Handbook of Solvents - George Wypych - ChemTech - Ventech!

1196 Michelle Bergin, Armistead Russell
17.4.3.3.1 Quantification of solvent emissions on ozone formation
Two methods are generally employed to quantify the role pollutants play in forming ozone:
experimental and computational. Both types of estimation approaches have their limitations.
In the case of physical experiments, it is difficult to fully simulate ambient conditions,
so the results do not have general applicability. In the case of computational approaches, uncertainties
and approximations in the model for airshed conditions, in its formulation, and in
the chemical mechanism cause uncertainties in the predicted ozone impacts. For these reasons,
modeling predictions and experimental measurements are used together.
17.4.3.3.1.1 Experimental analysis
Experimental analysis is performed using environmental ‘smog’ chambers, either with a series
of single hydrocarbons irradiated in the presence of NOx or using complex mixtures to
simulate, for example, automobile exhaust emitted into characteristic urban ambient conditions.
Such chambers are large reaction vessels (some with internal volumes of cubic meters),
in which air and small amounts of hydrocarbons and NOx are injected, and then
irradiated with real or artificial light. Both indoor and outdoor chambers are used so behaviors
can be evaluated under natural radiative conditions and under controlled conditions.
While these experiments 18-23 clearly indicate differences in ozone formation from individual
hydrocarbons, they do not represent some important physical systems of urban pollution
such as the mixing processes and continuing emissions cycles. Such experiments have focused
both on groups of compounds as well as specific VOCs, including solvents. A particular
limitation has been studying very low vapor pressure solvents because it is difficult to
get enough of the compound into the vapor phase in the chamber to appreciably change the
ozone levels. Another limitation is the expense of using smog chambers to simulate a large
range of conditions that might occur in the atmosphere. On the other hand, smog chambers
are very powerful, if not fundamental, for developing chemical mechanisms that describe
the reaction pathways that can be used in computational approaches.
17.4.3.3.1.2 Computational analysis (air quality models)
Given the limitations of physical experiments to simulate atmospheric conditions, computer
models have been developed to assess the impact of emissions on ozone. These models,
called airshed models, are computerized representations of the atmospheric processes responsible
for air pollution, and are core to air quality management. 23 They have been applied
in two fashions to assess how solvents affect ozone. One approach is to conduct a
number of simulations with varying levels of solvent emissions. 2 The second approach is to
evaluate individual compounds and then calculate the incremental reactivity of solvent mixtures.
19,21,24-28
Derwent and Pearson 2 examined the impact of solvent emissions on ozone by simulating
air parcel trajectories ending in the United Kingdom and perturbing the emissions to account
for an anticipated 30% mass reduction in VOCs from solvents between 1995 and
2007. They found a small decrease in ozone-from 78 to 77 ppb in the mean peak ozone in
the UK, and a 9 ppb reduction from 129 ppb outside of London. A more substantial decrease
of 33 ppb from the 129 ppb peak outside of London was found from reducing non-solvent
mass VOC emissions by 30% outside of the UK and 40% within the UK. This suggests that
the VOC emissions from sources other than solvents have a higher average reactivity, as is
discussed by McBride et al. 29
While the types of simulations conducted by Derwent and Pearson 2 are important to
understanding the net effect of solvent emissions on ozone, there is an unanswered associated
and important question, that being which specific solvents have the greatest impacts.