Opacity is the degree to which a plume blocks the transmission of light. A completely transparent plume has 0% opacity. A completely opaque plume has 100% opacity. Most air quality permits set opacity limits between 10% and 20% for normal operations, with brief exemptions for startup, shutdown, and malfunction events. Exceeding the opacity limit is one of the fastest ways to draw an enforcement action because opacity violations are visible to anyone who looks at your stack.
Opacity is not the same as particulate matter (PM) emissions, but it is closely correlated. The particles in the exhaust stream scatter and absorb light, creating the visible plume. Larger particles scatter more light per unit mass, so a plume heavy with coarse PM10 looks darker than a plume with the same mass of fine PM2.5. Regulatory agencies use opacity as a real-time, low-cost indicator of PM emissions. If the opacity is within limits, the PM emissions are presumed to be within limits. If the opacity spikes, the evidence is in the air for everyone to see.
EPA Method 9: How Opacity Is Measured
EPA Reference Method 9 is the official procedure for visual determination of opacity. A certified opacity observer stands at a specific position relative to the stack (at least 15 feet away, with the sun behind them, looking at the densest part of the plume against a contrasting background) and reads the opacity in 5% increments every 15 seconds for a minimum observation period, typically 6 minutes (24 readings). The reported opacity is the arithmetic average of the 24 readings.
Method 9 certification requires completing a certification course that includes reading simulated plumes of known opacity from a smoke generator. The observer must correctly read the opacity within ±7.5% on black plumes and ±15% on white plumes. Certification is valid for 6 months and must be renewed by passing the field test again.
The key regulatory nuance: most permits allow brief opacity exceedances. The federal standard in 40 CFR 60.11 allows one 6-minute period in any hour to exceed the opacity limit by up to 10 percentage points. So a source with a 20% opacity limit can have one 6-minute period per hour at up to 30% opacity without a violation. State regulations may differ significantly.
Continuous opacity monitoring systems (COMS) are required for large sources and provide real-time opacity data using a light beam transmitted across the stack diameter. COMS data is recorded continuously and reported to the regulatory agency. A COMS alarm at the permitted opacity limit gives operators immediate warning to adjust combustion or increase baghouse cleaning before a visible emissions complaint is filed.
Ringelmann 0 = 0% opacity (clear)
Ringelmann 1 = 20% opacity
Ringelmann 2 = 40% opacity
Ringelmann 3 = 60% opacity
Ringelmann 4 = 80% opacity
Ringelmann 5 = 100% opacity (solid black)
Stack Opacity to PM Estimator
Estimate particulate matter emissions from visible stack opacity readings. Convert Method 9 opacity observations to approximate PM emission rates using EPA correlation methods.
Opacity and PM Correlation
The relationship between opacity and particulate mass concentration follows Beer-Lambert law: opacity increases exponentially with particle concentration and path length (stack diameter). A stack with 0.01 grains per dry standard cubic foot (gr/dscf) of PM might show 5% opacity through a 4-foot diameter stack but 10% through an 8-foot diameter stack. The same mass loading produces different opacity readings depending on the stack geometry.
Particle size distribution is the other major variable. Particles in the 0.3 to 1.0 micron range scatter light most efficiently per unit mass (Mie scattering). A baghouse outlet with predominantly sub-micron particles from a condensable PM source can show 15% opacity at a mass concentration that would produce only 5% opacity if the particles were coarser. This is why opacity is a screening tool, not a substitute for mass emission testing.
Many permits include both an opacity limit and a PM mass limit. The opacity limit provides a continuous, real-time compliance indicator. The PM mass limit (typically from stack testing every 1 to 5 years) provides the quantitative verification. If your stack test shows 0.005 gr/dscf but your opacity is consistently at 18%, the disconnect suggests something changed between the test and normal operation: bag condition, combustion tuning, or feed material composition.
Condensable PM adds another complication. Gases that condense into fine aerosols after leaving the stack (sulfuric acid mist, organic condensables) create a visible plume that registers as opacity but may not be captured in a traditional filterable PM test. EPA Method 202 captures condensable PM separately. Sources with high condensable emissions (combustion of sulfur-bearing fuels, acid processes) often have visible plumes even with excellent filterable PM control.
Combustion Tuning and Opacity Control
For combustion sources (boilers, furnaces, kilns, incinerators), opacity is primarily a function of combustion completeness. Incomplete combustion produces soot (elemental carbon particles) that is highly light-absorbing and creates a dark plume. Complete combustion produces CO₂ and water vapor with minimal particulate. The difference between clean and dirty combustion often comes down to the fuel-to-air ratio, mixing quality, and combustion temperature.
Natural gas combustion at proper air-fuel ratio produces essentially zero opacity. If a gas-fired boiler shows visible emissions, the problem is almost always combustion-related: insufficient air (fuel-rich), poor mixing (stratified flow through the firebox), or flame impingement on cold surfaces. Adjusting the excess air to 10% to 15% O₂ at the economizer outlet typically eliminates visible emissions from gas-fired units.
Oil-fired units are more sensitive to atomization quality. The fuel oil must be broken into fine droplets (typically below 100 microns) for complete combustion. Worn or fouled burner nozzles produce large droplets that do not fully vaporize and burn, creating soot and opacity. Steam-atomizing burners generally produce better atomization than pressure-atomizing burners, especially at low loads. Heavy fuel oils (No. 6, residual) require preheating to 200–250°F for proper atomization viscosity.
Coal and solid fuel combustion produces fly ash that must be captured by an ESP or baghouse. The combustion tuning goal is to minimize the amount of unburned carbon in the fly ash (loss on ignition, or LOI) while maintaining opacity below the permit limit. LOI above 5% indicates incomplete combustion and typically correlates with higher opacity. Adjusting overfire air, undergrate air distribution, and fuel feed rate can reduce LOI and opacity simultaneously.
Permit Limits and Enforcement Triggers
Federal New Source Performance Standards (NSPS) in 40 CFR Part 60 set opacity limits for specific source categories. The default federal limit is 20% opacity (6-minute average) for most combustion sources, but many subparts set lower limits. Portland cement plants (Subpart F) are limited to 10%. Grain elevators (Subpart DD) are limited to zero percent opacity. State implementation plans and individual permits may impose limits different from the federal NSPS.
Opacity violations trigger enforcement through three main channels: scheduled inspections, complaint-driven inspections, and COMS data review. During a scheduled inspection, a certified opacity observer from the regulatory agency reads your plume using Method 9. If the 6-minute average exceeds your permitted limit, you receive a notice of violation (NOV). Repeat violations or violations exceeding 40% opacity can result in penalties of $10,000 to $50,000 per day per violation.
The most common reasons for opacity citations are baghouse failures (broken bags, failed cleaning systems), combustion problems (fuel-rich conditions, poor atomization), and fugitive dust (unpaved roads, uncovered stockpiles). A single broken bag in a multi-compartment baghouse can increase outlet opacity from 2% to 15% or more. Monthly self-inspections with a trained in-house observer catch these problems before the regulator arrives.
Fugitive emissions (dust from roads, stockpiles, material handling) also have opacity limits, typically 10% to 20% at the property line. Fugitive opacity is harder to measure because the plume is diffuse without a defined emission point. The practical implication: if you can see your plume from the parking lot, a regulator can see it from the highway, and a persistent visible plume invites scrutiny of your entire operation.