(302f) Oxidative Stress Induced By Ambient Air PMx: Which Are the Main Sources?

Particulate Matter (PM) can generate reactive oxygen species (ROS) through two different mechanisms. Through the oxidative constituents absorbed onto their surface and can cause oxidation and through their ability to induce cellular generation of ROS in target cells, such as pulmonary epithelial cells and pulmonary macrophages. Chemical components of ambient PM, such as quinones and transition metals can initiate the catalysis of redox reactions in a biological system. Few studies so far have focused on the link between ROS generation with the respective aerosol sources. Identifying sources of redox-active PM_x is important for environmental health and industrial technology development. A variety of methods are available for measuring the capacity of PM_x to catalyse redox reactions. The dithiothreitol (DTT) assay is a commonly used acellular assay due to its ability to correlate well with biological markers. Several source apportionment techniques (e.g., positive matrix factorization and chemical mass balance) have been applied for the identification of the major emission sources associated with PM-catalyzed ROS generation.

The present study aimed at investigating the most important sources that drive the redox activity of ambient PM_x (PM₁, PM_2.5, PM₁₀) at two sites in Thessaloniki, the 2^nd largest city in Greece, capturing different degrees of non-traffic and traffic-related activities. For this reason, an extensive campaign was carried out at two types of locations in the area of Thessaloniki to determine the chemical composition of urban aerosols and to correlate their toxicity with PM sources. From the chemical analysis of the PM_x sampled we found that oxidative potential of PM was not correlated to its mass concentration. On the contrary, a strong correlation between chemical composition and oxidation capacity of PM was observed. A rough estimation of ROS sources was obtained based on correlations between DTT activity and chemical species found in the respective PM samples and on connecting those species with sources. At the traffic site Cu and Zn were correlated with DTT activity, highlighting the presence of vehicular emissions while at the urban background site K, levoglucosan and PAHs were correlated with DTT activity of PM.

Ambient air data showed high levels of PMduring the sampling period. However, the contribution of traffic to PM levels in urban air is reduced in the last five years in Greece. Also it was observed that the mean concentrations of PM_x at the urban background site were higher than at the traffic site. Similarly, the average total PAH concentrations were significantly higher at the urban background site. Black carbon (BC) levels (associated to internal combustion sources) did not differ significantly between the two sites. During the sampling period the contribution of biomass-fueled domestic heating to ambient air PM mass concentration was calculated to rise up to 34%. The values of levoglucosan concentrations shed light on the relative contribution of the different sources at the two sampling sites. At the background site, the estimated mean concentrations of PM₁, PM_2.5 and PM₁₀ were 0.7 μg/m³, 1.1 μg/m³ and 1.4 μg/m³ respectively, whereas at the traffic site the corresponding values were 0.3 μg/m³, 0.4 μg/m³ and 0.5 μg/m³. Regarding the oxidative potential of PM_x, the one corresponding to PM₁ was the highest, while the values for PM_2.5 and PM₁₀ followed. On a mass basis, ultrafine particles were more potent than either the fine or coarse particles. UFPs are highly chemically reactive due to their small size and large surface area and thus more harmful to human health. During the current study, the average mass-normalized redox activity of PM_x measured in PM₁, PM_2.5 and PM₁₀ were 169 ± 90, 118 ± 50, 90 ± 50 pmol min^-1 μg^-1, respectively at the background site while at the traffic site were 135 ± 80 pmol min^-1 μg^-1 for PM_1, 100 ± 75 pmol min^-1 μg^-1 for PM_2.5 and 89 ± 50 for PM₁₀.

To identify the contribution of various sources to the oxidative properties of PM_1, PM_2.5 and PM₁₀, a PMF (positive matrix factorization) analysis was conducted using the DTT activity and measured chemical composition data at each sampling site. The pooled data of PM_x at each sampling site were used to increase the statistical power of PMF. A good stability was shown, for urban background data, by choosing 4 factors while for traffic data 5 factors were selected. At background site 4 factors were determined by PMF. The first had high loadings of crustal elements (Al, Si, Ca, Ti, Mn, Fe) and it was attributed to soil resuspension. The second contained high loadings of BC, Ca, V, Ni, Cu, Zn and Pb and represented vehicular traffic. Transition metals such as Cu, Fe, Mn, and Zn, which have been related to adverse health outcomes, were associated to this factor by 91%, 12 %, 52 % and 64 %, respectively. Mn was also associated with 45% to the soil resuspension factor. Zn was found in a mixture of factors, associated with 11 % with soil resuspension and 22 % with biomass burning. As third factor, secondary aerosols were identified, which were characterized with high loadings of S, BC NO₃^-, benz[a]anthracene and chrysene. BC was found to be associated with 23%, S with 73% and benz[a]anthracene and chrysene 92 and 99 % to secondary aerosols, respectively. The fourth factor had a clear signature of biomass burning with very high concentrations of BC, K and PAHs (BbF, BkF, BeP, BaP, IcdP, BghiP). Biomass burning appears to be a significant contributor to PAHs; the latter are responsible for a large part of its toxic effects. Based on these results, PAHs are important to the oxidative potential of PM due to their ability to be transformed into quinones through photochemical reactions with atmospheric oxidants. At the urban background site biomass burning dominated the ROS-generation potential contributing by 72% to the DTT activity of PM₁, while relatively low contributions were observed from vehicle emissions (25%) and secondary formation (1%). Soil dust made a close to null contribution to DTT activity (2%). The major drivers of the DTT activity of PM_2.5 at the urban background site were biomass burning and vehicle emissions, with 62% and 34% of contributions, respectively. This was followed by soil dust, which contributed 3%, while secondary aerosol formation contributed minimally (1%) to the DTT activity of PM_2.5. Moreover, biomass burning and vehicle emissions were the dominant sources of DTT activity of PM₁₀ at the urban background site with contributions of 47 and 38 %, respectively, while secondary oxidation processes and soil dust contributed 1% and 14 % to the DTT activity of PM₁₀.

At the traffic site five factors were identified as important by PMF. The first factor contained high loadings of BC, Ca, V, Ni, Cu, Zn and Pb and it was attributed to vehicular emissions. It is noted that V and Ni had high loading in this factor indicating that this factor was mixed with oil combustion. Ni and V originate from oil combustion which is linked to diesel vehicles and shipping emissions. The second factor was biomass burning characterized by very high concentrations of BC, K and PAHs. Secondary aerosol was third with very high concentrations of S, BC. The fourth factor had high loading of crustal elements (Al, Si, Ca, Ti, Mn, Fe) while the fifth factor had high loadings of Mn, Cu, Zn, Ca and Fe. Vehicle emissions were the dominant contributor to DTT activity of PM_x contributing 62% to PM₁, 66 % to PM_2.5 and 51% to PM₁₀. Biomass burning contributed 15% to the DTT activity of PM₁, 12% to PM_2.5 and 13% to PM₁₀. Secondary aerosol dominated the ROS-generation potential contributing 17% to DTT activity of PM₁, 16% to DTT activity of PM_2.5, 13% to DTT activity of PM₁₀. Soil dust had 5% contribution to DTT activity of PM₁, 5% to DTT activity of PM_2.5, 12% to DTT activity of PM₁₀ while road dust contributed 1% to DTT activity of both PM₁ and PM_2.5 and 11% to DTT activity of PM₁₀.

Vehicle emissions and biomass burning were the two most important sources that drive DTT redox activity of ambient PM_x in Thessaloniki, Greece. Biomass burning was the largest contributor at the urban background site while vehicle emissions at the traffic site.