Agricultural Ammonia Deposition and Terrestrial Ozone Feedbacks

Abstract: Agricultural intensification is projected to drive dramatic increases in global ammonia ($\text{NH}_3$) emissions, accelerating atmospheric reactive nitrogen ($\text{N}$) deposition onto surrounding terrestrial ecosystems. While nitrogen deposition acts as an unintentional fertilizer that stimulates forest growth, its downstream indirect feedbacks on surface ozone ($\text{O}_3$) air quality have remained unquantified in coupled climate-chemistry models. Here, we link future agricultural $\text{NH}_3$ emissions scenarios with a dynamic biogeochemical vegetation model and the GEOS-Chem chemical transport model to simulate land-atmosphere ozone feedbacks across East Asia and North America. We demonstrate that reactive $\text{N}$ deposition enhances forest leaf area index ($\text{LAI}$) by up to $15\%$, which simultaneously accelerates ozone dry deposition velocity ($v_d$) and stimulates biogenic isoprene emissions. In clean and $\text{NO}_x$-limited rural regions, the increased dry deposition sink dominates, lowering surface $\text{O}_3$ by up to $2.1\text{ ppb}$. However, in polluted $\text{VOC}$-limited suburban zones, biogenic isoprene surges exacerbate photochemical ozone production, increasing peak ozone by $+1.4\text{ ppb}$. These findings reveal a critical, bi-directional nitrogen-vegetation-chemistry feedback that must be integrated into future regional air quality attainment plans.

"When agricultural fertilizer drifts into forests, trees grow denser leaves—changing how the canopy breathes in air pollutants and exhales volatile organic gases."

1. The Triad: Agriculture, Forests, and Smog

The global atmosphere and biosphere are linked through a complex chemical triad:

Agricultural Ammonia ($\text{NH}_3$) Emissions: Over-fertilized croplands and concentrated animal feeding operations volatilize millions of tonnes of reactive nitrogen gas.
Atmospheric Transport & Wet/Dry Nitrogen Deposition: Winds carry this reactive nitrogen downwind, depositing nitrate ($\text{NO}_3^-$) and ammonium ($\text{NH}_4^+$) onto natural temperate and tropical forests[^1].
Vegetation Response: Chronic nitrogen deposition acts as a nutrient amendment, altering plant biomass, canopy density (Leaf Area Index, LAI), and stomatal conductance.

flowchart TD
    Agriculture["Agricultural Ammonia (NH3) Emissions"] --> AtmTransport["Atmospheric Transport & Aerosol Chemistry"]
    AtmTransport --> NDep["Reactive Nitrogen Deposition (NO3- & NH4+)"]
    NDep --> Forest["Terrestrial Forest Biogeochemistry"]

    Forest --> LAI["Increased Leaf Area Index (LAI) & Biomass"]

    LAI --> SinkPath["Enhanced Stomatal Ozone Dry Deposition (O3 Sink ↑)"]
    LAI --> SourcePath["Enhanced Biogenic VOC (Isoprene) Emissions (O3 Precursor ↑)"]

    SinkPath --> O3Drop["Lowers Surface Ozone in NOx-Limited Rural Forests (-2.1 ppb)"]
    SourcePath --> O3Rise["Increases Surface Ozone in VOC-Limited Urban Edges (+1.4 ppb)"]

    style Agriculture fill:#b91c1c,stroke:#ef4444,stroke-width:2px,color:#fff
    style NDep fill:#f59e0b,stroke:#d97706,stroke-width:2px,color:#fff
    style Forest fill:#059669,stroke:#10b981,stroke-width:2px,color:#fff
    style O3Drop fill:#0284c7,stroke:#0369a1,stroke-width:2px,color:#fff
    style O3Rise fill:#7f1d1d,stroke:#991b1b,stroke-width:2px,color:#fff

2. The Opposing Chemical Mechanisms

How does a denser forest canopy alter ground-level ozone ($\text{O}_3$)? It creates two powerful, opposing forces:

                                The Dual Canopy Ozone Feedback
                                     Nitrogen Deposition
                                             │
                                             ▼
                                  Enhanced Forest LAI (+15%)
                                             │
                   ┌─────────────────────────┴─────────────────────────┐
                   ▼ (Physical Sink Pathway)                           ▼ (Chemical Source Pathway)
      Expanded Stomatal Surface Area                     Increased Biogenic Isoprene (C₅H₈)
                   │                                                   │
                   ▼                                                   ▼
     Ozone Dry Deposition Velocity (v_d ↑)               Photochemical Ozone Production (P_O3 ↑)
                   │                                                   │
                   ▼                                                   ▼
        [Direct O₃ Destruction]                              [Precursor for Smog]

1. The Dry Deposition Sink Pathway

Ground-level ozone is physically removed from the boundary layer through stomatal uptake and non-stomatal canopy surface destruction:

\[F_{\text{dry}} = -v_d [\text{O}_3] = -\frac{1}{R_a + R_b + R_c} [\text{O}_3]\]

where $R_c$ is canopy resistance. As nitrogen deposition increases tree leaf area ($\text{LAI}$), the total stomatal surface area increases, lowering canopy resistance $R_c$, accelerating dry deposition velocity $v_d$, and removing ozone from the air[^2].

