Sulfuric acid outperforms boric acid for tracing ammonia back to its farm-field source

Atmospheric ammonia is the single most important alkaline gas in our air, and agriculture produces most of it. When ammonia reacts with sulfur dioxide and nitrogen oxides, it forms ammonium sulfate and ammonium nitrate - tiny particles that constitute a major fraction of PM2.5, the fine particulate matter linked to respiratory disease, cardiovascular damage, and premature death. Reducing ammonia emissions requires knowing exactly where they come from. That means distinguishing between a fertilized wheat field, a cattle feedlot, an apple orchard, and a vegetable greenhouse, often in the same agricultural region.

Nitrogen isotope ratios offer one of the most promising ways to do that. Different biological and chemical processes leave distinct isotopic fingerprints on the ammonia they release. But reading those fingerprints accurately depends on the sampling method, and a new study from the Chinese Academy of Sciences demonstrates that the choice of acid absorbent can make or break the measurement.

Two acids, very different results

The study, published in Nitrogen Cycling in January 2026 by a team led by Chaopu Ti, compared two standard methods for passively collecting atmospheric ammonia: sulfuric acid and boric acid absorption systems. Both use sponge samplers to capture ammonia gas, which is then analyzed for its nitrogen-15 to nitrogen-14 ratio - the isotopic signature that reveals the emission source.

In controlled laboratory experiments, the team volatilized ammonia from ammonium sulfate and three certified isotope reference materials (USGS-25, USGS-26, and IAEA-N1), each starting with 2.00 milligrams of ammonium nitrogen. They then captured the released gas using both acid systems and measured recovery efficiency, reproducibility, and isotope accuracy across ammonium concentrations ranging from 20 to 100 micromoles per liter.

The results were clear. Sulfuric acid at 0.1 moles per liter achieved ammonia recovery rates between 95.98% and 96.88%, with a mean of 96.43% and a coefficient of variation of just 0.47%. For the isotope reference standards, recovery ranged from 96.03% to 99.09%. Boric acid, by comparison, recovered only 80.47% to 86.48% of the ammonia, with a mean of 83.90% and noticeably greater variability.

Where boric acid falls short

The performance gap widened at low concentrations - exactly the conditions most common in real-world atmospheric sampling. When the researchers plotted conversion curves between the nitrogen isotope ratios of ammonium and nitrous oxide (the analytical intermediate), sulfuric acid maintained slopes close to the theoretical value of 0.5 across all concentrations, even before mathematical correction. Boric acid deviated sharply at 20 micromoles per liter, though its performance improved at higher concentrations after correction.

Both methods could reproduce certified isotope values within plus or minus 0.5 per mil, but sulfuric acid showed superior stability and lower interference from impurities. The practical implication is significant: boric acid can introduce isotope fractionation - a systematic bias in the measured ratio - particularly when ammonia concentrations are low. That fractionation can blur the isotopic differences between sources, making it harder to tell cropland emissions from livestock emissions.

From lab bench to farm field

The researchers validated their optimized sulfuric acid method in real agricultural environments, sampling ammonia from four source types. The isotopic signatures they measured were distinct and consistent with expected biological processes. Cropland emissions showed a nitrogen-15 value of -32.87 per mil, while livestock sources were even more depleted at -36.64 per mil. Orchards registered -19.63 per mil, and vegetable fields came in at -24.95 per mil.

The separation between these values is what makes source apportionment possible. Cropland and livestock emissions, both heavily depleted in nitrogen-15, reflect the strong kinetic fractionation that occurs during volatilization of urea and manure-derived ammonium. Orchard and vegetable emissions, less depleted, likely reflect different fertilization practices and soil chemistry. With sulfuric acid sampling, these distinctions emerge clearly. With boric acid, they risk being obscured by methodological noise.

Practical stakes for air quality policy

This is not an abstract methodological debate. China is the world's largest emitter of agricultural ammonia, and ammonia-driven secondary particulate formation is a dominant contributor to severe haze events across eastern China. Identifying which agricultural sectors contribute most to regional ammonia loads is essential for designing effective emission reduction strategies - and those strategies have direct consequences for public health.

If isotope-based source apportionment incorrectly attributes emissions to the wrong source, policy interventions may target the wrong practices. A fertilizer management regulation aimed at cropland does little good if the actual ammonia is coming primarily from livestock operations, or vice versa. The accuracy of the underlying measurement method determines whether policy targets the right problem.

Limitations and remaining questions

The study has constraints worth noting. The laboratory experiments used a limited set of reference materials and concentration ranges. Real atmospheric conditions introduce additional variables - humidity, temperature fluctuations, co-pollutants, and varying exposure times - that could affect absorbent performance in ways the controlled experiments did not capture.

The field validation, while demonstrating clear source separation, was conducted in a single agricultural region of China. Whether the isotopic signatures hold across different climates, soil types, and farming practices remains to be confirmed through broader geographic sampling. The study also did not compare sulfuric acid absorption against newer active sampling methods or spectroscopic techniques that bypass wet chemistry entirely.

And while the method excels at distinguishing between broad source categories, finer distinctions - between different fertilizer types, for instance, or between confined and free-range livestock operations - may require complementary approaches such as dual-isotope analysis or spatially resolved atmospheric modeling.

The broader context of nitrogen cycle research also matters here. Atmospheric ammonia is just one component of the global nitrogen problem, which includes nitrate contamination of groundwater, nitrous oxide emissions contributing to climate change, and eutrophication of coastal waters. Accurate source apportionment of ammonia is a prerequisite for integrated nitrogen management strategies that address multiple environmental endpoints simultaneously. A measurement method that performs reliably across the range of conditions encountered in field monitoring - including the low concentrations that prevail downwind of agricultural sources - is a necessary foundation for that work.

The choice of absorbent may seem like a minor technical detail in the context of a global pollution problem. But environmental science is built on measurement, and measurement quality determines whether the science is trustworthy enough to support policy. In this case, the difference between sulfuric acid and boric acid is the difference between a clear isotopic signal and a noisy one - between correctly identifying where ammonia comes from and guessing wrong.

For researchers and regulators working to untangle the sources of atmospheric ammonia in agricultural regions, the study offers a concrete, validated improvement to standard practice. Sulfuric acid at 0.1 moles per liter, used in passive sponge samplers, provides higher recovery, lower variability, and more reliable isotope measurements than the boric acid alternative - particularly at the low concentrations that typify ambient air monitoring.

The method is also inexpensive and field-deployable, requiring no specialized equipment beyond standard passive sponge samplers. That accessibility matters for monitoring networks in developing countries where agricultural ammonia emissions are rising rapidly but analytical resources are limited. A measurement upgrade that improves accuracy without increasing cost or complexity has the potential for widespread adoption - and the downstream consequences for nitrogen management and air quality policy could be substantial.