Chemical composition of pre-monsoon air in the Indo–Gangetic Plain measured using a new PTR-MS and air quality facility: high surface ozone and strong influence of biomass burning

Abstract

Abstract. One seventh of the world population lives in the Indo–Gangetic Plain (IGP) and the fertile region sustains agricultural food crop production for much of South Asia. Yet it remains one of the most under-studied regions of the world in terms of atmospheric composition and chemistry. In particular, the emissions and chemistry of volatile organic compounds (VOCs) that form surface ozone and secondary organic aerosol through photochemical reactions involving nitrogen oxides is not well understood. In this study, ambient levels of VOCs such as methanol, acetone, acetaldehyde, acetonitrile and isoprene were measured for the first time in the IGP. A new atmospheric chemistry facility that combines India's first high sensitivity proton transfer reaction mass spectrometer, an ambient air quality station and meteorological station, was used to quantify in-situ levels of several VOCs and air pollutants in May 2012 at a suburban site in Mohali (N. W. IGP). Westerly winds arriving at high wind speeds (5–20 m s−1) in the pre-monsoon season at the site, were conducive for chemical characterization of regional emission signatures. Average levels of VOCs and air pollutants in May 2012 ranged from 1.2–1.7 nmol mol−1 for aromatic VOCs, 5.9–37.4 nmol mol−1 for the oxygenated VOCs, 1.4 nmol mol−1 for acetonitrile, 1.9 nmol mol−1 for isoprene, 567 nmol mol−1 for carbon monoxide, 57.8 nmol mol−1 for ozone, 11.5 nmol mol−1 for nitrogen oxides, 7.3 nmol mol−1 for sulphur dioxide, 104 μg m−3 for PM2.5 and 276 μg m−3 for PM10. By analyzing the one minute in-situ data with meteorological parameters and applying chemical tracers (e.g. acetonitrile for biomass burning) and inter-VOC correlations, we were able to constrain major emission source activities on both temporal and diel scales. Wheat residue burning activity caused massive increases (> 3 times of baseline values) for all the measured VOCs and primary pollutants. Other forms of biomass burning at night were also a significant source for oxygenated VOCs and isoprene (r2 with acetonitrile ≥ 0.5 for night-time data), which is remarkable in terms of atmospheric chemistry implications. Surface ozone exceeded the 8 h national ambient air quality limit of 100 μg O3 m−3 on a daily basis, except for 17 May 2012, when a severe dust storm event (PM2.5 > 800 μg m−3; PM10 > 2700 μg m−3) characterized by long range transport from the west impacted the site. The novel dataset and results point to occurrence of high primary emissions of reactive VOCs. They also highlight the urgent need for establishing more comprehensive observational facilities in the IGP to constrain the spatial and seasonal variability of atmospheric chemical constituents. Such efforts will enable mechanistic level understanding of the in-situ chemical processes controlling formation of surface ozone, a necessary step for effective ozone mitigation and improvement of the regional air quality.

Source apportionment of NMVOCs in the Kathmandu Valley during the SusKat-ABC international field campaign using positive matrix factorization

“…All of the air samples were collected in 2 L glass flasks that had been validated for the stability of NMVOCs (Chandra et al, 2017) and were analyzed within 38 h of the collection (on 9 December 2014 between 03:42 and 04:05 LT). The whole air samples were diluted (dilution factor of 9.93) using zero air for the quantification of NMVOCs present in the grab samples using a proton transfer reaction quadrupole mass spectrometer (PTR-QMS) instrument (Sinha et al, 2014). The average background signals (zero air) were subtracted from each m/z channel and stable data of at least 10 cycles (∼ 10 min) were considered for the calculation of mixing ratios as per the protocol described by Sinha et al (2014).…”

Characteristics of Particulate-Phase Polycyclic Aromatic Hydrocarbons (PAHs) in the Atmosphere over the Central Himalayas

“…For example, biomass (such as agricultural waste, animal dung and wood) is burned throughout almost the whole year mainly for cooking. Burning of large amounts of agro-residue also occurs in the IGP, especially in the non-monsoon seasons , emitting a large amount of PAHs and other air pollutants (Sinha et al, 2014). Forest fires also increase significantly in the pre-monsoon season in this region (Vadrevu et al, 2012).…”

Characteristics and sources of polycyclic aromatic hydrocarbons in atmospheric aerosols in the Kathmandu Valley, Nepal

“…A considerable amount of agro-residue is also burned in northern India and southern Nepal (Ram and Sarin, 2010). These activities emit high quantities of PAHs and other air pollutants (Sinha et al, 2014). Furthermore, forest fires increase considerably in the pre-monsoon season in the region (Vadrevu et al, 2012).…”