AUS 32 and SCR Systems: How They Reduce NOx Emissions from Diesel Engines

What Is AUS 32?

AUS 32 stands for Aqueous Urea Solution 32.

Under ISO 22241, it is a standardized solution containing 32.5% technically pure urea by mass in pure water.

The product is specifically intended for use as a NOx-reduction agent in SCR converter systems.

The 32.5% concentration is not an arbitrary commercial formulation. It forms part of the technical specification of AUS 32 and is important for reliable SCR operation.

AUS 32 is stored in a dedicated tank on the vehicle or equipment.

It is not mixed with diesel fuel.

The SCR system draws the required quantity of AUS 32 from this separate tank and injects it into the exhaust stream based on engine operating conditions and the emission-control system's requirements.

AUS 32, DEF, and AdBlue®: Understanding the Terminology

Several names are encountered in the market, but they should not be treated identically.

AUS 32 is the technical designation used in the ISO 22241 standard.

DEF, or Diesel Exhaust Fluid, is another widely used term for this type of solution, particularly in North America. ISO documentation also recognizes DEF as terminology associated with AUS 32.

AdBlue®, however, is different from these generic technical terms.

AdBlue® is a registered trademark owned by the German Association of the Automotive Industry (Verband der Automobilindustrie — VDA).

The trademark is licensed by the VDA for commercial use in accordance with its licensing requirements.

For this reason, the term AdBlue® should not automatically be used as a generic substitute for AUS 32.

A product may conform to the applicable technical requirements for AUS 32 without necessarily being authorized to use the AdBlue® trademark.

Throughout this article, therefore, AUS 32 is used as the primary technical term.

What Is Selective Catalytic Reduction?

Selective Catalytic Reduction is an exhaust after-treatment technology designed primarily to reduce nitrogen oxide emissions.

NOx is produced during combustion, particularly under high-temperature conditions.

Rather than attempting to eliminate all NOx formation inside the engine, SCR treats NOx contained in the exhaust gas before it is released from the vehicle or machine.

A simplified process can be represented as:

Diesel engine → Exhaust gases → AUS 32 dosing → SCR catalyst → Reduced NOx emissions

AUS 32 is therefore one component of a larger emission-control system.

The SCR catalyst, dosing system, sensors, electronic controls, exhaust temperature, and engine-management strategy all contribute to overall system performance.

How Does AUS 32 Work?

AUS 32 is injected in a controlled quantity into the hot exhaust stream upstream of the SCR catalyst.

The water evaporates, while the urea undergoes thermal decomposition and hydrolysis, generating ammonia.

The ammonia then acts as the reducing agent inside the SCR catalyst.

Through catalytic reactions, ammonia reacts with nitrogen oxides and converts them primarily into:

• Nitrogen (N₂)
• Water (H₂O)

The dosing process must be carefully controlled.

Insufficient dosing can reduce NOx-conversion performance.

Excessive ammonia availability can lead to another problem known as ammonia slip, in which unwanted ammonia passes through the system.

Modern SCR systems therefore use sensors, dosing equipment, electronic control systems, and catalysts to regulate this process.

What Does AUS 32 Actually Reduce?

This is one of the most important distinctions when discussing AUS 32.

Its primary role within SCR technology is the reduction of:

Nitrogen oxides — NOx

NOx should not be confused with all other pollutants or greenhouse gases associated with diesel engines.

Modern diesel after-treatment systems may incorporate multiple technologies because different pollutants require distinct control mechanisms.

For example:

SCR systems primarily target NOx.

Diesel Particulate Filters (DPFs) primarily capture particulate matter.

Diesel Oxidation Catalysts (DOCs) help control carbon monoxide and unburned hydrocarbons and can support other after-treatment processes.

AUS 32 should therefore not be described as a fluid that independently eliminates "diesel emissions."

Its function is much more specific.

AUS 32 and Carbon Dioxide Emissions

NOx and carbon dioxide are fundamentally different environmental concerns.

NOx is an important air pollutant involved in processes that contribute to poor air quality, including the formation of ground-level ozone and secondary particulate matter.

Carbon dioxide, or CO₂, is a greenhouse gas closely associated with fuel combustion and climate change.

AUS 32 is primarily used to enable NOx reduction through SCR.

It should therefore not be presented as a direct CO₂-reduction technology.

