Aqueous Atmospheric Chemistry: Formation of 2,4-Dinitrophenol upon Nitration of 2-Nitrophenol and 4-Nitrophenol in Solution DAVIDE VIONE,* VALTER MAURINO, CLAUDIO MINERO, AND EZIO PELIZZETTI Dipartimento di Chimica Analitica, Universita ` di Torino, Via Pietro Giuria 5, 10125 Torino, Italy Field studies have shown that the powerful phytotoxic agent 2,4-dinitrophenol is very likely to form in the atmospheric aqueous phase upon nitration of 2-nitrophenol or 4-nitrophenol. However, until now, the nitration pathway and the relative importance of the two mononitrophenols as sources of 2,4-dinitrophenol were not known. The present study shows that 2,4-dinitrophenol formation from mononitrophenols can take place upon photolysis and photooxidation of nitrite/nitrous acid (NO 2 - /HONO) and that nitrogen dioxide plays a key role in the process. A possible pathway might be the reaction between light- excited mononitrophenols (both 2- and 4-isomers) and nitrogen dioxide, in the presence of oxygen. As an alternative, nitration might involve • NO 3 + • NO 2 . Possible sources of nitrogen dioxide in the atmospheric aqueous phase are dissolution from the gas phase and oxidation of NO 2 - . In the latter case, however, it is necessary that NO 2 - oxidation is faster than the oxidation of mononitrophenols. This would happen, for instance, in the presence of hematite under irradiation. Radiation absorption and scattering by hematite would also inhibit the direct photolysis of nitrophenols. The formation rate and the yield of 2,4- dinitrophenol are slightly higher when starting from 2-nitrophenol than those from 4-nitrophenol, but they are compensated by the higher concentration of 4-nitrophenol in the atmospheric aqueous phase. Introduction The occurrence of nitrophenols in the atmosphere and rainwater is a matter of concern, due to the phytotoxic properties of these compounds (1). Various studies suggest a correlation between nitrophenol levels in plant leaves and the observed degree of plant damage (2, 3). Such findings are supported by the fact that nitrophenol concentrations in the leaves of damaged trees are very near the ones exerting a phytotoxic effect in vitro (4). For this reason, nitrophenols are thought to be a contributory factor in the forest decline observed in central Europe and North America (5, 6). 2,4-Dinitrophenol (2,4-DNP) is the most phytotoxic compound among the atmospheric nitrophenols. It is a powerful uncoupling agent in oxidative phosphorylation and can harm plant metabolism at levels below 10 μM(7). The occurrence of this compound in surface waters is also of concern for the protection of aquatic organisms (8). 2,4- DNP has been detected in cloudwater in both remote (9) and polluted areas (10), at comparable levels (some μgL -1 ), which might explain the forest damage observed in rural regions (3). While a small percentage of the atmospheric 2,4-DNP originates from direct traffic emissions (10), most of the atmospheric occurrence of this compound is thought to result from the nitration of mononitrophenols in the atmospheric aqueous phase (11). This opinion is based on the results of various field studies, which suggest the existence of an efficient nitration pathway in solution yielding 2,4-DNP. A study into nitrophenol occurrence in the remote atmosphere of Great Dun Fell, Northern England, yielded an anticorrelation between 2-nitrophenol (2-NP) and 2,4-DNP during cloud events, while negligible concentrations of 2,4- DNP could be detected in the absence of clouds (9). Finding an anticorrelation between the concentration values of two chemical compounds indicates that, in the hypothesis of a rough mass conservation within the air masses, the trans- formation of the former occurs at the same time as the formation of the latter. In the present case, the anticorrelation between 2-NP and 2,4-DNP, only found in the presence of cloud events, is compatible with the existence of a nitration process transforming 2-NP into 2,4-DNP in cloud droplets. A similar conclusion has been reached in a later campaign carried out at Mount Brocken, Germany (10). An anticor- relation has also been found between 4-nitrophenol (4-NP) and 2,4-DNP in rainwater in the urban area of Milan, Italy (12). While it is thought that the main atmospheric source of 2,4-DNP is mononitrophenol nitration in the aqueous phase (11), a considerable lack of knowledge exists on the nature of such a process. The main obstacle is the difficulty to nitrate mononitrophenols, due to the electron-withdrawing character of the nitro group (9, 13). In a paper studying the reactivity of aromatic substrates in aqueous solution toward the • NO3 radical, produced upon irradiation of cerium(IV) ammonium nitrate, the formation of 2,4-DNP has been reported (14). However, a later paper studying the reactivity of phenol with • NO3 + • NO2 in solution only reported the formation of mononitrophenols, while 2,4-DNP was not detected (15). Various reactive species are present in atmospheric hydrometeors that might potentially take part to nitrophenol nitration. Examples are • NO2 (10 -11 -10 -10 M levels in average continental clouds), H2O2 (10 -5 -10 -4 M), O3 (10 -10 -10 -9 M), NO3 - (10 -4 -10 -3 M), NO2 - (10 -7 -10 -6 M), and HONO (10 -9 -10 -8 M). Note however that the concentration levels of the cited species in cloudwater and other atmospheric aqueous phases are extremely variable, even much beyond the reported intervals (16). Interestingly, the described situation for the formation of 2,4-DNP is very different from the one of phenol nitration. While a very wide range of processes is known for the nitration of phenol into 2-NP and 4-NP, both in the gas phase (17-19) and in aqueous solution (9, 20-23) (the photochemical phenol nitration in solution was described as early as 1988 (20)), there is still debate on the relative weight of the various atmospheric sources of mononitrophenols (direct traffic emissions or phenol nitration in the gas or in the liquid phase (24)). While a combination of field, laboratory, and modeling studies (25) will presumably be required for a deeper understanding of the mononitrophenol sources (24), there is a clear need for laboratory, mechanistic work on the processes that can lead to the formation of 2,4-DNP in the aqueous phase. Furthermore, various field studies suggest that 2,4-DNP can originate upon nitration of either 2-NP (9, 10) or 4-NP * Corresponding author phone: +39-011-6707633; fax: +39-011- 6707615; e-mail: davide.vione@unito.it. Environ. Sci. Technol. 2005, 39, 7921-7931 10.1021/es050824m CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7921 Published on Web 09/14/2005