Occurrence and Distribution of Nitrophenols in Air and Rain Water in Houston and Dallas
| Project Period: | 05/01/2009 - 08/31/2009 |
| Total Budget: | $45,000 |
| Sub-Contractors: | UT Arlington
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Early work has suggested that nitrophenols (NP’s) can be present in internal combustion engine exhaust in the rage of 10-40 mol/m3. Others compared the pattern of C0-, C1-, and C2- substituted NP’s in rain with the corresponding C0-, C1-, and C2- substituted benzene compounds in ambient air in Switzerland and concluded that atmospheric oxidation and nitration processes of alkylbenzenes are favored over unsubstituted benzene. NP’s, specifically 2-methyl-4-nitrophenol (2-Me-4-NP), 3-Me-4-NP and 4-NP have been extracted from diesel particulate matter; all, especially 3-Me-4-NP has been shown to be a potent vasodilator. 4-NP from diesel emission has been shown to be both estrogenic and anti-androgenic. NP’s are phytotoxic; higher concentrations have been found on damaged leaves than on controls but it is not known whether this is a cause or symptom. Thus far the data on NP’s and dinitrophenols (DNP’s) in the US are nonexistent. Because of their highly polar nature and generally high Henry’s law coefficient (high solubility), they are likely to be easily scavenged by precipitation. Measurements or estimates for Henry’s law coefficients (M/atm at 298 K) are available but can vary widely. Beyond primary emissions, NP’s are certainly atmospherically formed through secondary reactions, data on reaction kinetics exists but dearth of actual atmospheric data has inhibited modeling efforts
Apart from health related issues another major importance of NPs lies in their relevance to HONO related atmospheric chemistry. Recent data indicates that photolytic HONO formation may likely occur upon photolysis of 2NP. It is generally assumed that all phenolic aromatic groups with a nitro group in an ortho-position to the OH group may lead to HONO formation. This may include nitro-PAHs, polynitro- and polyhydroxy-aromatics. As outlined before an important direct source for 2NP is traffic exhaust. It has been found that the 2NP/4NP ratio is higher in gasoline vs diesel driven cars. Early morning rush hour emissions may likely contribute to enhanced ambient HONO concentration. Another potential source for elevated morning HONO levels may be its photolytic formation through 2NP which has been formed secondarily through preceding nighttime processes. These processes include reactions of phenol in the presence of NO3 and NO2. They may also include the interconversion of N2O5 and ClNO2 and subsequent reaction with phenols. ClNO2 has recently been suggested to be an important constituent of the nighttime atmosphere in the Houston Galveston area. Apart from nitrogen containing precursors, NP formation always requires aromatic hydrocarbon precursors. Under conditions of nitrite photolysis benzene undergoes hydroxylation to generate phenol and nitration to give nitrobenzene. Both, 2-NP and 4-NP, are observed as secondary intermediates in these processes. Aromatic hydrocarbon precursors have significant sources in traffic exhaust and industrial solvents.
Since reactions involving NPs often occur in aqueous phase, NPs are potential candidates as interferants in wet-chemistry based instruments, including HONO measurement systems. Data so far is virtually unavailable, but HONO intercomparison performed in Houston suggest an interferant under specific polluted conditions which so far has not been able to identify.
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