This distinction is important because statements such as:

"AUS 32 reduces greenhouse-gas emissions"

or

"AUS 32 directly supports decarbonization"

Can incorrectly combine two separate environmental issues.

The scientifically appropriate claim is more specific:

AUS 32 enables SCR-equipped diesel engines to control NOx emissions.

Does AUS 32 Improve Fuel Efficiency?

AUS 32 itself is not a fuel-efficiency additive.

It is injected into the exhaust after combustion and does not function by increasing the energy content or combustion efficiency of diesel fuel.

SCR technology can provide engine manufacturers with additional flexibility when designing the overall relationship between combustion strategy, engine performance, fuel consumption, and NOx control.

However, this does not mean that adding or consuming AUS 32 directly improves fuel economy.

Actual diesel consumption depends on many factors, including:

• Engine design
• Vehicle weight
• Payload
• Driving conditions
• Engine calibration
• Speed
• Aerodynamics
• Maintenance
• Operating environment

Claims that AUS 32 itself "improves fuel efficiency" should therefore be avoided unless they refer to a specifically documented vehicle or engine system and appropriate supporting evidence is provided.

Why NOx Reduction Matters

Nitrogen oxides are important regulated air pollutants.

NOx emissions contribute to atmospheric reactions involved in:

• Ground-level ozone formation
• Smog
• Secondary particulate matter
• Reduced air quality

Controlling NOx emissions from diesel engines is therefore an important component of vehicle and non-road engine emission regulation.

SCR has become widely used because properly designed systems can achieve substantial reductions in NOx emissions.

However, actual performance depends on the complete system, including:

• Catalyst design
• Exhaust temperature
• AUS 32 dosing
• Engine operating conditions
• Sensors
• System calibration
• Maintenance

It is therefore more accurate to describe SCR as an effective NOx-control technology than to claim that every SCR-equipped engine achieves an identical percentage reduction.

Where Is AUS 32 Used?

AUS 32 is used in many diesel-powered vehicles and machines equipped with SCR technology.

Applications can include:

• Heavy-duty trucks
• Buses
• Passenger diesel vehicles
• Agricultural machinery
• Construction equipment
• Industrial diesel equipment
• Certain non-road engines

Not every diesel engine requires AUS 32.

Its use depends on whether the engine and its exhaust after-treatment system were designed to operate with SCR technology using AUS 32.

Operators should therefore follow the specifications established by the vehicle or equipment manufacturer.

AUS 32 and Emission Regulations

The expansion of SCR technology has been closely associated with increasingly stringent NOx emission standards in major automotive and equipment markets.

SCR-equipped diesel engines are used across Europe, North America, and many other markets to help manufacturers meet applicable emission requirements.

However, an important distinction should be maintained:

AUS 32 itself does not make a vehicle compliant with an emission standard.

Regulatory compliance applies to the complete certified engine or vehicle and its emission-control architecture.

That system may include technologies such as:

• SCR
• Diesel particulate filters
• Oxidation catalysts
• Exhaust gas recirculation
• Sensors
• Electronic engine controls

AUS 32 is the required NOx-reduction agent for SCR systems designed to use it.

Why ISO 22241 Matters

The quality of AUS 32 is critical because modern SCR systems contain sensitive dosing and catalytic components.

The ISO 22241 series establishes requirements and guidance covering areas including:

• AUS 32 quality characteristics
• Test methods
• Handling
• Transportation
• Storage
• Refilling interfaces

The objective is not simply to standardize a chemical mixture.

Maintaining the correct composition and purity helps ensure reliable and stable operation of SCR converter systems.

The currently published ISO 22241-1:2019 specifies the quality characteristics of AUS 32. A new edition is under development as of 2026.

For buyers and distributors, this makes ISO conformity an important technical consideration when evaluating AUS 32 supply.

Why Agricultural Urea Should Not Be Used to Produce AUS 32 Informally

AUS 32 contains urea, but this does not mean that ordinary fertilizer-grade urea can simply be dissolved in water to create a suitable SCR fluid.

ISO defines the urea used for AUS 32 as technically pure urea with tightly controlled characteristics.

The specification excludes contaminants and substances that may be present in grades normally used for agricultural applications.

This distinction is particularly important for companies operating in both fertilizer and automotive-chemical markets.