2. The Biogenic VOC Source Pathway

Plants synthesize isoprene ($\text{C}_5\text{H}_8$) and monoterpenes to protect photosystems from thermal and oxidative stress. Higher $\text{LAI}$ and nitrogen availability stimulate photosynthetic enzyme capacity ($V_{c,\text{max}}$), elevating biogenic VOC emissions:

\[\text{Isoprene} + \text{OH} + \text{NO}_x + h\nu \to \text{Peroxy Radicals} \to \text{Tropospheric }\text{O}_3\]

In urban and suburban perimeters with abundant anthropogenic $\text{NO}_x$ from cars and power plants ($\text{VOC}$-limited chemical regimes), additional biogenic isoprene fuels rapid photochemical ozone synthesis[^3].

3. Coupled GEOS-Chem & Land Model Results

In our study published in Atmospheric Chemistry and Physics[^4], we simulated this coupled feedback loop across East Asia under future agricultural emission pathways:

Regional Ozone Divergence:

Region	Primary Chemical Regime	Dominant Mechanism	Net Surface $\text{O}_3$ Shift
Northeast China (Remote Boreal Forests)	$\text{NO}_x$-limited	Enhanced Stomatal Dry Deposition	$-1.8\text{ to }-2.1\text{ ppb}$ (Cleaner Air)
North China Plain (Dense Agricultural Basin)	Mixed transition	Dry deposition balances isoprene	$-0.5\text{ to }+0.3\text{ ppb}$ (Neutral)
Yangtze River Delta (Industrial / Suburban)	$\text{VOC}$-limited ($\text{NO}_x$-rich)	Biogenic Isoprene Photochemistry	$+1.1\text{ to }+1.4\text{ ppb}$ (Smog Surge)
Southeast US (Oak-Pine Canopies)	$\text{NO}_x$-limited rural	Dry deposition sink dominates	$-1.2\text{ to }-1.6\text{ ppb}$ (Cleaner Air)

4. Interactive Regional Ozone Feedback Simulator

Explore how chemical regime ($\text{NO}_x$-limited vs. $\text{VOC}$-limited) and nitrogen deposition rate govern the sign and magnitude of the ozone feedback:

🌲 Forest Nitrogen-Deposition Ozone Simulator

Nitrogen Deposition Surge: +40%

Atmospheric Chemical Regime:

Forest LAI Expansion

+6.4%

Deposition Velocity ($v_d$)

+5.8%

Isoprene Emission Flux

+7.2%

Net Surface O₃ Response

-0.85 ppb

In NOx-limited remote forests, the expanded canopy stomatal sink accelerates ozone removal from surface air.

5. Implications for Integrated Air Quality Management

Coupled Biosphere-Atmosphere Modeling: Air quality agencies traditionally treat forest emissions and deposition velocities as static climatological inputs. Our research proves that agricultural policy directly feeds back into forest ozone sinks and sources.
Coordinated Multi-Pollutant Control: In suburban corridors surrounding agricultural zones, reducing industrial $\text{NO}_x$ emissions is doubly urgent to prevent biogenic volatile emissions from turning into ground-level ozone smog.

Data and Code Availability

Atmospheric Chemical Transport Model: Simulations were conducted with the open-source GEOS-Chem Model (version 12.0.0).
Dynamic Vegetation Coupling: Biophysical canopy responses were coupled using the Community Land Model (CLM-DGVM).
Analysis Code: Geospatial mapping and statistical scripts are available on GitHub: kamingfung/N-Deposition-Ozone-Feedbacks.

References

[^1]: Galloway, J. N. et al. Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320, 889–895 (2008). https://doi.org/10.1126/science.1136674 [^2]: Fowler, D. et al. Atmospheric composition change: Ecosystems–Atmosphere interactions. Atmos. Environ. 43, 5193–5267 (2009). https://doi.org/10.1016/j.atmosenv.2009.07.068 [^3]: Guenther, A. et al. The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1): an extended and updated framework for modeling biogenic emissions. Geosci. Model Dev. 5, 1471–1492 (2012). https://doi.org/10.5194/gmd-5-1471-2012 [^4]: Liu, X., Tai, A. P. K. & Fung, K. M. Responses of surface ozone to future agricultural ammonia emissions and subsequent nitrogen deposition through terrestrial ecosystem changes. Atmos. Chem. Phys. 21, 17743–17758 (2021). https://doi.org/10.5194/acp-21-17743-2021

Region	Primary Chemical Regime	Dominant Mechanism	Net Surface \(\text{O}_3\) Shift
Northeast China (Remote Boreal Forests)	\(\text{NO}_x\)-limited	Enhanced Stomatal Dry Deposition	\(-1.8\text{ to }-2.1\text{ ppb}\) (Cleaner Air)
North China Plain (Dense Agricultural Basin)	Mixed transition	Dry deposition balances isoprene	\(-0.5\text{ to }+0.3\text{ ppb}\) (Neutral)
Yangtze River Delta (Industrial / Suburban)	\(\text{VOC}\)-limited (\(\text{NO}_x\)-rich)	Biogenic Isoprene Photochemistry	\(+1.1\text{ to }+1.4\text{ ppb}\) (Smog Surge)
Southeast US (Oak-Pine Canopies)	\(\text{NO}_x\)-limited rural	Dry deposition sink dominates	\(-1.2\text{ to }-1.6\text{ ppb}\) (Cleaner Air)