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, ourx
theoretical modeling approach remarkably identifies the most relevant PAHs, OPAHs
and NPAHs for climate forcing in this Chinese region.
