Srikanth S. Nadadur - Air Pollution and Health Effects

The concentration of oxidative species is generally measured using fluorogenic probes. Typically a non-fluorescent species is mixed with the extract obtained from a particulate matter sample and the probe is oxidized to a fluorescent form. The resulting fluorescence intensity can then be compared to a calibration curve developed from known concentrations of a specific oxidant such as H2O2. Hasson and Paulson () have produced a microfluidic method to implement the DTT analysis that may permit it to be more widely utilized in future studies.

Fairfull-Smith and Bottle () compared the responses of DCFH-DA, POPHAA (p-hydroxyphenylacetic acid) and DTT (dithiothreitol) to surrogate ROS compounds in ambient air.

Based on DCFH-DA, an automated ROS monitor was developed (Venkatachari and Hopke ). Concentrations from the online instrument generally agreed well with those from an intensive filter measurement of ROS. The measured ROS concentrations made with this instrument were lower than reported in other studies, often below the instruments average detection limit (0.15 nmol H2O2 equivalents m3). Mean ROS concentrations were 0.26 nmol H2O2 equivalents m3 at the Atlanta urban sites and 0.14 nmol H2O2 equivalents m3 at a rural site. Thus, it may be possible to obtain more detailed particle-bound ROS concentrations, but with a considerable effort to maintain its operation.

A non-fluorescent approach has been described by Mudway et al. (). Although these reports term this method as determining oxidative potential, the method is really examining the presence and possible abiotic formation of ROS. The DTT assay can also provide similar information depending on how it is implemented. However, there is also the potential for cellular processes to produce oxidants and that is described in the following section.

There are very little data available on the concentrations of ROS in ambient PM. Hung and Wang () used POPHAA to measure ambient gas- and aerosol-phase hydroperoxide levels in Los Angeles, CA. The values they measured were in the range 0.53:5 ppbv, and 013 ng m3, respectively. On average, about 40 % of aerosol-phase H2O2 was associated with fine particles.

Venkatachari et al. ().

The ROS concentrations were relatively low, with values of the order of 107 M/m3. However, they were found to be dominantly in the very fine and ultrafine particles, and therefore are capable of being deposited efficiently in the lower lung airways. A similar trend in diurnal variation was found in Flushing, NY although the magnitudes of the average total ROS concentrations were almost an order of magnitude less than the corresponding values in Rubidoux, as might be anticipated for winter conditions with lower sunlight intensity, lower ambient temperatures, and consequent lower ozone production rates.

The diurnal trends in Flushing, NY were, however, more pronounced. For the ambient aerosol in Flushing, the average total ROS concentrations expressed in terms of H2O2 concentrations were 0.919, 1.07, 0.941 and 0.845107 M/m3 during the sampling intervals between 811 AM, 123 PM, 47 PM and 9 PM12 AM respectively. Again, as in Rubidoux, the average nighttime concentrations were found to be comparable to the average daytime concentrations. The nighttime levels of particle-bound ROS suggest the presence of some long-lived ROS species, mostly organic peroxides (Docherty et al. ). A positive correlation between the O3 and the ROS concentrations was observed which indicated that the formation of ROS is promoted by enhanced photochemical activity. However, as in Rubidoux, intensity of photochemistry was found to be a moderate factor affecting the formation of particulate ROS in the daytime atmosphere. The absence of a more positive correlation in both these locations may be explained by the vertical mixing in the lower few kilometers, slow dry deposition to the surface, and local daytime photochemistry that destroys O3 and produces HOx.