What does opacity at the stack mean?

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A downward trend in the fine powder content in the ecat is a cause for concern when it is accompanied by losses within the unit. In such cases, it is necessary to check the amount and particle size distribution of these fine particles in the slurry circuit, flue gas system, and stack in order to determine where the catalyst is being lost. Two possible interim measures are to increase the fine powder content in the fresh catalyst and to recycle those fine particles back into the unit. It should be kept in mind that a catalyst with finer particles can lead to increased losses within the unit as well as higher opacity at the stack. A lower fine powder content in the ecat, combined with an increase in the average particle size, are typical signs of damaged cyclones. In this case, the slurry, flue gas system, and blackness should be checked to determine where the catalyst was lost. Two possible transitional measures are increasing the content of freshener powder and recycling the powder back to the device. Be aware that overly fine catalysts will increase equipment wear and flue gas darkness. A low content of balance agent fine powder, along with an increased particle size, is a typical sign of damage to the cyclone separator. This passage is about catalytic cracking units. What does \"opacity at the stack\" mean? I’m not sure if \"smoke darkness\" is the right translation