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Response to Comment on “Semiempirical Model for Organic Aerosol Growth by Acid-Catalyzed Heterogeneous Reactions of Carbonyls” The previously reported semiempirical model was only an exploratory step to predict the rate constants relating to heterogeneous organic aerosol growth of carbonyls in diverse experimental conditions (1). We believe that the development of predictive models and the characterization of secondary organic aerosol (SOA) formation considering both partitioning and heterogeneous reactions in the particle phase is a challenging subject to the atmospheric aerosol community. The criteria of the statement given by Jang et al. (1), “When only simple partitioning theory [log(iKpCiMseed)] is implemented to predict observed organic aerosol mass [log(Mmix - Mseed)], no predictivity power was achieved, as shown in Table 4 (R2 ) 0.0069 for 101 data points)”, was meant to describe this specific experimental setup, which was chosen to maximize heterogeneous reactions specifically. This statement is not related to the general partitioning theory and its classic application. We did not mean to suggest that thermodynamic partitioning is not important in the determination of atmospheric organic aerosol but to emphasize the fact that is does not describe the kinetic processes that are taking place heterogeneously. As we stated, semiempirical model approaches will improve our ability to predict organic aerosol growth. We were not suggesting that the model, as it stands, is directly able to predict aerosol growth in the atmosphere. This format however can be used to determine the apparent rate constant for heterogeneous aerosol growth, which can then be further applied to determine subsequent aerosol formation for heterogeneous reactions. The definitions of Y1 and Y2 were questioned in the comments of Barsanti et al. The main role of the aerosol yield (Y1), called “first-order relative aerosol yields” (1), is to simply estimate the potential capability of heterogeneous organic aerosol production of carbonyls. The deviation from the fundamental partitioning process indicated the excess gain of organic aerosol growth by heterogeneous reactions. The main goal in defining Y2 (1) is to achieve the apparent rate constant of heterogeneous reactions of carbonyl in the presence of an acid catalyst. We have modified and updated the equation for Y2(kapp,i′) in a more recent study and applied it to SOA formation from the ozonolysis of R-pinene in weakly acidic conditions, which are more relevant to realistic atmospheric aerosols (2, 3). In the modified apparent rate constant, the environmental parameter related to particle acidity is decoupled from the structure parameter to provide flexibility in the statistical regression. To demonstrate acid catalytic effects on organic aerosol production, enormous efforts have been given to this study, changing inorganic seed composition, humidity, and model carbonyls. There have been questions relating to the experimental conditions because the applied concentrations of carbonyl in the flow reactor were very high and the time scale was relatively short. We fully understand the comments for the requirement of additional studies varying concentrations. However, we emphasize that the previous study using the flow reactor associated with model carbonyls is compromised at best, based on the current limitation in choosing model carbonyls to represent secondary organic aerosol products as well as the effectiveness of collecting an extensive 8110
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set of data varying diverse experimental parameters. Currently, most secondary multifunctional aldehydes are not commercially available due to limitations in synthesis and instability in ambient conditions. For example, pinonaldehyde, a well-known secondary organic product from ozonolysis of R-pinene is unstable when heated and can decompose by ring-opening reactions. Glyoxal is the only dialdehyde which is commercially available but only in the form of polymeric aqueous solutions due to its instability. These examples in themselves indicate the tremendous driving forces of such compounds toward polymeric type products. Another important aspect of heterogeneous reactions in the particle phase is the concentration of carbonyls in the particle phase. In general, the formation of secondary organic aerosol from atmospheric oxidations of reactive organic precursors (e.g., terpenes and aromatics) progresses near saturation vapor pressure. The concentration of secondary organic products in the particle phase is much higher due to their lower volatility than the model carbonyls used in the previous study when the same gas-phase concentration is applied to the system. To study the kinetic behavior of carbonyls in the particle phase under the similar concentration ranges with secondary organic compounds, the gas-phase concentration of the model carbonyl used in the previous study was elevated to the reported experimental concentration. It is equally telling that the study of heterogeneous reactions of volatile carbonyls at low concentrations is not applicable for secondary organic products with concentrations near saturation vapor pressure. Furthermore, the high concentration of volatile model carbonyls in the previous study provides enough concentration of heterogeneous reactions of carbonyls within a short time using a flow reactor. We acknowledge your comments that the seed aerosol is not at dilute conditions; however there is limited data on partitioning to a concentrated seed aerosol. The partitioning coefficients of carbonyls in concentrated seed aerosols will be lower than those with the seed aerosol in dilution. This does not discredit this work; it only introduces more uncertainty into the determined partitioning coefficients. In fact, it gives more credence to our assertion that heterogeneous reactions are important aerosol processes that create more mass than predicted by solely partitioning. Up to now, the partitioning process has been welldeveloped by Pankow (4) and used by many research groups, but the organic aerosol formation by heterogeneous reactions is an unquantified area. We have strongly believed that both partitioning and heterogeneous reactions in the particle phase are important to predict secondary organic aerosol formation. This effort has been reflected in our recent study (2, 3) of the SOA formation from ozonolysis of R-pinene. We have attempted to predict SOA formation including both partitioning and heterogeneous reactions. The target of the previous semiempirical model is to evaluate the approximate range of apparent rate constants considering both particle acidity and molecular structures of the carbonyls. The resulting rate constants can be applied to predict SOA formation by heterogeneous reaction. This type of the study is fundamental to understand the heterogeneous reaction potential of complicated secondary organic products. We also recognize that the current model approach is not final and needs to be modified as we and the community understand the phenomena of heterogeneous reactions. We anticipate that many approaches associated with different and various experimental data including different levels of the carbonyl concentration, residence time, non-steady-state 10.1021/es0515699 CCC: $30.25
2005 American Chemical Society Published on Web 09/07/2005
status, and temperature conditions will be required in the evolution of such a model. As such, it requires enormous experimental and theoretical efforts on SOA formation in the future. We appreciate the comments by Barsanti et al. and also believe that we need further studies and updating including (1) organic aerosol growth in the presence of nondiluted inorganic seed aerosol, (2) heterogeneous reactions in nonsteady-state conditions, and (3) development of diverse ways to represent molecular structure effects.
Acknowledgments This work was supported by a grant from the National Science Foundation (ATM-0314128).
Literature Cited (1) Jang, M.; Czoschke, N. M.; Northcross, A. L. Semiempirical model for organic aerosol growth by acid-catalyzed heterogeneous reactions of organic carbonyls. Environ. Sci. Technol. 2005, 39, 164-174.
(2) Czoschke, N. M.; Jang, M. Effect of acidity parameters on the formation of heterogeneous aerosol mass in the R-pinene ozone reaction system. Environ. Sci. Technol., submitted for publication. (3) Jang, M.; Czoschke, N. M.; Northcross, A. L.; Cao, G. Secondary organic aerosol formation from partitioning and heterogeneous reactions: Model study. Environ. Sci. Technol., submitted for publication. (4) Pankow, J. F. An absorption model of the gas/aerosol partitioning involved in the formation of secondary organic aerosol. Atmos. Environ. 1994, 28, 189-193.
Myoseon Jang,* Nadine M Czoschke, Amanda L. Northcross, and Gang Cao Department of Environmental Sciences and Engineering CB# 7431, Rosenau Hall The University of North Carolina at Chapel Hill Chapel Hill, North Carolina, 27599 ES0515699
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