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Overview of VOC Emissions and Chemistry from PTR-TOF-MS Measurements During the Suskat-ABC Campaign

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Overview of VOC Emissions and Chemistry from PTR-TOF-MS Measurements During the Suskat-ABC Campaign Atmos. Chem. Phys., 16, 3979–4003, 2016 www.atmos-chem-phys.net/16/3979/2016/ doi:10.5194/acp-16-3979-2016 © Author(s) 2016. CC Attribution 3.0 License. Overview of VOC emissions and chemistry from PTR-TOF-MS measurements during the SusKat-ABC campaign: high acetaldehyde, isoprene and isocyanic acid in wintertime air of the Kathmandu Valley Chinmoy Sarkar1, Vinayak Sinha1, Vinod Kumar1, Maheswar Rupakheti2,3, Arnico Panday4, Khadak S. Mahata2, Dipesh Rupakheti5, Bhogendra Kathayat3, and Mark G. Lawrence2 1Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, S. A. S. Nagar, Manauli PO, Punjab, 140306, India 2Institute for Advanced Sustainability Studies (IASS), Berliner Str. 130, 14467 Potsdam, Germany 3Himalayan Sustainability Institute (HIMSI), Kathmandu, Nepal 4International Centre for Integrated Mountain Development (ICIMOD), Khumaltar, Lalitpur, Nepal 5Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China Correspondence to: Vinayak Sinha ([email protected]) Received: 10 August 2015 – Published in Atmos. Chem. Phys. Discuss.: 15 September 2015 Revised: 19 February 2016 – Accepted: 8 March 2016 – Published: 24 March 2016 Abstract. The Kathmandu Valley in Nepal suffers from ing our wintertime deployment was acetaldehyde (8.8 ppb) severe wintertime air pollution. Volatile organic compounds > methanol (7.4 ppb) > acetone + propanal (4.2 ppb) > (VOCs) are key constituents of air pollution, though their benzene (2.7 ppb) > toluene (1.5 ppb) > isoprene (1.1 ppb) specific role in the valley is poorly understood due to > acetonitrile (1.1 ppb) > C8-aromatics (∼1 ppb) > furan insufficient data. During the SusKat-ABC (Sustainable (∼0.5 ppb) > C9-aromatics (0.4 ppb). Distinct diel profiles Atmosphere for the Kathmandu Valley–Atmospheric Brown were observed for the nominal isobaric compounds isoprene Clouds) field campaign conducted in Nepal in the winter (m = z D 69.070) and furan (m = z D 69.033). Comparison of 2012–2013, a comprehensive study was carried out to with wintertime measurements from several locations else- characterise the chemical composition of ambient Kath- where in the world showed mixing ratios of acetaldehyde mandu air, including the determination of speciated VOCs, (∼ 9 ppb), acetonitrile (∼ 1 ppb) and isoprene (∼ 1 ppb) by deploying a proton transfer reaction time-of-flight mass to be among the highest reported to date. Two “new” spectrometer (PTR-TOF-MS) – the first such deployment ambient compounds, namely formamide (m = z D 46.029) in South Asia. In the study, 71 ion peaks (for which mea- and acetamide (m = z D 60.051), which can photochemically sured ambient concentrations exceeded the 2σ detection produce isocyanic acid in the atmosphere, are reported in this limit) were detected in the PTR-TOF-MS mass scan data, study along with nitromethane (a tracer for diesel exhaust), highlighting the chemical complexity of ambient air in the which has only recently been detected in ambient studies. valley. Of the 71 species, 37 were found to have campaign Two distinct periods were selected during the campaign average concentrations greater than 200 ppt and were for detailed analysis: the first was associated with high identified based on their spectral characteristics, ambient wintertime emissions of biogenic isoprene and the second diel profiles and correlation with specific emission tracers with elevated levels of ambient acetonitrile, benzene and as a result of the high mass resolution (m = 1m > 4200) isocyanic acid from biomass burning activities. Emissions and temporal resolution (1 min) of the PTR-TOF-MS. The from biomass burning and biomass co-fired brick kilns concentration ranking in the average VOC mixing ratios dur- were found to be the dominant sources for compounds such Published by Copernicus Publications on behalf of the European Geosciences Union. 3980 C. Sarkar et al.: Wintertime high acetaldehyde, isoprene and isocyanic acid in Kathmandu Valley as propyne, propene, benzene and propanenitrile, which Table 1. Principal operational settings for PTR-TOF-MS parame- correlated strongly with acetonitrile (r2 > 0:7), a chemical ters. tracer for biomass burning. The calculated total VOC OH reactivity was dominated by acetaldehyde (24.0 %), isoprene Parameter Value (20.2 %) and propene (18.7 %), while oxygenated VOCs Overall drift voltage (Udrift) 600 V ◦ and isoprene collectively contributed to more than 68 % Temperature at drift tube (Tdrift) 60 C of the total ozone production potential. Based on known Pressure at drift tube (Pdrift) 2.2 mbar secondary organic aerosol (SOA) yields and measured Length of the drift tube (Ldrift) 9.3 cm ambient concentrations in the Kathmandu Valley, the Reaction time .t/ 92 µs ∗ relative SOA production potential of VOCs were ben- Field strength of the drift tube (E=N) 135 Td zene > naphthalene > toluene > xylenes > monoterpenes > ∗ E is the electric field strength (Vcm−1) and N is the gas number trimethylbenzenes > styrene > isoprene. The first ambient density (moleculecm−3). 