Emission drivers and variability of ambient isoprene, formaldehyde and acetaldehyde in north-west India during monsoon season

Highlights

• First monsoon time in-situ measured dataset of isoprene, formaldehyde and acetaldehyde from India.

• Evidence of strong biogenic isoprene emission during monsoon season.

• Temperature was the main driving factor for biogenic isoprene.

• Photo-oxidation of hydrocarbons were the main source of formaldehyde and acetaldehyde.

Abstract

Isoprene, formaldehyde and acetaldehyde are important reactive organic compounds which strongly impact atmospheric oxidation processes and formation of tropospheric ozone. Monsoon meteorology and the topography of Himalayan foothills cause surface emissions to get rapidly transported both horizontally and vertically, thereby influencing atmospheric processes in distant regions. Further in monsoon, Indo-Gangetic Plain is a major rice growing region of the world and daytime hourly ozone can frequently exceed phytotoxic dose of 40 ppb O₃. However, the sources and ambient variability of these compounds which are potent ozone precursors are unknown. Here, we investigate the sources and photochemical processes driving their emission/formation during monsoon season from a sub-urban site at the foothills of the Himalayas. The measurements were performed in July, August and September using a high sensitivity mass spectrometer. Average ambient mixing ratios (±1σ variability) of isoprene, formaldehyde, acetaldehyde, and the sum of methyl vinyl ketone and methacrolein (MVK+MACR), were 1.4 ± 0.3 ppb, 5.7 ± 0.9 ppb, 4.5 ± 2.0 ppb, 0.75 ± 0.3 ppb, respectively, and much higher than summertime values in May. For isoprene these values were comparable to mixing ratios observed over tropical forests. Surprisingly, despite occurrence of anthropogenic emissions, biogenic emissions were found to be the major source of isoprene with peak daytime isoprene driven by temperature (r ≥ 0.8) and solar radiation. Photo-oxidation of precursor hydrocarbons were the main sources of acetaldehyde, formaldehyde and MVK+MACR. Ambient mixing ratios of all the compounds correlated poorly with acetonitrile (r ≤ 0.2), a chemical tracer for biomass burning suggesting negligible influence of biomass burning during monsoon season. Our results suggest that during monsoon season when radiation and rain are no longer limiting factors and convective activity causes surface emissions to be transported to upper atmosphere, biogenic emissions can significantly impact the remote upper atmosphere, climate and ozone affecting rice yields.

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.