- October 30, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
“Removes 90% of particulates.” It is a clean, confident sentence, and it appears on brochures, in tender responses and, increasingly, in sustainability reports that want to show a DG fleet is cleaner than it was last year. The trouble is that it answers none of the questions a careful reader should ask. Ninety per cent of what, measured how, on which engine, at what load, with which fuel, and on the day the device was new or after two years in service?
Those questions matter most when “particulates” is quietly swapped for “black carbon”. RECD black carbon reductions and RECD particulate mass reductions are related, but they are rarely equal, and a report that treats them as interchangeable can overstate a climate benefit or understate a health one.
Below, we take one headline figure apart: the chemistry of diesel particulate, the instruments that see different slices of it, the test conditions behind the percentage, and a worked example in which a filter removes 98% of black carbon while total PM falls by far less. We finish with what Indian compliance checks and how to word your own reporting.
Start with the number, then ask what it hides
Every particulate reduction claim is a ratio of after to before, subtracted from one (our guide on how to calculate RECD particulate reduction covers the formula). The formula is simple; what goes into it is not.
A single percentage can conceal at least five choices:
- The substance: total PM mass, solid carbon only, smoke darkness or particle count.
- The instrument: filter papers, smoke meter, absorption photometer, thermal-optical analyser or particle counter.
- The operating point: full load, a weighted multi-mode cycle, or whatever the DG was doing that day.
- The fuel and oil: sulphur and lube consumption change what the particulate is made of.
- The device’s age: a clean new substrate behaves differently from one carrying ash or a cracked segment.
Change any one and the same hardware can produce a different headline. That is not dishonesty; it is what happens when a mixture is reduced to one figure.
What diesel particulate is actually made of
“Soot” and “particulate matter” are often used as synonyms. They are not. Soot is the dark, mostly solid carbon part. Particulate matter, as regulators and labs define it, is whatever collects on a filter paper under specified sampling conditions, and that includes liquids and condensed vapours alongside the solid carbon.
DieselNet’s overview of diesel particulate filters makes the point plainly: filters are very effective on the solid fraction but can be much less effective on organic and sulphate fractions, which is why many designs add catalyst coatings. Proportions vary widely by engine, load, fuel and oil, so treat any brochure split as a snapshot.
| Component | What it is | Where it comes from | What typically reduces it in an aftertreatment system |
|---|---|---|---|
| Black carbon / elemental carbon (EC) | Solid, light-absorbing carbon aggregates; the “black” in black smoke | Incomplete combustion of fuel in rich zones of the cylinder | Physical capture in a filter (wall-flow captures it very efficiently); later burnt off during regeneration |
| Organic carbon (OC) / soluble organic fraction (SOF) | Heavy hydrocarbons from unburnt fuel and lube oil, partly as vapour at exhaust temperature, condensing on cooling | Fuel and oil that escape full combustion; worse at light load and with worn rings or guides | An oxidation catalyst (on a DOC or a catalysed filter) oxidises the vapour phase when hot enough; a bare filter catches only what is already adsorbed on soot |
| Sulphates and bound water | Sulphuric acid droplets and the water they hold | Sulphur in fuel and oil, oxidised to SO3 | Largely not removed by a filter; a catalyst can actually increase sulphate formation at high temperature with higher-sulphur fuel |
| Nitrates | Condensed nitrogen compounds | NOx chemistry in the exhaust and during dilution | Not meaningfully addressed by particulate filters |
| Ash and metals | Incombustible inorganic residue: calcium, zinc, magnesium, phosphorus compounds, plus wear metals | Lubricant additives, fuel trace elements, engine wear | Captured by a wall-flow filter and retained permanently until cleaning |
Two things follow. A filter is a mechanical sieve for solids and a catalyst is a chemical converter for vapours, so a “particulates” claim depends on which the device has and how hot it runs. And ash is captured but never burnt; DieselNet’s page on ash in particulate filters explains how it can eventually outweigh the soot load, one reason an aged device may test differently from a new one.
