Diurnal variation of nitroaromatic compounds (NACs) in the eastern Indo-Gangetic Plain.

Nitroaromatic compounds (NACs) are known to contribute significantly to the light absorption characteristics of aerosol brown carbon (BrC); however, their distribution over the Indian region is hitherto unexplored. Here we report the first measurements of 9 NACs along with methanol-soluble BrC (BrCme) in daytime and nighttime PM2.5 samples collected during summer, post-monsoon and winter seasons of 2019 from a rural receptor location in the Indo-Gangetic Plain (IGP). BrCme was enriched in daytime samples compared to nighttime (10.2±2.8 vs 5.8±1.3 Mm-1) during the summer, possibly due to secondary formation, while the profile was opposite for post-monsoon (16.6±5.2 vs 37.4±9.7 Mm-1). The 24-h average of ΣNACs across the 3 seasons was 0.24±0.13 µg m-3, with the highest levels in the post-monsoon (0.29±0.13 µg m-3) and lowest in summer (0.15±0.05 µg m-3). Daytime and nighttime values of ΣNACs for post-monsoon and winter were similar, and were significantly higher (p<0.05) compared to summer. Contribution of ΣNACs to PM2.5-associated organic carbon (OC) ranged from 0.62-1.64% during nighttime and 0.74-0.85% during daytime. The overall abundance profile of individual NACs during the study period was 5-Nitrosalicyclic acid (5NSA)>3-Nitrosalicyclic acid (3NSA)>4-Nitrocatechol (4NC)>2-Methyl-4-Nitrophenol (2M4NP)>4-Nitrophenol (4NP)>4-Methyl-5-Nitrocatechol (4M5NC)>2,6-Dimethyl-4-Nitrophenol (2,6DM4NP)>3-Methyl-4-Nitrophenol (3M4NP)>5-Nitroguaiacol (5NG). 5NSA contribution to ΣNACs was 66% during summer daytime, suggesting secondary formation via photooxidation. On the other hand, moderately good correlations of 5NSA with NO3- (r=0.46, p<0.05) during post-monsoon and with nss-K+ (r=0.49, p<0.01) during winter indicated biomass burning (BB) origins, possibly via secondary processes involving BB-related volatile organics during post-monsoon and primary BB emissions during winter. A good correlation (r= 0.51, p<0.05) between 4M5NC and 5NSA during post-monsoon and that between 4NC and 5NSA (r=0.53, p<0.01) during winter showed co-emission and evolution of these NACs. Overall, this study presents the important first step in relating BrC optical properties with constituent NACs in the Indian context.