Two products may both contain chemically recognizable urea while having very different purity requirements and intended applications.

Using unsuitable urea or contaminated water can compromise AUS 32 quality and potentially interfere with SCR system performance.

Professional production therefore requires appropriate raw materials, controlled water quality, suitable equipment, testing, and quality assurance.

Contamination and Product Quality

Contamination can occur during production, transportation, storage, or dispensing.

Potential sources include:

• Diesel fuel
• Lubricants
• Dust
• Minerals
• Metals
• Tap water
• Residues from unsuitable tanks or containers
• Contaminated pumps and hoses

SCR systems can be sensitive to impurities.

Contamination may interfere with dosing equipment, injectors, sensors, catalysts, or overall system performance.

AUS 32 should therefore be handled using dedicated, compatible equipment.

Quality must be protected throughout the entire supply chain:

Production → Storage → Transport → Distribution → Dispensing → Vehicle tank

A compliant product can lose its required quality if it is handled improperly after manufacturing.

Storage and Handling

Correct storage is necessary to preserve AUS 32 quality.

ISO 22241-3 provides requirements and best-practice recommendations for handling, transportation, and storage.

Important considerations include:

• Temperature
• Storage duration
• Protection from contamination
• Suitable tank materials
• Dedicated pumps and hoses
• Clean filling equipment
• Protection from unnecessary heat exposure

AUS 32 freezes at approximately −11°C.

Freezing does not automatically make correctly formulated AUS 32 unusable. Once properly thawed and homogenized according to appropriate handling guidance, it can generally continue to be used.

Prolonged exposure to elevated temperatures, however, can affect storage life and product quality.

Operators and distributors should therefore follow supplier instructions and applicable ISO requirements.

Crystallization and Deposits

White crystalline deposits can sometimes appear around AUS 32 filling points or where spilled solution has dried.

Because the product contains dissolved urea, water evaporation can leave solid material behind.

Deposits may also become an operational concern within poorly functioning SCR dosing systems.

The correct response is proper system diagnosis and maintenance.

Operators should not attempt to solve SCR problems by:

• Diluting AUS 32 with ordinary water
• Adding unauthorized chemicals
• Using unsuitable substitute fluids
• Bypassing emission-control equipment

Such actions can compromise both equipment performance and regulatory compliance.

SCR System Reliability and Maintenance

An SCR system depends on more than the quality of the fluid.

Reliable operation also requires functioning:

• Pumps
• Injectors
• Sensors
• Catalysts
• Heaters were required
• Electronic control systems
• Diagnostic systems

Failures can lead to warning messages, reduced emission-control performance, or operating restrictions depending on the equipment and applicable regulations.

The United States has recently reconsidered aspects of DEF system diagnostics and inducement strategies in response to operator concerns about sensor failures and severe engine derates.

These regulatory developments do not change the fundamental chemical role of AUS 32.

They demonstrate that successful emission-control technology must combine:

Effective NOx reduction + reliable hardware + accurate diagnostics + practical serviceability

The Trademark Issue: Why Correct Terminology Matters

The automotive-chemical market frequently uses brand names in ways that can make them appear generic.

This should be avoided when intellectual property rights apply.

AdBlue® is a registered trademark of the VDA.

The VDA operates a licensing system that governs the commercial use of the trademark, and authorized licensees must meet applicable conditions.

AUS 32, by contrast, is the technical terminology associated with the standardized aqueous urea solution.

This distinction matters for:

• Manufacturers
• Distributors
• Traders
• Product labels
• Websites
• Marketing materials
• Commercial offers
• Technical documentation

Companies selling or discussing AUS 32 should therefore distinguish clearly between the technical product specification and third-party trademarks.

Using AUS 32 as the principal technical term also provides clearer communication across international markets.

Purchasing AUS 32: What Buyers Should Evaluate

Commercial buyers should evaluate more than price.

Important considerations include:

Product Specification

Verify conformity with applicable ISO 22241 requirements.

Raw-Material Quality

The solution requires technically appropriate high-purity urea and pure water.

Quality Documentation

Suppliers should provide appropriate technical and quality documentation.

Traceability

Reliable batch and supply-chain traceability can help identify a product's origin and handling history.

Storage and Transportation

Appropriate tanks, containers, pumps, hoses, and transportation systems should protect product purity.