Soot vs particulate matter in everyday terms
The grey or black tint on a used test filter paper is mostly elemental carbon, but its weight gain includes organics, sulphate, water and ash too. A paper can look nearly white after an RECD and still carry measurable mass. “How dark” and “how heavy” are separate questions.
Why engine condition shifts the mix
An engine that burns oil or idles for long periods at light load pushes more organic carbon into the exhaust (see how engine oil consumption affects RECD suitability). The richer the exhaust is in organics, the lower its PM mass reduction tends to be through a filter, even if black carbon reduction stays high.
Black carbon vs PM: five measurement lenses
Each instrument sees a different slice of the mixture. None is wrong; mixing them in one sentence is where confusion starts.
PM mass (gravimetric)
Exhaust is sampled, usually diluted, and drawn through a pre-weighed filter paper at controlled temperature. The paper is conditioned and re-weighed. The result is reported in units such as mg/Nm³ or g/kWh. This captures everything that ends up on the paper: EC, OC, sulphate, water, nitrates, ash. It is the metric most regulations, including India’s RECD procedure, are built on. If you are unsure how the concentration and specific-emission units relate, see our explainer on mg/Nm³ vs g/kWh for RECD emissions.
Because organic vapours condense differently with dilution ratio and temperature, dilution can mislead before-and-after measurements.
Smoke opacity
An opacimeter reports how much light the exhaust blocks. It responds strongly to dark solids and weakly to organics and sulphate; it is a quick field check, not a mass measurement. See smoke opacity vs particulate mass.
Black carbon (optical absorption)
Aethalometers and micro-soot sensors measure light absorption and convert it to an equivalent black carbon mass using a calibration factor, hence the label “equivalent black carbon”. They are very sensitive to dark solid carbon and almost blind to colourless organics and sulphate.
Elemental carbon (thermal-optical)
A quartz filter sample is heated in stages, first in inert gas to drive off organic carbon, then with oxygen to burn the elemental carbon, with an optical correction for charring. The output is separate EC and OC masses. EC and optical BC usually track each other closely in diesel exhaust, but they are defined differently and need not match exactly.
Particle number
Condensation particle counters count particles, often after removing volatiles so only solids are counted. An ultrafine particle weighs almost nothing but counts the same as a large one, so number can move very differently from mass; see whether RECDs capture ultrafine particles.
The lenses side by side
| Metric | What it responds to | Typical units | Strongly affected by organics and sulphate? | Typical use |
|---|---|---|---|---|
| PM mass | Everything collected on the paper | mg/Nm³, g/kWh | Yes | Regulation, type approval |
| Smoke opacity | Light blocked by dark particles | %, m⁻¹ | Weakly | Field checks, maintenance |
| Black carbon (optical) | Light absorption by dark carbon | µg/m³, mg/Nm³ equivalent | Very little | Climate studies, research |
| Elemental carbon (thermal-optical) | Carbon that burns only with oxygen | µg/m³, mg/Nm³ | No (OC is reported separately) | Source apportionment, research |
| Particle number | Count of particles, often solids only | particles per cm³ or per kWh | Depends on volatile removal | Ultrafine and health studies |
Why a BC reduction can differ from a PM mass reduction
If a device removes almost all the black carbon but only part of the organic and sulphate fractions, the black carbon percentage will be high and the PM mass percentage lower. How much lower depends on how big the non-carbon slices were. On a healthy engine at high load on low-sulphur diesel the gap may be small; on an oil-burning engine at light load it can be large.
The reverse can also happen in principle. A device that is strong on organics (an oxidation catalyst on its own, say) but weak on solids could show a decent PM mass reduction with a modest black carbon reduction. That is one reason catalysed and uncatalysed filters need to be compared on the right metric.
A worked example: one device, two percentages
The following example is illustrative. The composition and the per-component efficiencies are chosen to make the arithmetic clear; they are not measurements of any specific engine or product.
Baseline (upstream of the device): total PM of 100 mg/Nm³, made up of:
- Black carbon / elemental carbon: 70 mg/Nm³
- Organic carbon: 20 mg/Nm³
- Sulphate (with bound water): 8 mg/Nm³
- Ash: 2 mg/Nm³
Check: 70 + 20 + 8 + 2 = 100 mg/Nm³.
