust performance of the custom, open-path sensors located on top of the vehicle where the optics are exposed to the harsh on-road environment. The mobile sensing system has been integrated and installed on top of common passenger vehicles and participated in extensive field campaigns (>400 h on-road time with >18,000 km total distance) in both the USA and China. The simultaneous detection of multiple trace gas species makes the mobile sensing platform a unique and powerful tool to identify and quantify different emission sources through mobile mapping.

Satellite observations of tropospheric ammonia and carbon monoxide: Global distributions, regional correlations and comparisons to model simulations

Ammonia (NH3) and carbon monoxide (CO) are primary pollutants emitted to the Earth's atmosphere from common as well as distinct sources associated with anthropogenic and natural activities. The seasonal and global distributions and correlations of NH3 and CO from the Tropospheric Emission Spectrometer (TES) satellite observations and GEOS-Chem model simulations for 2007 are investigated to evaluate how well the global and seasonal pollutant sources are prescribed in the model. Although the GEOS-Chem simulations of NH3 and CO atmospheric mixing ratio values are lower than the TES satellite observations, the global distribution patterns from the model reasonably agree with the observations, indicating that the model represents the general location of the source regions and the seasonal enhancements of NH3 and CO globally over large regional scales. In regions and seasons where biomass burning is the dominant source of both NH3 and CO emissions into the atmosphere, there are strong NH3:CO correlations, which is consistent with the relationship demonstrated by surface measurements over fires. In regions where the enhanced NH3 and CO are known to be produced by different sources, the NH3:CO correlations from TES observations and model simulations are weak or non-existent. For biomass burning regions the NH3:CO ratios are 0.015 (TES) and 0.013 (GEOS-Chem). In regions of high-population density, known heavy traffic, and limited biomass burning sources, such as the rapidly developing areas of South Asia and northern China, which include mixtures of megacities, industrial, and agricultural areas, the two species show weaker but still positive correlations and NH3:CO ratios of 0.051 (TES) and 0.036 (GEOS-Chem). These enhancement ratios of NH3 relative to CO are useful in constraining NH3 emission inventories when CO emission inventories are better known for some events or regions (i.e. biomass burning).

Soil–atmosphere exchange of ammonia in a non-fertilized grassland: measured emission potentials and inferred fluxes

Abstract. A 50-day field study was carried out in a semi-natural, non-fertilized grassland in south-western Ontario, Canada during the late summer and early autumn of 2012. The purpose was to explore surface–atmosphere exchange processes of ammonia (NH3) with a focus on bi-directional fluxes between the soil and atmosphere. Measurements of soil pH and ammonium concentration ([NH4+]) yielded the first direct quantification of soil emission potential (Γsoil = [NH4+]/[H+]) for this land type, with values ranging from 35 to 1850 (an average of 290). The soil compensation point, the atmospheric NH3 mixing ratio below which net emission from the soil will occur, exhibited both a seasonal trend and diurnal trend. Higher daytime and August compensation points were attributed to higher soil temperature. Soil–atmosphere fluxes were estimated using NH3 measurements from the Ambient Ion Monitor Ion Chromatograph (AIM-IC) and a simple resistance model. Vegetative effects were ignored due to the short canopy height and significant Γsoil. Inferred fluxes were, on average, 2.6 ± 4.5 ng m−2 s−1 in August (i.e. net emission) and −5.8 ± 3.0 ng m−2 s−1 in September (i.e. net deposition). These results are in good agreement with the only other bi-directional exchange study in a semi-natural, non-fertilized grassland. A Lagrangian dispersion model (Hybrid Single-Particle Lagrangian Integrated Trajectory – HYSPLIT) was used to calculate air parcel back-trajectories throughout the campaign and revealed that NH3 mixing ratios had no directional bias throughout the campaign, unlike the other atmospheric constituents measured. This implies that soil–atmosphere exchange over a non-fertilized grassland can significantly moderate near-surface NH3 concentrations. In addition, we provide indirect evidence that dew and fog evaporation can cause a morning increase of [NH3]g. Implications of our findings on current NH3 bi-directional exchange modelling efforts are also discussed.

