While the US government and state regulators have been trying to reduce VOC emissions as part of air quality improvement measures, the US solvents industry - led by The American Chemistry Council's (ACC) Solvents Industry Group - believes that traditional US approaches to VOC regulations have reached a point of diminishing returns.
It argues that a policy that distinguishes between the actual ozone forming contributions of different VOCs could significantly improve regulatory strategies.
The largest contributor to ground level ozone is NOx (oxides of nitrogen) emissions from vehicles and industrial activity. VOCs from a variety of sources, including products that contain solvents - such as paints and coatings - can also contribute to the problem but all VOCs are not the same.
Photochemical reactivity
"Current regulations in the US use a mass-based approach to VOC controls, which simply limits the overall percentage of VOCs contained in a product or formulation such as paint," explains David Laucella (pictured above left), Solvents Technical Manager - Americas."
"The mass-based approach treats all VOCs the same whereas in fact they vary significantly in their potential to impact ozone levels, based on their photochemical reactivity," says Laucella, who is the Shell Chemicals representative on the ACC's Solvents Industry Group.
"The higher the reactivity of a VOC, the greater the potential contribution to ozone. Two formulations that meet mass-based limits can have very different ozone-forming profiles, depending on the solvents used."
The ACC believes that a policy that distinguishes between the actual ozone forming contributions of different VOCs could improve regulatory strategies.
"We have worked for many years with state and federal regulators as well as industry to improve the understanding of the differential contributions of VOCs to ozone formation, and to develop an improved scientific basis for development of reactivity-based regulatory policies," says Barbara Francis (pictured above right), Managing Director of the ACC's Solvents Industry Group.
"Many scientists and regulators believe we now have sufficient understanding to implement a more efficient approach to VOC control in the US, which could result in faster progress toward meeting ambient air quality standards.
"A system that recognises differences in the impact of various VOCs encourages formulators to select lower reactivity solvents, and so decreases the ozone creation potential of the final products."
Not a new concept
Photochemical reactivity is not a new concept - scientists have known for years that various VOCs can make profoundly different contributions to ozone formation. Since 1998, the US EPA (Environmental Protection Agency) has supported a study of reactivity through the Reactivity Research Working Group, which consists of representatives from academia and industry as well as regulators.
It has also addressed the need for developing scientifically valid "reactivity scales". One of the most common scales in use today is the Maximum Incremental Reactivity (MIR) scale. The MIR scale measures the relative photochemical reactivity of solvents on a common, continuous scale.
"Using a reactivity-based approach provides an incentive for formulators to move from high to medium reactivity solvents, or from medium to low reactivity solvents," says Laucella.
"There are many lower reactivity solvents to choose from that achieve and/or exceed targeted reductions in ozone creation potential while still providing the technical attributes necessary for a particular application."
Blanket constraints
He says that by not imposing blanket constraints on formulation flexibility that might impair a product's performance, it could also result in further benefits to the environment.
"For example, maintaining the performance of a coating could mean a substrate may need to be repainted less often since the paint can be formulated to last longer, saving the materials and energy associated with re-applications."
The current mass approach to VOC limits on the other hand poses significant reformulation challenges. So much so that California's Air Resources Board (ARB), which has implemented some of the most stringent air quality regulations in the world, has concluded that it may not be feasible to achieve the required VOC limits in aerosol coatings by using this approach.
Working with industry experts and scientists, the ARB has developed a reactivity-based rule for aerosol coatings, which has been approved by the EPA.
The rule encourages reductions in the use of higher reactivity VOCs in aerosol coatings and the ARB estimates that this will achieve the equivalent of an additional 3.1 tons per day of VOC reductions in California compared to a mass-based approach. It is now working on inclusion of reactivity-based approaches for other types of coatings.
The EPA also believes that reactivity-based approaches, such as the one developed in California, should be more efficient and effective than traditional approaches that make no distinction between VOCs.
As a result, it has updated its VOC policies by endorsing photochemical reactivity as a sound science-based approach for VOC control and has encouraged other states to "consider how they may incorporate VOC reactivity information to make future VOC control measures more effective and efficient".
A number of US industry associations that represent solvent-using sectors are also working to position reactivity as a fundamental principle of VOC management. "Recognising the relevance of reactivity will encourage a shift from 'low VOC paints' to 'low ozone paints' by allowing formulators the flexibility to utilise the most appropriate solvents," says Laucella.
"There is sufficient scientific information to support conversion from mass-based to reactivity-based regulation of VOCs in many applications."
VOCs and ground level ozone
Ozone in the upper atmosphere or stratospheric ozone, absorbs UV light and protects the earth from harmful ultraviolet radiation.
On the other hand, ground level or tropospheric ozone, is the main component of smog and can have adverse effects on human health.
Troposheric ozone forms when UV light from the sun reacts with NOx emissions, which mainly come from automotive vehicles. This happens even when VOCs are not present.
When VOCs are also present in the atmosphere, the ozone breaks down more slowly, because of competing chemical reactions, and so more ground level ozone may accumulate.
There are many contributors to VOC emissions in the atmosphere, including natural or "biogenic" sources such as trees and vegetation. Man-made or anthropogenic sources include vehicle emissions, petroleum refining, industrial manufacturing plants and power generation.
Solvents may contribute to VOC levels in applications such as paints and coatings, where they evaporate into the air.
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