Contribution of PAHs and NACs to brown carbon light absorption in PM2.5 in Nanchang, Southern China

Polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing aromatic compounds (NACs) are pivotal chromophores in the light-absorbing component of organic carbon, specifically brown carbon (BrC). This study delves into the light absorption properties of 16 PAHs and 8 NACs, determining their mass absorption efficiency (MAE). Focusing on Nanchang, China, we conducted a case study from March to April 2023, examining the PM2.5's light absorption contribution, from these chromophores. Notably, Benzo[g,h,i]perylene exhibited the highest MAE at 365 nm, with values reaching 53.9 m2/g and 5-Nitrosalicylic acid at 70.7 m2/g. Our findings supplement the MAE across most PAH and NAC monomers with methanol as a solvent at 365 nm and average MAE at 300-500 nm. In Nanchang, methanol-soluble organic carbon (MSOC) demonstrated distinct light absorption coefficients on hazy versus clean days, 3.48 Mm-1 and 2.71 Mm-1 respectively, suggesting that secondary BrC formation or aging processes augment light absorption on hazy days. The & Aring;ngstro<spacing diaeresis>m exponent (AAEMSOC) of 4.85 for MSOC underscores the influence of a variety of regional pollutants and the diversity of chromophores formed through aging processes. MAEMSOC,365 was higher on clean days (3.23 m2 g- 1) compared to hazy days (1.53 m2 g- 1), likely attributable to MSOC's increased concentration during haze events. The light absorption contribution of PAHs at 365 nm accounted for 9.20 %, substantially exceeding their mass contribution by a factor of approximately 5.2, underscoring their dominant role. In contrast, NACs, at 2.68%, surpassed their mass contribution by a factor of 1.9, indicating a substantial yet lesser impact. This study accentuates the substantial influence of PAHs as chromophores in regional carbonaceous aerosols.