Determination of Atmospheric Ammonia Deposition: Sampling Approaches, Analytical Methods, and Applic

Atmospheric ammonia deposition determination quantifies the transfer of reactive nitrogen from the atmosphere to terrestrial and aquatic ecosystems through both wet and dry pathways. Reliable measurement of ammonia concentrations in air and precipitation, combined with validated flux calculation methods, is essential for controlling eutrophication, secondary PM2.5 formation, and ecosystem degradation. This article reviews the sampling techniques, analytical procedures, applicable standards, and practical applications of atmospheric ammonia deposition determination.

Technical Overview

Atmospheric ammonia (NH3) is the most abundant alkaline trace gas in the ambient atmosphere. It is emitted predominantly from agricultural activities, including livestock manure management and volatilization from nitrogen fertilizers, with additional contributions from biomass burning, industrial processes, and motor vehicles equipped with three-way catalysts or selective catalytic reduction systems. Once released, ammonia reacts with acidic gases such as sulfuric acid and nitric acid to form ammonium salts, which are major components of secondary fine particulate matter (PM2.5).

Ammonia returns to the Earth's surface through two principal pathways. Wet deposition occurs when gaseous NH3 and particulate ammonium (NH4+) are scavenged by precipitation and delivered to the surface in rain, snow, or hail. Dry deposition occurs through turbulent transfer and direct absorption of gaseous NH3 and ammonium-containing aerosols onto surfaces during periods without precipitation. Deposited reactive nitrogen drives soil acidification, water eutrophication, biodiversity loss in sensitive ecosystems, and exceedance of empirical critical loads for nitrogen.

The determination of atmospheric ammonia deposition therefore involves two complementary tasks: measuring ammonia concentrations in air and ammonium concentrations in precipitation using standardized sampling and analytical techniques, and converting these concentrations into deposition fluxes, typically expressed in kilograms of nitrogen per hectare per year (kg N ha⁻¹ yr⁻¹). Because dry deposition cannot be measured as directly as wet deposition, concentration measurements are commonly combined with modeled deposition velocities to estimate total nitrogen inputs to ecosystems.

Testing Samples

Gaseous Ammonia in Ambient Air

Airborne NH3 is captured on acid-impregnated substrates, such as filters coated with citric acid or phosphoric acid, or on the inner walls of coated denuder tubes. Exposed substrates are desorbed in the laboratory, and the extracted ammonium is quantified to determine the time-averaged atmospheric NH3 concentration.

Precipitation Samples

Rainwater and snowmelt are collected using wet-only collectors, which open automatically during precipitation events, or bulk collectors, which remain open continuously. Precipitation samples are transferred to clean polyethylene bottles, preserved as required, and analyzed for dissolved ammonium and related major ions.

Particulate Ammonium Aerosols

Ammonium bound to PM2.5 or PM10 particles is collected on Teflon or quartz fiber filters using dichotomous or gravimetric samplers. These samples allow the ammonium contribution of aerosols to total dry deposition to be distinguished from gaseous NH3.

Throughfall and Surface Deposition Samples

In ecological studies, throughfall collectors placed beneath vegetation canopies capture precipitation modified by canopy interception of both wet and dry deposited ammonia, providing an integrated estimate of total nitrogen deposition to the ecosystem.

Testing Items

Typical parameters determined in atmospheric ammonia deposition studies include:

• Ambient gaseous NH3 concentration, expressed in micrograms per cubic meter (µg/m³)
• Particulate NH4+ concentration in PM2.5 and PM10 fractions
• Dissolved ammonium concentration in precipitation, expressed in milligrams per liter (mg/L)
• Wet deposition flux of ammonium nitrogen (kg N ha⁻¹ yr⁻¹)
• Dry deposition flux of gaseous NH3 and particulate ammonium
• Total reactive nitrogen deposition and critical load exceedance values
• Deposition velocities used in inferential flux calculations
• Ancillary ions (sulfate, nitrate, calcium, potassium) for source apportionment and ion balance verification

Testing Methods

Passive Diffusive Sampling

Passive samplers, such as diffusion tubes and membrane-based high-absorption samplers, expose an acid-coated absorbent to ambient air for periods of one week to one month. Ammonia diffuses along a defined geometry and is trapped quantitatively, allowing the time-averaged concentration to be calculated from Fick's law of diffusion. Passive samplers are inexpensive, require no power, and are well suited to dense spatial monitoring networks. The European standard EN 17346:2020 specifies a standardized method for the determination of ammonia in ambient air using diffusive samplers.

Active Denuder Sampling

Annular denuder systems draw air at a controlled flow rate through coated glass tubes that selectively strip gaseous NH3 before particles are collected on a downstream filter. Long-term denuder-based systems provide monthly-resolution speciation of gaseous NH3 and particulate NH4+, while continuous-flow denuder analyzers coupled to online detection can deliver hourly or sub-hourly data with high sensitivity. Denuder methods minimize the positive and negative artifacts that affect simpler filter techniques.