Assumed removal by a filter-based device (illustrative): 98% of black carbon, 50% of organic carbon, 10% of sulphate, 90% of ash.
| Component | Before (mg/Nm³) | Assumed removal | Calculation | After (mg/Nm³) |
|---|---|---|---|---|
| Black carbon | 70 | 98% | 70 × (1 − 0.98) = 70 × 0.02 | 1.4 |
| Organic carbon | 20 | 50% | 20 × (1 − 0.50) = 20 × 0.50 | 10.0 |
| Sulphate | 8 | 10% | 8 × (1 − 0.10) = 8 × 0.90 | 7.2 |
| Ash | 2 | 90% | 2 × (1 − 0.90) = 2 × 0.10 | 0.2 |
| Total PM | 100 | 1.4 + 10.0 + 7.2 + 0.2 | 18.8 |
PM mass reduction: (100 − 18.8) ÷ 100 × 100 = 81.2%.
Black carbon reduction: (70 − 1.4) ÷ 70 × 100 = 68.6 ÷ 70 × 100 = 98.0%.
So the same device, on the same test, achieves “98% black carbon reduction” and “81.2% PM reduction”. Both are true; neither is complete alone.
The composition also flips. Black carbon was 70% of PM before; after, it is 1.4 ÷ 18.8, about 7.4%. Organic carbon (10.0 ÷ 18.8, about 53.2%) and sulphate (7.2 ÷ 18.8, about 38.3%) now dominate, which is why a post-RECD paper can look pale yet still weigh something.
Same device, different engine
Now keep the illustrative efficiencies exactly the same but change the baseline mix to represent an engine burning more oil and running on fuel with more sulphur. Total PM is still 100 mg/Nm³, but it is made up of 50 black carbon, 35 organic carbon, 13 sulphate and 2 ash (50 + 35 + 13 + 2 = 100).
| Component | Before | Calculation | After |
|---|---|---|---|
| Black carbon | 50 | 50 × 0.02 | 1.0 |
| Organic carbon | 35 | 35 × 0.50 | 17.5 |
| Sulphate | 13 | 13 × 0.90 | 11.7 |
| Ash | 2 | 2 × 0.10 | 0.2 |
| Total PM | 100 | 1.0 + 17.5 + 11.7 + 0.2 | 30.4 |
PM mass reduction: (100 − 30.4) ÷ 100 × 100 = 69.6%. Black carbon reduction: (50 − 1.0) ÷ 50 × 100 = 98.0%.
Black carbon reduction has not moved, but PM mass reduction has fallen 11.6 percentage points, from 81.2% to 69.6%, purely because the engine and fuel changed. Real devices, especially with catalyst stages, do not have fixed per-component efficiencies, but the direction is real: organics and sulphate pull PM reduction below BC reduction. That is why fuel and engine condition confound RECD tests and should be recorded with every result.
What exactly is the claim based on? A checklist
When a percentage lands on your desk, run it through these questions before it goes into a report.
- Which test procedure? A named type-approval procedure, a lab’s in-house method, or an informal field test? Ask for the document reference, and read it with our guide on how to read an RECD test certificate beside you.
- Which engine? Make, model, rating, age and hours. A result on a test-bed engine in good condition does not automatically transfer to a fleet of older sets.
- Which load points? A single high-load point, several modes averaged with weightings, or whatever load was available? Load changes both the PM mix and exhaust temperature, which in turn changes catalyst activity. See why load must be matched in comparison tests.
- Which fuel? Sulphur content directly affects sulphate PM and catalyst behaviour.
- How was the sample taken? Raw or diluted, at what dilution ratio and temperature, and at which point in the exhaust line?
- Which metric? PM mass, opacity, black carbon, elemental carbon or particle number. If the claim says “particulates”, ask which.
- How were results averaged? One run or several? Mean, median or best result? Were any runs excluded, and why?
- New or aged device? Was the device fresh from the factory, or had it accumulated hours, ash and regeneration cycles? Was a regeneration happening during sampling?
- Who measured? An accredited or recognised agency, the supplier’s own team, or a consultant with a portable instrument? Each can be legitimate, but they carry different weight.
