Modeling the Formation, Degradation, and Spatiotemporal Distribution of 2-Nitrofluoranthene and 2-Nitropyrene in the Global Atmosphere

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants and known to cause adverse health effects. Nitrated PAHs (NPAHs) are formed in combustion activities and by nitration of PAHs in the atmosphere and may be equally or more toxic, but their spatial and temporal distribution in the atmosphere is not well characterized. Using the global EMAC model with atmospheric chemistry and surface compartments coupled, we investigate the formation, abundance, and fate of two secondarily formed NPAHs, 2-nitrofluoranthene (2-NFLT) and 2-nitropyrene (2-NPYR). The default reactivity scenario, the model with the simplest interpretation of parameters from the literature, tends to overestimate both absolute concentrations and NPAH/PAH ratios at observational sites. Sensitivity scenarios indicate that NO₂-dependent NPAH formation leads to better agreement between measured and predicted NPAH concentrations and that photodegradation is the most important loss process of 2-NFLT and 2-NPYR. The highest concentrations of 2-NFLT and 2-NPYR are found in regions with strong PAH emissions, but because of continued secondary formation from the PAH precursors, these two NPAHs are predicted to be spread across the globe.

Figure 1

Comparison between simulated and measured near-surface concentrations [pg m^–3] at rural sites of (a) 2-NFLT and (b) 2-NPYR. The dashed lines marked 1:1, 100:1, and 1:100 represent where the measured concentrations are predicted, underestimated by a factor of 100, and overestimated by a factor of 100, respectively.

Figure 2

Comparison between the simulated and measured ratios of NPAH/PAH near-surface concentrations at rural sites of (a) 2-NFLT/FLT and (b) 2-NPYR/PYR. The dashed lines marked 1:1, 100:1, and 1:100 represent where the measured concentrations are predicted, underestimated by a factor of 100, and overestimated by a factor of 100, respectively.

Figure 3

Comparison between simulation and observations for the Noto Peninsula in Japan (January 2006 to December 2007) for (a) 2-NFLT and (b) 2-NPYR particle-phase concentrations [pg m^–3]. Measurement data are from Tang et al. (2014) and presented as monthly mean concentrations (solid red line). Error bars in the measurement data are the minimum and maximum weekly values for each month. The upper and lower quartiles of simulated concentrations each month are bounded by the shaded region. The modified normalized mean bias (MNMB) and Pearson correlation coefficient (R) are shown for the default simulation (α = 0.05, blue line).

Figure 4

Comparison between simulation and observations for the Noto Peninsula in Japan (January 2006 to December 2007) for (a) 2-NFLT/FLT and (b) 2-NPYR/PYR ratio of particle-phase concentrations. Measurement data are from Tang et al. (2014) and presented as monthly mean concentrations (solid red line). The modified normalized mean bias (MNMB) and Pearson correlation coefficient (R) are shown for the default simulation (α = 0.05, blue line).

Figure 5

(a) Atmospheric concentrations at the near-surface level [pg m^–3] of 2-NFLT and 2-NPYR and (b) column densities [kg m^–2], averaged over 2006–2008, using the default reactivity scenario.

Figure 6

Ratio of NPAH to parent PAH near-surface concentrations, (a) 2-NFLT/FLT and (b) 2-NPYR/PYR, averaged over 2006–2008.