Filter Pack Method

Multi-stage filter packs combine a particle-collecting Teflon filter with subsequent acid-impregnated backup filters. The first stage retains particulate ammonium while the backup stages capture gaseous NH3 that volatilizes from the particles or passes through. The method is widely used in monitoring networks because of its simplicity, although volatilization artifacts from ammonium nitrate must be carefully evaluated.

Wet Deposition Collection and Analysis

Wet-only and bulk collectors following the sampling principles of the GB 13580 series for precipitation collection and analysis are used to gather precipitation samples. Ammonium in the filtered samples is determined by spectrophotometry or ion chromatography, and the wet deposition flux is calculated as the precipitation-weighted ammonium concentration multiplied by precipitation depth.

Analytical Determination Techniques

Several validated analytical methods are available for quantifying extracted ammonium:

• Nessler's reagent spectrophotometry and sodium salicylate spectrophotometry, as specified in HJ 533-2009 and HJ 534-2009 for ammonia determination in ambient air and exhaust gas
• Indophenol blue (Berthelot reaction) spectrophotometry, widely applied to precipitation and extract solutions
• Ion chromatography with conductivity detection, which simultaneously quantifies ammonium and other major ions with low detection limits
• Flow injection analysis with gas diffusion, providing automated high-throughput determination

Micrometeorological Flux Measurement

At research sites, the eddy covariance technique directly measures vertical NH3 fluxes using fast-response laser spectrometers based on quantum cascade laser, tunable diode laser, or cavity ring-down spectroscopy. The aerodynamic gradient method offers an alternative in which vertical concentration profiles measured at multiple heights are combined with turbulence parameters to derive fluxes.

Inferential Flux Calculation

In most routine networks, dry deposition fluxes are estimated by multiplying measured air concentrations of NH3 and NH4+ by modeled deposition velocities. Deposition velocities are derived from resistance-based models that account for atmospheric turbulence, surface characteristics, vegetation type, and seasonal canopy behavior. Total deposition is then obtained as the sum of wet, dry particulate, and dry gaseous components.

Applications

Determination of atmospheric ammonia deposition supports a wide range of environmental management and scientific objectives:

• Assessment of nitrogen critical load exceedance in nature reserves, wetlands, forests, and other sensitive ecosystems
• Verification of ammonia emission reduction measures around livestock, poultry, and fertilizer production facilities
• Development of PM2.5 control strategies by quantifying the ammonia available for secondary inorganic aerosol formation
• Long-term trend analysis within national and international deposition monitoring networks
• Environmental impact assessment and permitting for agricultural and industrial projects
• Validation and refinement of chemical transport models and atmospheric nitrogen budget studies
• Eutrophication and acidification risk evaluation for soils, lakes, and coastal waters

FAQ

What is the difference between wet and dry ammonia deposition?

Wet deposition refers to ammonia and ammonium removed from the atmosphere by precipitation and delivered in rain or snow, and it is measured by chemically analyzing collected precipitation. Dry deposition refers to the direct turbulent transfer and surface absorption of gaseous NH3 and ammonium aerosols during non-precipitation periods, and it is typically estimated from measured concentrations combined with modeled deposition velocities.

How are concentrations converted into deposition fluxes?

Wet deposition flux is calculated as the precipitation-weighted mean ammonium concentration multiplied by the precipitation depth. Dry deposition flux is calculated as the measured air concentration multiplied by the deposition velocity, which is derived from resistance models considering meteorology, surface roughness, and vegetation uptake characteristics. Total deposition is the sum of all pathways.

What sampling durations are typically used?

Passive samplers are usually exposed for two weeks to one month to accumulate sufficient ammonia for accurate analysis. Active denuder and filter pack systems commonly operate on weekly or monthly cycles. Wet deposition samples are collected per precipitation event or as weekly composite samples. Continuous optical analyzers can provide hourly or sub-hourly resolution.

Which standards and methods govern the determination?

Relevant references include EN 17346:2020 for ammonia determination using diffusive samplers, the HJ 533-2009 and HJ 534-2009 spectrophotometric methods for ammonia in ambient air, and the GB 13580 series for the collection and analysis of precipitation samples. Ion chromatographic procedures following recognized standard methods are also widely accepted for ammonium quantification.

What quality assurance measures are applied?

Standard quality assurance includes field blanks and travel blanks, duplicate or co-located samplers, breakthrough checks on backup absorbent stages, certified reference materials, multi-point calibration curves, ion balance and conductivity checks for precipitation samples, and periodic intercomparison between passive and active measurement systems. Accredited third-party laboratories typically report method detection limits, recovery rates, and measurement uncertainty with each data set.

What detection limits can be achieved?

Spectrophotometric determination of ammonium in aqueous extracts typically achieves detection limits on the order of 0.01 mg/L, while ion chromatography can reach lower levels with excellent precision. For monthly passive sampler exposures, ambient NH3 concentrations as low as approximately 0.05 to 0.1 µg/m³ can be reliably quantified, which is sufficient for background rural sites as well as source-impacted areas.