1 Td D 10−17 Vcm2 molecule−1: measurements from any site in South Asia of compounds with significant health effects such as isocyanic acid, formamide, acetamide, naphthalene and nitromethane have been reported in this study. Our results suggest that ley and found that the brick kiln region south-east of the mitigation of intense wintertime biomass burning activities, valley and cities were most severely affected. With regard in particular point sources such biomass co-fired brick kilns, to quantification of volatile organic compounds (VOCs) in would be important to reduce the emission and formation downtown Kathmandu and a rural site in Nagarkot, data per- of toxic VOCs (such as benzene and isocyanic acid) in the taining to light C2–C6 compounds were obtained in a study Kathmandu Valley. in November 1998 using 38 whole air samples analysed of- fline with a GC-FID (Sharma et al., 2000). Subsequently Yu et al.(2008) measured mixing ratios of seven monocyclic aromatic hydrocarbons, using long-path differential optical 1 Introduction absorption spectroscopy (DOAS) at a suburban site in Kath- mandu during January–February 2003. All these initial stud- The Kathmandu Valley is a bowl-shaped basin at an alti- ies highlighted that traffic sources were major contributors tude of ∼ 1300 m that is surrounded by the Shivapuri, Phul- to air pollution in the Kathmandu Valley (Yu et al., 2008). chowki, Nagarjun and Chandragiri mountains, which have In the time since these studies, due to rapid urbanisation and an altitude range of 2000–2800 m above mean sea level, population growth over the last decade, the wintertime air (a.m.s.l.) and is prone to poor air quality and air pollution quality has deteriorated severely. However, very little infor- episodes (Panday et al., 2009). In particular during the win- mation is currently available with regard to the emissions and ter mornings, due to the combination of suppressed mix- chemistry of volatile organic compounds in the Kathmandu ing, katabatic wind flows and the topography of the basin, Valley. Except for a handful of species, most of which were pollutants remain trapped under an inversion layer close to measured only periodically using offline sampling methods, the surface of the valley (Kitada et al., 2003; Regmi et al., virtually no in situ data are available from the region with re- 2003). Previous studies in similar valley sites such as Santi- gard to the concentration and speciation of several important ago de Chile and Mexico City have investigated the coupling volatile organic compounds. of topography, meteorology, atmospheric dynamics, emis- VOCs, in particular the reactive ones, have atmospheric sions and chemical processes in exacerbating air pollution lifetimes ranging from minutes to days (Atkinson, 2000) and episodes and suggested ways to mitigate the air pollution and exert a profound influence on regional air quality through improve air quality (Molina et al., 2007; de Foy et al., 2006; their participation in chemical reactions leading to the forma- Schmitz, 2005; Rappenglück et al., 2005). In contrast, only tion of secondary pollutants such as tropospheric ozone and few such studies have been carried out within the Kathmandu secondary organic aerosol (SOA). Both tropospheric ozone Valley. Previous studies in the Kathmandu Valley have exam- and secondary organic aerosol are important from the stand- ined pollution in relation to carbon monoxide (CO), nitrogen point of air quality and climate due to their impact on health oxides (NOx), sulfur dioxide (SO2), ozone (O3)(Panday and and the radiative forcing of the atmosphere (IPCC, 2013). Prinn, 2009; Larssen et al., 1997; Yu et al., 2009; Ramana Further, through reactions with the hydroxyl radicals (the de- et al., 2004; Pudasainee et al., 2006) and particulate matter tergent of the atmosphere; Lelieveld et al., 2004), photodis- (Gurung and Bell, 2012; Sharma et al., 2012). An early study sociation reactions and radical recycling reactions, VOCs by Davidson et al.(1986) reported ambient average concen- strongly influence ambient OH reactivity and the budget of trations of 2 ppm CO during the winter season of 1982–1983. HOx (OH, HO2) radicals which control the removal rates of Offline measurements of nitrogen dioxide (NO2) and sulfur gaseous pollutants, including most greenhouse gases from dioxide (SO2) performed by Larssen et al.(1997) examined
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