- What were the absolute numbers? A percentage without the before and after values in units hides whether the baseline was unusually high.
If a supplier cannot answer most of these, the percentage is a marketing figure. That does not make the product bad; it makes the number unusable for formal reporting. For contracts, see how to tie RECD performance guarantees to operating conditions.
RECD black carbon claims, climate impact and health
Sustainability teams care about black carbon for a reason that PM mass alone does not capture. Black carbon is a short-lived climate pollutant. The Climate and Clean Air Coalition’s black carbon page describes it as staying in the atmosphere for only days to weeks, yet absorbing far more heat per unit mass than CO2 while it is there, and darkening snow and ice when it settles. It also notes that diesel engines are a dominant black carbon source in some regions.
Short lifetime plus strong warming is why black carbon reduction is discussed as a fast-acting climate lever. But converting it into CO2-equivalent requires choosing a metric and time horizon, and published factors vary widely. If your team does this, have a specialist set the method and state it openly.
Do not confuse a black carbon benefit with a CO2 benefit. A particulate filter does not cut CO2; burning trapped soot produces a little CO2 and back-pressure can nudge fuel use up. See whether an RECD reduces CO2 emissions.
On health, the case for cutting diesel particulate is strong in its own right. In June 2012 the International Agency for Research on Cancer classified diesel engine exhaust as carcinogenic to humans (Group 1), based on evidence linking exposure to lung cancer. That classification covers the whole exhaust, not just black carbon, which is a useful reminder that both the solid carbon and the organic fraction matter for people standing near a DG set. For background on particle sizes and why fine particles travel deep into the lungs, see our explainer on PM2.5 vs PM10.
Where black carbon and PM mass priorities diverge
| Priority | Metric that best represents it | What to ask of a device |
|---|---|---|
| Regulatory compliance in India | PM mass | Type-approval PM capture efficiency under the prescribed procedure |
| Near-field climate effect | Black carbon / EC | Solid carbon capture and how it holds up as the device ages |
| Visible smoke complaints | Opacity | Reduction in smoke at start-up and transient load |
| Ultrafine particle exposure | Particle number | Capture of solid particles in the smallest size ranges |
| Odour and irritant organics | OC, plus gaseous HC | Catalyst activity and the temperature at which it starts working |
A device may score very differently across these rows. The mistake is quoting the best row as if it represented all of them.
What Indian compliance actually checks
In India, RECDs for in-use DG engines are assessed under the “System and Procedure for Emission Compliance Testing of Retro-fit Emission Control Devices (RECD)” issued by the Central Pollution Control Board, which covers engines up to 800 kW gross mechanical power. The headline requirement is a particulate matter capture efficiency of at least 70%, demonstrated through type approval by a recognised testing agency such as ARAI or ICAT. You can find CPCB notifications on the CPCB website.
Three points follow for anyone reading a black carbon claim:
- The regulatory metric is PM mass. The 70% threshold refers to particulate matter measured under the procedure, not to black carbon, opacity or particle number.
- Black carbon and particle number are not RECD requirements at the time of writing. Check CPCB and your State Pollution Control Board for updates.
- A high black carbon figure does not prove compliance. Compliance evidence is the device’s type-approval documentation and its fitment to your engine range, not a BC number.
See what an RECD is and how it works and our page on DG set emission regulations. SPCB consent conditions, and CAQM directions in Delhi NCR, may add monitoring requirements of their own.
What “70% PM reduction” means, and does not mean
It means that under the prescribed test conditions the device reduced PM mass by at least 70%. It does not mean every component fell by 70%, that black carbon fell by 70% (it may be much higher), that particle number did, or that your engine on your day will reproduce the figure. It is a type-approval benchmark, not a promise for every operating point.
Does the filter type change the black carbon story?
Yes. A wall-flow filter forces all exhaust through porous walls, so solid particles, including the smallest, are captured with high efficiency. A partial-flow design lets some exhaust bypass the media, so solid capture is lower and varies with load and loading state; see wall-flow vs partial-flow RECD filtration. Electrostatic and wet scrubbing designs split their removal between black carbon, organics and sulphate differently again. Ask for multi-metric data if your reporting needs it, and accept that many suppliers will only hold PM mass data from type approval.
