Variations in surface ozone and carbon monoxide in the Kathmandu Valley and surrounding broader regions during SusKat-ABC field campaign: role of local and regional sources

Abstract

Abstract. Air pollution resulting from rapid urbanization and associated human activities in the Kathmandu Valley of Nepal has been leading to serious public health concerns over the past 2 decades. These concerns led to a multinational field campaign SusKat-ABC (Sustainable atmosphere for the Kathmandu Valley – Atmospheric Brown Clouds) that measured different trace gases, aerosols and meteorological parameters in the Kathmandu Valley and surrounding regions during December 2012 to June 2013 to understand local- to regional-scale processes influencing air quality of the Kathmandu Valley. This study provides information about the regional distribution of ozone and some precursor gases using simultaneous in situ measurements from a SusKat-ABC supersite at Bode, Nepal, and two Indian sites: a high-altitude site, Nainital, located in the central Himalayan region and a low-altitude site, Pantnagar, located in the Indo-Gangetic Plain (IGP). The diurnal variations at Bode showed a daytime buildup in O3 while CO shows morning and evening peaks. Similar variations (with lower levels) were also observed at Pantnagar but not at Nainital. Several events of hourly ozone levels exceeding 80 ppbv were also observed at Bode. The CO levels showed a decrease from their peak level of about 2000 ppbv in January to about 680 ppbv in June at Bode. The hourly mean ozone and CO levels showed a strong negative correlation during winter (r2 = 0.82 in January and r2 = 0.71 in February), but this negative correlation gradually becomes weaker, with the lowest value in May (r2 = 0.12). The background O3 and CO mixing ratios at Bode were estimated to be about 14 and 325 ppbv, respectively. The rate of change of ozone at Bode showed a more rapid increase ( ∼ 17 ppbv h−1) during morning than the decrease in the evening (5–6 ppbv h−1), suggesting the prevalence of a semi-urban environ. The lower CO levels during spring suggest that regional transport also contributes appreciably to springtime ozone enhancement in the Kathmandu Valley on top of the local in situ ozone production. We show that regional pollution resulting from agricultural crop residue burning in northwestern IGP led to simultaneous increases in O3 and CO levels at Bode and Nainital during the first week of May 2013. A biomass-burning-induced increase in ozone and related gases was also confirmed by a global model and balloon-borne observations over Nainital. A comparison of surface ozone variations and composition of light non-methane hydrocarbons among different sites indicated the differences in emission sources of the Kathmandu Valley and the IGP. These results highlight that it is important to consider regional sources in air quality management of the Kathmandu Valley.

Molecular characterization of organic aerosols in the Kathmandu Valley, Nepal: insights into primary and secondary sources

“…The ratio of levoglucosan to mannosan (Lev/Man) has been applied to distinguish the possible categories of biomass burnt. Previously, higher Lev/Man ratios were reported for emissions from combustion of hardwood (ranging from 12.9 to 35.4 with an average of 21.5 ± 8.3) and agricultural residues (range from 12.7 to 55.7 with an average of 32.6 ± 19.1; Sang et al, 2013;Bhattarai et al, 2019). For the softwood burning, the average ratio was 4.0 ± 1.0 (ranging from 2.5 to 5.8).…”

“…Anhydrosugars of levoglucosan (1,6-anhydro-β-Dglucopyranose) and its two isomers (mannosan and galactosan) have been used as ideal molecular tracers for BB emissions (Simoneit, 2002;Bhattarai et al, 2019). They are exclusively emitted from the combustion and pyrolysis of cellulose and hemicelluloses.…”

“…Levoglucosan to OC ratios (Lev/OC) detected in source samples have been used in a wide range for quantitative estimations of the BB contribution to OC (Stone et al, 2012;Zhang et al, 2015;Wan et al, 2017), although the ratios vary among different types of biomass burnt and combustion conditions (Bhattarai et al, 2019). An average of 8.14 % (8.0 %-8.2 %) for Lev/OC from the burning sites of biofuel, savanna, crop residues, tropical forests, and so on was documented in Andreae and Merlet (2001).…”

final response

“…South Asia is considered to be a major air pollution hotspot (Monks et al, 2009) and it is expected to be one of the most polluted regions in the world for surface ozone (O 3 ) and other pollutants by 2030 (Dentener et al, 2006;IEA, 2016;OECD, 2016). Past studies have shown that the air pollution from this region affects not only the region itself, but is also transported to other parts of the world, including comparatively pristine regions such as the Himalayas and the Tibetan Plateau (Bonasoni et al, 2010;Ming et al, 2010;Lüthi et al, 2015), as well as to other distant locations such as northern Africa and the Mediterranean (Lawrence and Lelieveld, 2010).The pollutants are also uplifted to the tropopause by convective air masses and transported to the extratropical stratosphere during the monsoon season (Tissier and Legras, 2016;Lawrence and Lelieveld, 2010;Fueglistaler et al, 2009;Highwood and Hoskins, 1998). Air pollution is particularly alarming in many urban areas of South Asia, including in the city of Kathmandu and the broader Kathmandu Valley, Nepal (Chen et al, 2015;Putero et al, 2015;Kim et al, 2015;Sarkar et al, 2016;Shakya et al, 2017).…”

Ambient air quality in the Kathmandu Valley, Nepal during the pre-monsoon: Concentrations and sources of particulate matter and trace gases

“…andPhys., 11, 9, 4039-4072, doi:10.5194/acp-11-4039-2011, 2011. (Simoneit et al, 1999) Fossil fuel combustion/evaporation Hopanes (Schauer et al, 1999) Food cooking Sterols (Rogge et al, 1991) Cow dung burning Stigmastanol (Sheesley et al, 2003) Garbage/plastic burning 1,3,5-Triphenylbenzene (Simoneit et al, 2005) Vegetative detritus n-Alkanes with odd carbon preference (Rogge et al, 1993) Isoprene SOA Methyltetrols (Kleindienst et al, 2007) Bhardwaj et al (2018); b) monthly average for January 2013; c) dash denotes data not available; d) standard error, e) NMVOC were measured 1305 continuously by PTR-MS, f) CO and CH 4 were measured continuously Atmos. Chem.…”