Variability of atmospheric ammonia related to potential emission sources in downtown Toronto, Canada

The variability of atmospheric ammonia (NH3) in downtown Toronto was investigated through the analysis of 24 hr integrated denuder NH3 samples collected at a site above a street canyon every third day from July 2003 to September 2011 (Dataset 1). The measured NH3 mixing ratios ranged from below the detection limit to 14.7 ppb during the eight-year period. Distinctive seasonal variations were observed with summer averages (±standard deviation) of 3.9 ± 1.6 ppb and winter averages of 1.1 ± 0.6 ppb. Two other datasets, weekly/biweekly passive samples of NH3 monitored at 74 agricultural and remote sites across southern Ontario during the period of June 2006 to March 2007 (Dataset 2) and semi-continuous measurements of NH3 and ammonium (pNH4+) in PM2.5 collected at a site inside a street canyon approximately 170 m away from the downtown site in December 2008 and February, March and May 2009 (Dataset 3), were further used to evaluate the potential NH3 sources in downtown Toronto. The NH3 mixing ratios at the downtown location were higher than those at the surrounding agricultural sites, and the mixing ratios within the street canyon were almost double those measured above it. These results suggested that the observed NH3 at the downtown location was mainly from local sources within the street canyon. Analysis of Dataset 3 showed that the diurnal cycles of NH3 at the downtown location cannot be simply explained by traffic and water consumption patterns. A green space inside the street canyon was identified as a potential important source contributing to the observed NH3 when T > 0 °C. However, the major NH3 emission sources at T ≤ 0 °C are yet to be investigated.

Fast In Situ Airborne Measurement of Ammonia Using a Mid-Infrared Off-Axis ICOS Spectrometer

A new ammonia (NH3) analyzer was developed based on off-axis integrated cavity output spectroscopy. Its feasibility was demonstrated by making tropospheric measurements in flights aboard the Department of Energy Gulfstream-1 aircraft. The ammonia analyzer consists of an optical cell, quantum-cascade laser, gas sampling system, control and data acquisition electronics, and analysis software. The NH3 mixing ratio is determined from high-resolution absorption spectra obtained by tuning the laser wavelength over the NH3 fundamental vibration band near 9.67 μm. Excellent linearity is obtained over a wide dynamic range (0-101 ppbv) with a response rate (1/e) of 2 Hz and a precision of ±90 pptv (1σ in 1 s). Two research flights were conducted over the Yakima Valley in Washington State. In the first flight, the ammonia analyzer was used to identify signatures of livestock from local dairy farms with high vertical and spatial resolution under low wind and calm atmospheric conditions. In the second flight, the analyzer captured livestock emission signals under windy conditions. Our results demonstrate that this new ammonia spectrometer is capable of providing fast, precise, and accurate in situ observations of ammonia aboard airborne platforms to advance our understanding of atmospheric compositions and aerosol formation.

Dimethylamine and ammonia measurements with ion chromatography during the CLOUD4 campaign

Abstract. The CLOUD project investigates the influence of galactic cosmic rays on the nucleation of new particles in an environmental chamber at CERN. Dimethylamine (DMA) was injected intentionally into the CLOUD chamber to reach atmospherically relevant levels away from sources (up to 100 pptv) in order to study its effect on nucleation with sulphuric acid and water at 278 K. Quantification of DMA and also background ammonia (NH3) was performed with ion chromatography (IC). The IC method used together with the sampling line developed for CLOUD in order to measure NH3 and DMA at low pptv levels is described; the overall sampling efficiency of the method is discussed; and, finally, mixing ratios of NH3 and DMA measured during CLOUD4 are reported.