How to word your own reporting honestly
If an RECD appears in your sustainability report or client submission, the wording matters as much as the number, so getting RECD black carbon language right is worth the effort. The table contrasts statements that overreach with alternatives that hold up.
| Statement that overreaches | Why it is a problem | Better wording |
|---|---|---|
| “Our RECDs eliminate black carbon from our generators.” | “Eliminate” implies 100% under all conditions; no test supports that | “Our DG sets are fitted with CPCB type-approved RECDs; supplier test data indicates high capture of the solid carbon fraction under test conditions.” |
| “96% reduction in particulate pollution.” | No metric, conditions or source given | “The supplier reports up to 96% PM reduction depending on DG condition and fuel; type-approval testing confirms at least the 70% PM capture threshold.” |
| “Black carbon reduced by 98%, equivalent to X tonnes CO2e saved.” | Mixes a single test result with a fleet estimate and an undisclosed climate metric | “Estimated black carbon reduction is based on [method], [hours run] and [emission factor]; CO2-equivalent conversion uses [metric and time horizon] and is indicative only.” |
| “RECDs cut our carbon emissions.” | Ambiguous; readers will assume CO2, which a filter does not reduce | “RECDs reduce particulate emissions; they do not reduce CO2. Fuel-related CO2 is reported separately.” |
| “Our generators now meet BC norms.” | No BC norm currently applies to RECDs in India | “Our RECDs are type approved under CPCB’s PM-based procedure. There is no separate black carbon standard for RECDs at the time of writing.” |
| “Smoke-free generators.” | Opacity and PM are different; organics and sulphate may remain | “Visible smoke from fitted DG sets is substantially reduced; compliance is based on PM mass testing.” |
Always name the metric, quote absolute values alongside percentages, state the data source, keep fleet estimates separate from single-test results, and revisit figures when devices age or engines are overhauled.
FAQ
Is black carbon the same as particulate matter?
No. Black carbon is the dark, light-absorbing solid carbon part of particulate matter. PM mass also includes organic carbon, sulphates, nitrates, water and ash. In diesel exhaust black carbon is often a large share of PM, but the proportion varies with engine condition, load and fuel.
Why is a black carbon reduction usually higher than the PM reduction for the same RECD?
Filters capture solid carbon very efficiently but are less effective on organic vapours and sulphates, which pass through or form downstream. Those leftover fractions keep the PM mass figure lower than the black carbon figure.
Does the CPCB require black carbon testing for RECDs?
At the time of writing, no. The CPCB procedure for RECDs on engines up to 800 kW is based on PM mass with at least 70% capture efficiency. Check the CPCB website and your SPCB for any later changes.
Can I report RECD black carbon reductions in an ESG report?
You can, provided you state the source and metric, separate test results from fleet estimates, and disclose any method used to convert black carbon into CO2-equivalent. Avoid implying a CO2 reduction from the RECD itself.
What does elemental carbon vs organic carbon mean in a diesel test report?
Elemental carbon is the solid, soot-like carbon that burns only with oxygen; organic carbon is carbon in hydrocarbon compounds from unburnt fuel and oil. Thermal-optical analysis separates the two from one filter sample.
Does a pale filter paper after the RECD mean zero emissions?
No. A pale paper shows that most dark solid carbon has been removed. The paper can still carry weight from organics, sulphate and water, which is why gravimetric PM is measured separately from appearance or opacity.
Getting a Particulate Claim You Can Actually Stand Behind
Aceget supplies CPCB type-approved retrofit emission control devices for DG sets from 25 kVA to 2500 kVA. Aceget’s product page describes a silicon carbide ceramic filter with a precious-metal coating, an oxidation catalyst stage and NO2-assisted regeneration, with up to around 96% particulate reduction depending on DG condition and fuel. If you need to know what a figure like that covers for your engines, ask us for the test basis, metric and conditions behind it. Start with our RECD product range, review the type-approval certifications, and contact our team with your DG ratings, load profile and reporting needs.