Emission drivers and variability of ambient isoprene, formaldehyde and acetaldehyde in north-west India during monsoon season☆
Graphical abstract
Introduction
Isoprene is the most abundant hydrocarbon emission into the atmosphere after methane (Goldstein and Galbally, 2007; Palmer et al., 2006). Owing to its high chemical reactivity (daytime atmospheric lifetime typically < 90 min) isoprene plays a critical role in global oxidant chemistry (Lelieveld et al., 2008) and is a major hydrocarbon precursor of tropospheric ozone (Chameides et al., 1988; Fuentes et al., 2000; Pierce et al., 1998) and organic aerosols (Andreae and Crutzen, 1997; Claeys et al., 2004), which influence the Earth’s climate and are atmospheric pollutants.
During the daytime isoprene oxidation is driven by hydroxyl radicals (OH) forming oxygenated VOCs such as methyl vinyl ketone (MVK), methacrolein (MACR) as major products of its oxidation as well as formaldehyde and acetaldehyde in varying amounts (Sprengnether et al., 2002). It has been reported that plants synthesize isoprene using the methylerythritolphosphate pathway (MEP) using carbon pools of photosynthesis which is mainly dependent on enzymatic activities (Bamberger et al., 2017; Sharkey and Singsaas, 1995; Sharkey et al., 2008). As a consequence, the biogenic isoprene emissions are strongly dependent on environmental and physiological conditions (i.e., temperature and radiation, soil moisture, leaf area index (LAI), and leaf age) (Guenther et al., 1993, 2012; Niinemets et al., 1999; Stavrakou et al., 2014), and type of vegetation (i.e., grassland, forest, shrubland) (Guenther et al., 1995).
Oxygenated organic compounds such as acetaldehyde and formaldehyde have strong photochemical sources and are formed by oxidation of precursor hydrocarbons such as alkenes and alkanes (Fortems-Cheiney et al., 2012; Millet et al., 2010). In the case of acetaldehyde direct biogenic emissions from poplar and mosses and lichens have also been reported. These oxygenated volatile organic compounds are among the most reactive and abundant ambient oxygenated compounds Fall, 2003. In addition to fueling formation of surface ozone in combination with nitrogen oxides, they also act as a tropospheric source of hydroxyl and hydroperoxy radicals. By forming peroxy acetyl nitrates (PAN) they move reactive nitrogen far away from its emission sources to the upper troposphere and downwind regions, playing significant roles in regional oxidant chemistry.
In addition to the biogenic sources of isoprene and photochemical sources of acetaldehyde, formaldehyde, MVK+MACR, all of these compounds can also be emitted from biomass burning sources (Andreae, 2019; Sinha et al., 2019; Sinha et al., 2014). The northern Indo-Gangetic plains are one of the world’s highly populated (∼400 million people) and productive rice growing regions. It therefore has potentially significant biomass burning sources in addition to biogenic sources. In addition, during most of the year the temperature and radiation (Kumar et al., 2016) are sufficient for photochemically forming acetaldehyde, formaldehyde, methacrolein and methyl vinyl ketone. Thus, this densely populated region of the world with large anthropogenic and natural sources can be a significant source of isoprene through biogenic and biomass burning sources and for formaldehyde, acetaldehyde, MVK+MACR additionally through photochemical sources. In fact, hourly daytime ozone mixing ratios frequently exceed the phytotoxic dose of 40 ppb (Mills et al., 2018) during monsoon season from July to September (see Fig. S3, of Kumar et al., 2016), when paddy crops cover a vast part of the land use in the region.
Though not as reactive as acetaldehyde and isoprene, methanol and acetone are also among the most abundant compounds in ambient air and can have biogenic and photochemical sources in addition to strong anthropogenic sources (Schade and Goldstein, 2006). Based on a quantitative source apportionment study carried out during summer in the north-west Indo-Gangetic plain (Pallavi et al., 2019), anthropogenic sources such as industrial emissions and solvent use as well as biofuel use and waste disposal and traffic (cars and two-wheelers) were the main drivers controlling the ambient mixing ratios of these compounds in the region, with both biogenic and photochemical sources contributing less than 25%. The industrial and traffic sources are expected to be as active during the monsoon season as well. Hence in this study where our main focus is to examine biogenic and photochemically formed compounds during the monsoon season, we did not consider them further.
