Impacts of photochemical ageing on the half-lives and diagnostic ratio of polycyclic aromatic hydrocarbons intrinsic to PM2.5 collected from 'real-world' like combustion events of wood and rice straw burning

Highlights: • PAHs quantified in ${\text{P}\text{M}}_{2.5}$ from 'real world like' wood and rice straw burning events. • Photochemical ageing impacts on particulate PAH half lives and diagnostic ratio (DR). • PAH-substrate interaction leads to distinguishable DR values for wood and straw. • Estimated half lives of carcinogenic PAHs under sun can affect global air quality. • Results demand immediate regulations against uncontrolled biomass burning in India. -- Abstract: The present experimental study describes the characteristics of polycyclic aromatic hydrocarbons (PAHs) emitted with ${\text{P}\text{M}}_{2.5}$ particles during wood and rice straw burning as well as impacts of photochemical ageing on the half lives of particulate PAHs and their diagnostic ratio values. The photochemical degradation kinetics experiments were carried out by exposing the ${\text{P}\text{M}}_{2.5}$ to light and synthetic air flow. Pseudo first order rate constants were calculated based on PAH loss as a function of exposure time. Relatively quick degradation of lighter PAHs (3-rings) [$(0.2–0.5){\text{h}}^{-1}$] than heavier PAHs (4–6 rings) [$(0.0005–0.03){\text{h}}^{-1}$] indicates substantial impact of PAH–substrate interaction through $\pi -\pi$ stacking with the carbonaceous substrates. Moreover, our results showed distinct PAH diagnostic ratios (DR) for wood and rice straw burnings which, however, change with time due to photochemical degradation. The later may add uncertainties in the applications of DR values for source apportionment. Furthermore, considerably large half lives (100–3000 h) of the carcinogenic PAHs as estimated under ambient solar radiation may cause poor and adverse air quality in long range and therefore demands immediate regulations against uncontrolled biomass burning.