Ammonia sources in the California South Coast Air Basin and their impact on ammonium nitrate formation

Observations from the NOAA WP‐3D aircraft during CalNex in May and June 2010 are used to quantify ammonia (NH3) emissions from automobiles and dairy facilities in the California South Coast Air Basin (SoCAB) and assess their impact on particulate ammonium nitrate (NH4NO3) formation. These airborne measurements in the SoCAB are used to estimate automobile NH3 emissions, 62 ± 24 metric tons day−1, and dairy facility NH3 emissions, 33 ± 16 to 176 ± 88 metric tons day−1. Emission inventories agree with the observed automobile NH3:CO emission ratio, but substantially underpredict dairy facility NH3 emissions. Conditions observed downwind of the dairy facilities were always thermodynamically favorable for NH4NO3 formation due to high NH3 mixing ratios from the concentrated sources. Although automobile emissions generated lower NH3 mixing ratios, they also can thermodynamically favor NH4NO3 formation. As an aerosol control strategy, addressing the dairy NH3 source would have the larger impact on reducing SoCAB NH4NO3 formation.

TES ammonia retrieval strategy and global observations of the spatial and seasonal variability of ammonia

Abstract. Presently only limited sets of tropospheric ammonia (NH3) measurements in the Earth's atmosphere have been reported from satellite and surface station measurements, despite the well-documented negative impact of NH3 on the environment and human health. Presented here is a detailed description of the satellite retrieval strategy and analysis for the Tropospheric Emission Spectrometer (TES) using simulations and measurements. These results show that: (i) the level of detectability for a representative boundary layer TES NH3 mixing ratio value is ~0.4 ppbv, which typically corresponds to a profile that contains a maximum level value of ~1 ppbv; (ii) TES NH3 retrievals generally provide at most one degree of freedom for signal (DOFS), with peak sensitivity between 700 and 900 mbar; (iii) TES NH3 retrievals show significant spatial and seasonal variability of NH3 globally; (iv) initial comparisons of TES observations with GEOS-CHEM estimates show TES values being higher overall. Important differences and similarities between modeled and observed seasonal and spatial trends are noted, with discrepancies indicating areas where the timing and magnitude of modeled NH3 emissions from agricultural sources, and to lesser extent biomass burning sources, need further study.

Atmospheric ammonia measurements in Houston, TX using an external-cavity quantum cascade laser-based sensor

Abstract. In order to improve the current understanding of the dynamics of ammonia (NH3) in a major industrial and urban area, intensive measurements of atmospheric NH3 were conducted in Houston during two sampling periods (12 February 2010–1 March 2010 and 5 August 2010–25 September 2010). The measurements were performed with a 10.4-μm external cavity quantum cascade laser (EC-QCL)-based sensor employing conventional photo-acoustic spectroscopy. The mixing ratio of NH3 ranged from 0.1 to 8.7 ppb with a mean of 2.4 ± 1.2 ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1 ± 2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH3 mixing ratios existed between weekdays and weekends. This study suggests that NH3 mixing ratios in Houston occasionally exceeded previous modeling predictions when sporadic and substantial enhancements occurred, potentially causing profound effects on particulate matter formation and local air quality.

Airborne observations of ammonia and ammonium nitrate formation over Houston, Texas