Supplier Reliability

Consistent production and quality-control systems are particularly important for a fluid used in sensitive emission-control equipment.

The cheapest supply can become expensive if poor quality contributes to equipment problems, downtime, rejected product, or customer claims.

AUS 32 in International Trade

AUS 32 has become an established automotive and industrial chemical traded across multiple regions.

Demand is closely linked to the installed base and utilization of SCR-equipped diesel engines.

Major demand sectors can include:

• Road freight
• Bus transportation
• Agriculture
• Construction
• Passenger vehicles
• Industrial and non-road equipment

Market demand can therefore be influenced by:

• Diesel fleet size
• SCR penetration
• Freight activity
• Agricultural mechanization
• Construction activity
• Emission regulations
• Distribution infrastructure
• Vehicle utilization

This provides a more technically accurate explanation of AUS 32 demand than simply attributing market growth to broad "green transportation" trends.

AUS 32 and Environmental Claims

Environmental communication surrounding automotive chemicals should be specific and measurable.

For AUS 32, the relevant claims relate to its function within SCR technology for NOx control.

Claims should not imply that AUS 32 independently:

• Decarbonizes transportation
• Eliminates diesel emissions
• Makes diesel engines zero-emission
• Directly reduces CO₂
• Replaces particulate filters
• Guarantees improved fuel economy
• Makes fossil-fuel use environmentally neutral

A technically accurate description is stronger than an exaggerated sustainability claim.

AUS 32 performs a specific and important function:

It supplies the standardized urea solution required by SCR systems to reduce NOx emissions from compatible diesel engines.

The Future of AUS 32

The future demand for AUS 32 will depend heavily on the continued operation and production of SCR-equipped diesel engines.

Large diesel fleets remain active globally in transportation, agriculture, construction, and industrial sectors.

Where those engines use SCR technology, AUS 32 will continue to be required for proper operation of the emission-control system.

At the same time, the broader transportation and equipment sectors are evolving.

Technologies including:

• Battery-electric powertrains
• Hybrid systems
• Alternative fuels
• Hydrogen technologies
• Advanced combustion systems
• Improved exhaust after-treatment

may gradually change the composition of global vehicle and machinery fleets.

AUS 32 should therefore not be presented as the final solution to sustainable transportation.

Its role is more specific and technically defined:

AUS 32 is an important NOx-reduction agent for current diesel SCR technology.

Conclusion

AUS 32 plays an important but clearly defined role in modern diesel emission control.

It is a standardized aqueous urea solution containing 32.5% technically pure urea by mass, used as the NOx-reduction agent in Selective Catalytic Reduction systems.

Within the SCR process, AUS 32 enables the formation of ammonia, which reacts with nitrogen oxides over the catalyst, converting them primarily into nitrogen and water.

Its principal function is therefore NOx control.

AUS 32 should not be presented as a fluid that independently reduces all vehicle emissions, directly lowers CO₂ emissions, improves fuel economy, or delivers broad climate objectives.

Quality is equally important.

ISO 22241 establishes technical requirements designed to support reliable SCR operation, and the use of unsuitable raw materials, contaminated fluids, or improper storage and handling can compromise product quality and equipment performance.

Terminology must also be used correctly.

AUS 32 is the technical designation for the standardized product. AdBlue® is a registered trademark owned by the VDA and should not be treated as the generic name of the product.

For manufacturers, traders, distributors, and end users, the correct approach is therefore based on four principles:

• Correct specification.
• Correct terminology.
• Correct handling.
• Correct technical claims.

When these principles are followed, AUS 32 can perform its intended function: supporting effective NOx control in compatible SCR-equipped diesel engines.

References

1. International Organization for Standardization (ISO). ISO 22241-1:2019 — Diesel engines — NOx reduction agent AUS 32 — Quality requirements.
2. International Organization for Standardization (ISO). ISO 22241-3:2017 — Diesel engines — NOx reduction agent AUS 32 — Handling, transportation, and storage.
3. International Organization for Standardization (ISO). ISO 22241-4:2023 — Diesel engines — NOx reduction agent AUS 32 — Refilling interface.
4. U.S. Environmental Protection Agency (EPA). Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards.
5. Verband der Automobilindustrie (VDA). AdBlue® — Trademark, Technical Information and Licensing.