Nitro Aromatic Compounds and Levoglucosan as Biomass Burning Tracers of Atmosphere in New Delhi, India

Abstract

Accurate quantification of atmospheric particle phase compounds is essential for understanding their environmental and health impacts. Biomass Burning (BB) is an identified source of Nitro Aromatic Compounds (NACs). In the presence of NOX, alcoholic substituents in biomass transform into NACs. NACs are significant constituents of brown carbon (BC) due to their ability to absorb light in visible and near-UV regions. This study aimed to trace the presence of four groups of NACs namely nitrophenol, nitrocatechol, dinitrophenol and nitrobenzene in the atmosphere of New Delhi, India. To confirm the biomass burning origins of these NACs, the biomarker molecule levoglucosan (C6H10O5) was utilized. Levoglucosan derived from cellulose pyrolysis was specifically selected for its stability. and resistance against photochemical oxidation. To achieve this, onsite sampling and analysis of both gas and particle phases were performed, supplemented by offline filter desorption in the laboratory. The analytical methodology deployed Chemical Ionization Mass Spectrometry (CIMS) coupled with a Filter Inlet for Gas and Aerosol (FIGAERO). Time series analysis identified three distinct pollution episodes corresponding to biomass burning, as indicated by elevated levoglucosan levels. However, significant peaks in particle phase NACs were observed even when levoglucosan levels were low or absent, suggesting secondary formation pathways of NACs other than BB. Gas phase NACs had similar daytime behavior with a reference study (Salvador et al., 2021) but showed additional nighttime peaks during early morning and afternoon hours, further confirming secondary formation processes. The observed highest median daytime and nighttime NAC concentrations were 6000 ng m-3 and 2000 ng m-3, respectively . These concentrations are above the reported levels in the reference study. The study also evaluated the credibility of offline analysis by comparing it with online measurements, considering factors like background signal, filter size, data reproducibility, and desorption methods. Onsite blanks provided reliable background correction, and proper filter size ensured sufficient reagent ion concentration. Uniform particle collection was confirmed in the results, and over 70% of NACs evaporated in the first desorption cycle except for C6H4N2O5. Both offline and online methods showed distinct Tmax values occasionally with bimodal peaks, during the measurements. These bimodal peaks suggest the presence of isomers and fragments, leading to multiple Tmax values due to their different volatilities.