Anthropogenic emissions of NOx (nitric oxide (NO) + nitrogen dioxide (NO2)), which in sunlight can be oxidized to form nitric acid (HNO3), can react with ammonia (NH3) to form ammonium nitrate particles. Ammonium nitrate formation was observed from the NOAA WP‐3D aircraft over Houston during the 2006 Texas Air Quality Study with fast‐response measurements of NH3, HNO3, particle composition, and particle size distribution. Typically, NH3 mixing ratios over the urban area ranged from 0.2 to 3 ppbv and were predominantly from area sources. No NH3 enhancements were observed in emission plumes from power plants. The few plumes with high NH3 levels from point source emissions that were sampled are analyzed in detail. While the paucity of NH3 data in emission inventories made point source identification difficult, one plume was traced to NH3 release from an industrial accident. NH3 mixing ratios in these plumes ranged from 5 to 80 ppbv. In these plumes, the NH3 enhancement correlated with a decrease in HNO3 mixing ratio and an increase in particulate NO3− concentration indicating ammonium nitrate formation. The ammonium nitrate aerosol mass budget in the plumes was analyzed to assess the quantitative agreement between the gas and aerosol phase measurements. The thermodynamic equilibrium between the gas and aerosol phase was examined for one flight by comparing the modeled dissociation constant for ammonium nitrate with NH3 and HNO3 measurements. The high levels of NH3 in these plumes shifted the equilibrium toward favorable thermodynamic conditions for the condensation of ammonium nitrate onto particles.

A Chemical Ionization Mass Spectrometer for ambient measurements of Ammonia

Abstract. This study presents a chemical ionization mass spectrometer (CIMS) for fast response, in-situ measurements of gas phase ammonia (NH3). The NH3 background level detected with the CIMS ranged between 0.3–1 ppbv, with an uncertainty of 30 pptv under optimized conditions. The instrument sensitivity varied from 4–25 Hz/pptv for >1 MHz of reagent ion signals (protonated ethanol ions), with a 30% uncertainty estimated based on variability in calibration signals. The CIMS detection limit for NH3 was ~60 pptv at a 1 min integration time (3 sigma). The CIMS time response was <30 s. This new NH3-CIMS has been used for ambient measurements in Kent, Ohio, for several weeks throughout three seasons. The measured NH3 mixing ratios were usually at the sub-ppbv level and higher in spring (200 ± 120 pptv) than in winter (60 ± 75 pptv) and fall (150 ± 80 pptv). High emissions of SO2 from power plants in this region, and thus possible high acidity of aerosol particles, may explain these low NH3 mixing ratios in general.

Variations in Mixing Ratios of Ambient Ammonia, Nitric Oxide and Nitrogen Dioxide in Different Environments of India

° 38'N, 77 ° 10'E; 220m m.s.l.), India (Figure 1) during July 2009. Delhi, the capital city of India, located in the northern part of India. It is one of the ten most polluted cities of the world and third most polluted city of the country having numerous industries in and around the city. The total number of registered vehicles in the city Abstract Mixing ratios of ambient NH3, NO and NO2 were measured over three different environments viz Delhi, Dibrugarh and Thiruvananthapuram during 2009-10 to study their diurnal variations, concentration and source strength. The average mixing ratio of NH 3 over Delhi, Dibrugarh and Thiruvananthapuram were recorded as 13.24 ± 0.39, 13.46 ± 2.52 and 12.65 ± 1.51 ppb respectively. The average mixing ratio of NO2 was recorded as 12.69 ± 0.39 with maxima of 20.22 ppb and minima of 0.75 ppb over Delhi whereas, the mixing ratio of ambient NO 2 ranges from 0.65 to 3.65 ppb and 0.83 to 3.02 over Dibrugarh and Thiruvananthapuram respectively. The mixing ratio of NO and NO2 varying significantly at all the three locations except NH 3 might be due to source strength and meteorological conditions of the locations. Result reveals that the mixing ratio of ambient NH 3 is positively correlated with ambient temperature (r 2 = 0.872) and NO2 (r 2 = 0.975) over Delhi whereas non-significant at other locations.

