Investigation of sources and sinks of isocyanic acid, formamide and acetamide in the north- western Indo-Gangetic Plain (NW-IGP) during winter season

Abstract

Biomass burning/fossil fuel combustion are considered to be the major sources of atmospheric isocyanic acid and its precursors (formamide and acetamide). This study provides new insights into the sources and sinks of these Volatile Organic Compounds (VOCs) during winter time. All VOCs measurements were carried out using a Proton Transfer Reaction-Mass Spectrometer (PTR-MS) from 14.12.2014 to 03.02.2015 at a 4 minute temporal resolution at a representative suburban site in the NW-IGP (north-western Indo-Gangetic Plain). Average VOC concentrations during winter time were formamide (5.1 ppbV) > acetamide (1.1 ppbV) > isocyanic acid (0.4 ppbV). Good correlation of ICA with solar radiation (r = 0.7) suggests photochemical production to be its predominant source. Diel profile analysis of acetamide and formamide suggests that these compounds are the dominant precursors of isocyanic acid. Inter gas phase specie correlation reveals that biomass burning/fossil fuel combustion does not appear to be major source of isocyanic acid and its precursors (r ≤ 0.5 for correlation with CH 3 CN, CO, NOx and C 6 H 6 ). A strong correlation (r ≥ 0.9) of isocyanic acid, formamide and acetamide with SO 2 and NH 3 suggests aerosol-gas phase bidirectional transfer. Strong positive correlation of formamide and acetamide was observed (r = 0.8) with relative droplet volume (H 2 O(g)/24hr Average), indicating aqueous phase uptake plays a significant role in regulating their levels whereas weak correlation (r = 0.4) in the case of isocyanic acid indicates partial role of aqueous phase. Comparison of d[isocyanic acid]/dt (from sunrise to daytime maxima) in three distinct periods during winter followed the order sunny > overcast > fog which was opposite to levels of RH (a proxy for aerosol liquid water) consistent with the above hypothesis. The ambient concentration of isocyanic acid exceeded 1ppbV (exposure level of documented concern) on 28 out of 46 days highlighting the need for controlling the sources of its precursor compounds regionally.