Theoretical Prediction of Nocturnal/Diurnal and Seasonal Real World Absorption Spectra of Polycyclic Aromatic Hydrocarbons and their Derivatives in Two Chinese Polluted Cities

Abstract

Brown carbon (BrC) is an organic aerosol, released during combustion of organic matter. It strongly absorbs solar radiations hence, plays an important role in radiative forcing. Neglecting brown carbon from climate models leads to difference between theoretical and experimental works. Polycyclic aromatic hydrocarbons (PAHs) and their derivatives such as oxygenated polycyclic aromatic hydrocarbons (OPAHs) and nitrated polycyclic aromatic hydrocarbons (NPAHs) are the major constituents of the BrC and they are persistent environmental pollutants. Their study is the need of hour as they are ubiquitous in atmosphere and carcinogenic in nature. Our strategy here is to utilize time dependent-density functional theory (TD-DFT) to model the absorption spectra of PAHs and their derivatives in two Chinese industrial sites: Qingcheng district (site A) and Longtang town of Qingyuan (site B). These data are corrected for “Real-world” experimental concentrations of PAHs over these cities. For the first time, nocturnal/diurnal and seasonal variations of PAHs are being studied at a time in these Chinese regions. These findings show that most relevant absorption regions for climate forcing are ~320nm, ~380nm and ~430nm. Moreover, almost all the absorption at site A and B takes place mainly due to PAHs while OPAHs and NPAHs play a very negligible part in absorption. Site A is highly affected by climate forcing caused by these PAHs. Absorption in winter is higher as compared to that of in summer. Furthermore, this model infers that relevant contributors to absorption at ~320nm are PAHs and OPAHs while absorption at ~380nm is attributed only to PAHs. Absorption at ~430nm is mainly explained by PAHs and NPAHs. Most of the absorption takes place in UVA and visible regions of the electromagnetic spectrum. More specifically, from the 35 studied PAHs and their derivatives, the major contributors towards radiative forcing are DBA (dibenzo[ah]anthracene), BaA (benz[a]anthracene) and BkF (benzo[k]fluoranthene) (PAHs). While among OPAHs and NPAHs, major contributors are Bpone (6H-benzo[c,d]pyrene-6-one), Bzdion (Benzo[a]anthracene-7,12-dione) and 7NBaA (7-nitrobenz[a]anthracene). Thus, our x theoretical modeling approach remarkably identifies the most relevant PAHs, OPAHs and NPAHs for climate forcing in this Chinese region.