Abstract
Abstract. The tropical forests are Earth's largest source of biogenic volatile organic compounds (BVOCs) and thus also the largest atmospheric sink region for the hydroxyl radical (OH). However, the OH sink above tropical forests is poorly understood, as past studies have revealed large unattributed fractions of total OH reactivity. We present the first total OH reactivity and volatile organic compound (VOC) measurements made at the Amazon Tall Tower Observatory (ATTO) at 80, 150, and 320 m above ground level, covering two dry seasons, one wet season, and one transition season in 2018–2019. By considering a wide range of previously unaccounted for VOCs, which we identified by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS), the unattributed fraction was with an overall average of 19 % within the measurement uncertainty of ∼ 35 %. In terms of seasonal average OH reactivity, isoprene accounted for 23 %–43 % of the total and oxygenated VOCs (OVOCs) for 22 %–40 %, while monoterpenes, sesquiterpenes, and green leaf volatiles combined were responsible for 9 %–14 %. These findings show that OVOCs were until now an underestimated contributor to the OH sink above the Amazon forest. By day, total OH reactivity decreased towards higher altitudes with strongest vertical gradients observed around noon during the dry season (−0.026 s−1 m−1), while the gradient was inverted at night. Seasonal differences in total OH reactivity were observed, with the lowest daytime average and standard deviation of 19.9 ± 6.2 s−1 during a wet–dry transition season with frequent precipitation; 23.7 ± 6.5 s−1 during the wet season; and the highest average OH reactivities during two dry-season observation periods with 28.1 ± 7.9 s−1 and 29.1 ± 10.8 s−1, respectively. The effects of different environmental parameters on the OH sink were investigated, and quantified, where possible. Precipitation caused short-term spikes in total OH reactivity, which were followed by below-normal OH reactivity for several hours. Biomass burning increased total OH reactivity by 2.7 to 9.5 s−1. We present a temperature-dependent parameterization of OH reactivity that could be applied in future models of the OH sink to further reduce our knowledge gaps in tropical-forest OH chemistry.
Highlights
The Amazon rainforest, with its area of over 5.8 × 106 km2, contains more than half of Earth’s tropical forests (Morley, 2000), a quarter of global biodiversity (Dirzo and Raven, 2003), and nearly 15 % of terrestrial biomass (Houghton et al, 2001; Bar-On et al, 2018)
Noontime OH reactivity was higher in the dry seasons than in the wet or transition seasons, which is consistent with earlier studies at lower heights (Nölscher et al, 2016)
We found 31 biogenic volatile organic compounds (BVOCs) that are probably direct emissions according to the literature, which is a large number compared to previous rainforest OH reactivity studies but a small number in comparison with a study that found 264 different volatile organic compound (VOC) in the emissions of tropical trees directly measured at the leaf/bark level (Courtois et al, 2009)
Summary
The Amazon rainforest, with its area of over 5.8 × 106 km, contains more than half of Earth’s tropical forests (Morley, 2000), a quarter of global biodiversity (Dirzo and Raven, 2003), and nearly 15 % of terrestrial biomass (Houghton et al, 2001; Bar-On et al, 2018). Once released to the atmosphere, BVOCs undergo oxidation reactions within seconds to days, mainly reacting with OH radicals, which are formed during the daytime from the ozone photoproduct O1D and water as well as through recycling reactions (Taraborrelli et al, 2012) This oxidation pathway influences regional tropospheric ozone and secondary organic aerosol formation (Palm et al, 2018; Wyche et al, 2014; Hamilton et al, 2013; Goto et al, 2008; Schulz et al, 2018), thereby impacting oxidative stress to ecosystems, as well as cloud formation and global climate (Bates and Jacob, 2019; Scott et al, 2018; Engelhart et al, 2011; Pöschl et al, 2010; Heald and Spracklen, 2015). The reaction of BVOCs with OH affects the regional atmospheric oxidation capacity, which in turn controls the residence times of longerlived greenhouse gases (e.g., CH4) and pollutants (e.g., CO) (Bates and Jacob, 2019; Arneth et al, 2010; Peñuelas and Staudt, 2010)
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