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"Development of Statewide Inventory Estimates of Ammonia Emissions from Native Soils and Crop Fertilizers in California"
Produced in collaboration with Charles Krauter, CIT California State University Fresno and Michael Benjamin, California Air Resources Board

This work was funded by the California Air Resources Board & NASA Ames Research Center.

Ammonia (NH3) is the dominant gaseous base in the atmosphere and a principal neutralizing agent for atmospheric acids.

Volatilized ammonia:

  • represents a reduction in fertilizer use efficiency by crops;
  • acts as an atmospheric buffer by reacting with NOx and SOx from combustion.

NH4NOx and NH4SOx are components of PM2.5, an air quality particulate problem that may require regulation.

In California, the estimated patterns of N deposition suggest that for areas close to photochemical smog areas, concentrations of the oxidized forms of N (NO2, HNO3) dominate, whereas in areas near agricultural activities the importance of reduced N forms (NH3 and NH4+) increases.

The magnitude and distribution of NH3 emissions from fertilizer application and other agricultural sources are still largely undetermined.


Overall: To estimate NH3 emissions from a variety of major crop-fertilizer combinations and from native soils in California.

Phase One:

  1. Develop a simulation model for NH3 emissions from native soils using satellite image drivers and other spatial data.
  2. Evaluate an active sampler for monitoring NH3 emissions from fertilized fields and background soil emission levels.

Modeling Methods California Model Diagram

As a regional model application to the state of California, we built on the foundation of the NASA-CASA ecosystem trace gas model. Vegetation greenness estimates from satellite data were used as inputs to estimate productivity of ecosystems and associated nitrogen mineralization rates in soils, which provide the substrate for potential NH3 volatilization. Predicted NH3 fluxes from the model were constrained by a new series of field measurements from cropped areas in the Central Valley.

Field Methods

  1. Ammonia Trap Preparation
    • Glass filters impregnated with 3% citric acid in 95% ethanol were assembled in the laboratory.
    • NH3 forms ammonium citrate on the treated filter.
    • In the lab, micrograms (µg) of NH3 on the filters were determined by: (1) dissolving the ammonium in de-ionized water and (2) measuring concentrations using Nessler reagent and a spectrophotometer.
Ammonia Field Trap
Ammonia Field Trap Bottom
  • Estimate NH3 at four heights
    • Wind speed was monitored at each elevation and values averaged for the sampling period.
    • Sample concentration of NH3 (µg m-3) of air multiplied by the average wind speed (m s-1) to get the NH3 flux (µg NH3 m-2s-1) during the sampling period.

  • Determine NH3 Flux Rates
  • Ammonia Flux Tower
    Ammonia Flux Plot
    Initial Field Results
    Fertilizer Application on Tomatoes NH3 Flux over Tomatoes

    Fertilizer Application on Cotton NH3 Flux over Cotton

    • Time series plots show peak emissions shortly after fertilizer application (filled symbols).
    • Emission factors are highly consistent among most fertilizer application forms and methods.

    Modeling Results

    Soil pH   NPP   N Mineralization

    Soil N-NH3 Emission   Fertilizer N-NH3 Emission

    • Estimated annual emissions of N-NH3 from native soils in California were 14 x 106 kg from cropland and
      16 x 106 kg from non-cropland (mainly coniferous forest) areas.
    • Estimated annual emissions of N-NH3 from chemical fertilizer were 23 x 106 kg.

    Future Work

    • Improve the characterization of the NH3 flux gradients with additional crop sites.
    • Technique will be used to monitor emissions from livestock operations.

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