Intercomparison of ammonia measurement techniques at an intensively managed grassland site (Oensingen, Switzerland)

Abstract. As part of a field campaign in the framework of the NitroEurope project, three different instruments for atmospheric ammonia (NH3) measurements were operated side-by-side on a managed grassland site in Switzerland: a modified Proton Transfer Reaction Mass Spectrometer (PTR-MS), a GRadient of AErosol and Gases Online Registrator (GRAEGOR), and an Automated Ammonia Analyzer (AiRRmonia). The modified PTR-MS approach is based on chemical ionization of NH3 using O2+ instead of H3O+ as ionizing agent, GRAEGOR and AiRRmonia measure NH4+ in liquids after absorption of gaseous NH3 in a rotating wet-annular denuder and through a gas permeable membrane, respectively. Bivariate regression slopes using uncorrected data from all three instruments ranged from 0.78 to 0.97 while measuring ambient NH3 levels between 2 and 25 ppbv during a 5 days intercomparison period. Correlation coefficients r2 were in the range of 0.79 to 0.94 for hourly average mixing ratios. Observed discrepancies could be partly attributed to temperature effects on the GRAEGOR calibration. Bivariate regression slopes using corrected data were >0.92 with offsets ranging from 0.22 to 0.58 ppbv. The intercomparison demonstrated the potential of PTR-MS to resolve short-time NH3 fluctuations which could not be measured by the two other slow-response instruments. During conditions favoring condensation in inlet lines, the PTR-MS underestimated NH3 mixing ratios, underlining the importance of careful inlet designs as an essential component for any inlet-based instrument.

A chemical ionization mass spectrometry technique for airborne measurements of ammonia

A chemical ionization mass spectrometer (CIMS) utilizing protonated acetone dimer ion chemistry to measure gas‐phase ammonia (NH3) from the NOAA WP‐3D aircraft is described. The average sensitivity determined from in‐flight standard addition calibrations ranged from 2.6 to 5 ion counts s−1 pptv−1, depending on flow conditions, for 1 MHz of reagent ion signal. The instrument time response was determined to be 5 s from the 2 e‐folding signal decay time after removal of a standard addition calibration. The instrumental background varied from flight to flight ranging from 0.5 to 1.3 ppbv. The variability between successive background measurements ranged from 50 pptv to 100 pptv. Total uncertainty for the 5 s data was conservatively estimated to be ±(30% + 125 pptv). Two NH3 sources were sampled during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS‐ITCT) 2004 campaign, one urban and one agricultural. During the 25 July flight, enhancements in NH3 mixing ratios were coincident with enhancements in CO, NOx, and SO2 mixing ratios downwind of New York City. The NH3 mixing ratios in the urban outflow plume ranged from 0.4 to 1 ppbv, or enhancements of 0.2 to 0.8 ppbv above local background. During the 15 August flight, NH3 mixing ratios were enhanced 0.3 to 0.45 ppbv above local background directly downwind of an agricultural area northeast of Atlanta, Georgia. The NH3 CIMS instrument has shown the ability to measure sub‐ppbv NH3 levels at high time resolution from an aircraft.

Analysis of urban gas phase ammonia measurements from the 2002 Atlanta Aerosol Nucleation and Real‐Time Characterization Experiment (ANARChE)

Gas phase ammonia (NH3) measurements were made in July and August 2002 during the Atlanta Aerosol Nucleation and Real‐Time Characterization Experiment with two different chemical ionization mass spectrometry techniques. Correlations between the 1 min data from both instruments yielded a slope of 1.17 and an intercept of −0.295 ppbv, with a linear correlation coefficient (r2) of 0.71. Ambient NH3 mixing ratios ranged from 0.4 to 13 ppbv. NH3 observations were compared to the Community Multiscale Air Quality (CMAQ) modeling system as well as a thermodynamic equilibrium model, ISORROPIA, used by CMAQ to predict NH3 partitioning. A morning rise in both observed and modeled NH3 mixing ratios strongly suggests a regional influence due to automobile emissions. However, at midday the predicted NH3 decreased to less than 0.5 ppbv, while the observations remained around 3 ppbv. Both observed and modeled ammonium nitrate levels were too low to support the observed midday NH3 mixing ratios. ISORROPIA calculations of NH3 constrained by the total measured ammonia mass (NH3 + ammonium (NH4+)) agreed well with the observations (slope of 1.25 and r2 of 0.75). For times when the net aerosol charge was near zero the agreement was excellent (slope of 1.22 and r2 of 0.88). These results indicate that for most of the observed conditions, ISORROPIA could accurately predict NH3 partitioning. The observations suggest that local sunlight‐ or temperature‐driven NH3 sources, such as soil emissions, may be responsible for the discrepancy between the model results and measured values.

