4 Sensitivity of Ambient Atmospheric Formaldehyde to VOC and NO x …
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are used to evaluate and improve isoprene inventories [1], so it is important to accurately characterize how HCHO is formed from isoprene versus from other biogenic
VOCs. Overall, these results predict that HCHO has much smaller sensitivity to NO x
than hydrocarbons, indicating that HCHO formation is not limited by NO x , although
O 3 can be. When only anthropogenic emissions are considered, HCHO is found to
be sensitive to both NO x and hydrocarbons, with mobile sources contributing substantially. Condensed chemical mechanisms are used to develop emission control
strategies, so it is important to confirm these sensitivities using detailed mechanisms
and dynamic evaluations.
The goal of multipollutant control strategies is to develop emission reductions
targeting one important pollutant, but provide co-benefits in reducing other harmful
pollutants. While both O 3 and HCHO are formed under similar conditions (high
actinic flux, availability of reactive hydrocarbons and NO x ), they show strikingly
different sensitivities. NO x reduction strategies will have the most impact overall on
O 3 . Hydrocarbon-focused strategies would be the most optimal for reducing HCHO,
although NO x controls are predicted to have some small positive impact in summer.
Questioner: Paul Makar
Question: To what extent does the gas-phase mechanism employed conserve carbon
number, and how much of the missing HCHO could be due to losses of carbon on
the way down the reaction sequence towards HCHO?
Answer: The mechanism used is CB05 with aromatic updates. While it does not conserve carbon, the carbon that the mechanism drops would not participate strongly
in ozone formation, i.e. relatively non-reactive (such as CO 2 ) or produced in small
yields. Some of these could eventually form a small amount of HCHO further downwind, but it would not likely account for the 30% under prediction.
Disclaimer Although this work was reviewed by EPA and approved for publication, it may not
necessarily reflect official agency policy.
References
1. M. Bauwens, T. Stavrakou, J.F. Müller, I. De Smedt, M. Van Roozendael, G.R. van der Werf,
C. Wiedinmyer, J.W. Kaiser, K. Sindelarova, A. Guenther, Nine years of global hydrocarbon
emissions based on source inversion of OMI formaldehyde observations. Atmos. Chem. Phys.
16, 10133–10158 (2016)
2. A. Hakami, M.T. Odman, A.G. Russell, High-order, direct sensitivity analysis of multidimensional air quality models. Environ. Sci. Technol. 37, 2442–2452 (2003)
3. D.J. Luecken, S.L. Napelenok, M. Strum, R. Scheffe, S. Phillips, Sensitivity of ambient atmospheric formaldehyde and ozone to precursor species and source types across the U.S. Environ.
Sci. Tech. 52(8), 4668–4675 (2017). https://doi.org/10.1021/acs.est.7b05509
4. S.L. Napelenok, D.S. Cohan, M.T. Odman, S. Tonse, Extension and evaluation of sensitivity
analysis capabilities in a photochemical model. Environ. Mod. Softw. 23, 994–999 (2008)
2 7
are used to evaluate and improve isoprene inventories [1], so it is important to accurately characterize how HCHO is formed from isoprene versus from other biogenic
VOCs. Overall, these results predict that HCHO has much smaller sensitivity to NO x
than hydrocarbons, indicating that HCHO formation is not limited by NO x , although
O 3 can be. When only anthropogenic emissions are considered, HCHO is found to
be sensitive to both NO x and hydrocarbons, with mobile sources contributing substantially. Condensed chemical mechanisms are used to develop emission control
strategies, so it is important to confirm these sensitivities using detailed mechanisms
and dynamic evaluations.
The goal of multipollutant control strategies is to develop emission reductions
targeting one important pollutant, but provide co-benefits in reducing other harmful
pollutants. While both O 3 and HCHO are formed under similar conditions (high
actinic flux, availability of reactive hydrocarbons and NO x ), they show strikingly
different sensitivities. NO x reduction strategies will have the most impact overall on
O 3 . Hydrocarbon-focused strategies would be the most optimal for reducing HCHO,
although NO x controls are predicted to have some small positive impact in summer.
Questioner: Paul Makar
Question: To what extent does the gas-phase mechanism employed conserve carbon
number, and how much of the missing HCHO could be due to losses of carbon on
the way down the reaction sequence towards HCHO?
Answer: The mechanism used is CB05 with aromatic updates. While it does not conserve carbon, the carbon that the mechanism drops would not participate strongly
in ozone formation, i.e. relatively non-reactive (such as CO 2 ) or produced in small
yields. Some of these could eventually form a small amount of HCHO further downwind, but it would not likely account for the 30% under prediction.
Disclaimer Although this work was reviewed by EPA and approved for publication, it may not
necessarily reflect official agency policy.
References
1. M. Bauwens, T. Stavrakou, J.F. Müller, I. De Smedt, M. Van Roozendael, G.R. van der Werf,
C. Wiedinmyer, J.W. Kaiser, K. Sindelarova, A. Guenther, Nine years of global hydrocarbon
emissions based on source inversion of OMI formaldehyde observations. Atmos. Chem. Phys.
16, 10133–10158 (2016)
2. A. Hakami, M.T. Odman, A.G. Russell, High-order, direct sensitivity analysis of multidimensional air quality models. Environ. Sci. Technol. 37, 2442–2452 (2003)
3. D.J. Luecken, S.L. Napelenok, M. Strum, R. Scheffe, S. Phillips, Sensitivity of ambient atmospheric formaldehyde and ozone to precursor species and source types across the U.S. Environ.
Sci. Tech. 52(8), 4668–4675 (2017). https://doi.org/10.1021/acs.est.7b05509
4. S.L. Napelenok, D.S. Cohan, M.T. Odman, S. Tonse, Extension and evaluation of sensitivity
analysis capabilities in a photochemical model. Environ. Mod. Softw. 23, 994–999 (2008)