Unfortunately, due to paucity of in-situ measurements of these compounds from the region, there is a lack of understanding concerning the major seasonal processes that control the ambient variability of these compounds, which can act as potent ozone precursors. Till date only a few studies have reported measurements of these compounds in summer and in winter season from the South Asian atmospheric environment (Chandra and Sinha, 2016; Hakkim et al., 2019; Sarkar et al., 2016; Sinha et al., 2014). To the best of our knowledge no measurements of isoprene and acetaldehyde have been reported for the monsoon season from any region in India. The monsoon season is one of the most important seasons of the year over this region as it represents a period characterized by convective activity that can uplift surface emissions to the upper atmosphere (Lawrence and Lelieveld, 2010), facilitating long range transport of surface emissions. Furthermore, from the point of view of biogenic emissions from vegetation and oxidation chemistry, in contrast to the summer season, this a period when one would expect stronger biogenic emissions as the vegetation is no longer limited by availability of moisture.
Here, we report and analyze a two-month long dataset to investigate the emission processes and ambient variability of isoprene, acetaldehyde, formaldehyde, MVK+MACR, at a sub-urban site in the foothills of the Himalayas during the monsoon season. By use of a chemical marker compound for biomass burning namely acetonitrile, which was measured simultaneously, we first examined the relative importance of biomass burning sources and biogenic sources during the monsoon season. Next we examined the temperature and radiation dependence in addition to tracer data, for investigating the biogenic and photochemical sources of these compounds. Finally, we analyzed the daytime variability of the compounds and influence of boundary layer growth to assess the role of atmospheric dilution on the observed variability during monsoon and summer seasons. After comparison of the meteorological and emission drivers of these compounds we discuss the larger implications of our findings.
Section snippets
Materials and methods
The site and climatology of air masses for different seasons at the site has been described in detail in previous published works (Kumar et al., 2016; Pawar et al., 2015). The study was conducted at a sub-urban site called IISER Mohali (Pawar et al., 2015; Sinha et al., 2015) which is located in the Himalayan foothills (Fig. 1 a). To its north east, lie urban agglomerates such as Chandigarh and Panchkula city, whereas the north west fetch region consists of mainly rural and agricultural land.
General meteorology and temporal characteristics of isoprene, MVK+MACR, acetaldehyde, formaldehyde, acetonitrile
The measurements reported in this work were acquired from July 21, 2012 to September 19, 2012 during the monsoon season. In this season, the north-west Indo Gangetic Plain is characterized by high average daily relative humidity (∼65%), heavy rainfall events during the wet spells, thunderstorm activity and diffused solar radiation even though the average daily temperatures are circa 30 °C (Kumar et al., 2016). During the measurement period the average hourly daytime temperature always remained
Conclusion
This study presents the first monsoon time dataset of isoprene, MVK+MACR, formaldehyde, and acetaldehyde from India. Although the analyzed dataset is from the year 2012, due to continued similar land use and agro-forestry practices as well as similar monsoon season meteorology every year, the findings have great contemporary relevance. The influence of biomass burning sources was found to be negligible/absent when compared with summertime conditions at the same site. Further, it was found that
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We acknowledge the Central Atmospheric Chemistry facility of IISER Mohali. The technical assistance rendered by previous and current Atmospheric Chemistry and Emissions group members, in particular Chinmoy Sarkar, Vinod Kumar, Prafulla Chandra and Harshita Pawar is gratefully acknowledged. We thank the two anonymous reviewers for their helpful suggestions and comments.
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This paper has been recommended for acceptance by Pavlos Kassomenos.