High precision determinations of NH3 concentration by means of diode laser spectrometry at 2.005 μm

This paper reports on the development of a compact analyzer for ammonia monitoring in air, based on a distributed feedback diode laser at 2.005 μm. A dual-beam long-path technique was combined with wavelength modulation detection of absorption in order to reach a detection limit of about 25 ppb, with a 0.2-Hz equivalent noise bandwidth. Retrieval of NH3 concentration was accomplished through a careful spectra analysis procedure based on the formalism of Fourier expansion of the 2nd harmonic signals, also taking into account residual-amplitude-modulation effects. The system was tested on certified gas-mixtures and revealed a good performance in terms of accuracy and reproducibility. Particularly, the short-term precision was found to be about 1 ‰, for NH3 mixing ratios of 10 ppm. Finally, the possibility to make use of the analyzer for measurements of ammonia fluxes from soils is discussed.

Application of a widely electrically tunable diode laser to chemical gas sensing with quartz-enhanced photoacoustic spectroscopy.

A near-infrared diode laser with sample-grating distributed Bragg reflectors was used as a widely tunable spectroscopic source for multispecies chemical sensing. Quartz-enhanced photoacoustic spectroscopy was utilized to obtain high absorption sensitivity in a compact gas cell. CO2, H2O C2H2, and NH3 were monitored. A noise equivalent sensitivity of 8 x 10(-9) cm(-1) W(-1) Hz(-1/2)for NH3 detection was achieved, which corresponds to a NH3 mixing ratio of 4.4 parts in 10(6) by volume (ppmv) with a 1-s time constant and available 5.2-mW optical power in the gas cell.

The far-infrared spectra of Jupiter and Saturn

We present far-infrared, disk-averaged spectra of Jupiter and Saturn that were measured with the Long Wavelength Spectrometer aboard the Infrared Space Observatory. The Long Wavelength Spectrometer covered the wavelength region between 43 and 197μm and achieved in grating mode a resolving power of 150–300.The spectra observed in the far infrared are compared to synthetic spectra calculated from atmospheric radiative transfer models using expected values for the constituent vertical concentration profiles. Rotational transitions of ammonia and phosphine are responsible for the broad absorption features observed: Strong ammonia absorption manifolds are obvious against the background continuum slope, appearing at 63, 72, 84, 101 and 125μm in both Jupiter and Saturn. Also phosphine absorption features are clearly present; we found them at the expected wavelengths of 113 and 141μm in both planets and at 81, 87, 94, 103, 126 and 160μm on Saturn only.Our findings are in good agreement with the results obtained with other instruments, in particular the Short Wavelength Spectrometer of the Infrared Space Observatory. On Jupiter we obtained the best fit to the observations with models having an NH3 mixing ratio of 2×10−4. We fitted Saturn's measured spectrum with models where the ammonia mixing ratio amounted to 10−4. For phosphine we found an abundance of 7−3.5+7×10−6 in the troposphere of this planet.

Ternary nucleation of H2SO4, NH3, and H2O in the atmosphere

Classical theory of binary homogeneous nucleation is extended to the ternary system H2SO4‐NH3‐H2O. For NH3 mixing ratios exceeding about 1 ppt, the presence of NH3 enhances the binary H2SO4‐H2O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H2SO4‐NH3‐H2O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H2SO4‐NH3‐H2O ternary nucleation rate is independent of relative humidity over a large range of H2SO4 concentrations has wide atmospheric consequences. The limiting component for ternary H2SO4‐NH3‐H2O nucleation is, as in the binary H2SO4‐H2O case, H2SO4; however, the H2SO4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.