Total OH reactivity over the Amazon rainforest: variability with temperature, wind, rain, altitude, time of day, season, and an overall budget closure

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.

An improved representation of fire non-methane organic gases (NMOGs) in models: emissions to reactivity

“…Figure 5 compares OHR measurements made at the ATTO site during the fire seasons in October 2018 and Septem- ber 2019 (Pfannerstill et al, 2021) with the updated GEOS-Chem model simulation. ATTO is situated ∼ 150 km northeast of Manaus, Brazil.…”

“…ATTO is situated ∼ 150 km northeast of Manaus, Brazil. We use total OHR measurements taken at 80, 150, and 320 m on the tower during two intensive observation periods in October 2018 and September 2019 using the comparative reactivity method (CRM; Sinha et al, 2008), which is described in more detail by Pfannerstill et al (2021). We confirm that simulated OHR is mostly driven by isoprene during this campaign, as Pfannerstill et al (2021) show for the observations, and find that the model (in red; median = 21.5 s −1 ) captures the overall observed (in black; median = 22.4 s −1 ) cOHR (Fig.…”

“…5a). Pfannerstill et al (2021) assessed that fires contribute 17 % of their OHR measurements (shown in black in Fig. 5b).…”

An Improved Representation of Fire Non-Methane Organic Gases (NMOGs) in Models: Emissions to Reactivity

“…4). This is an Figure 5 compares OHR measurements made at the ATTO site during the fire seasons in October 2018 and September 2019 (Pfannerstill et al, 2021) with the updated GEOS-Chem model simulation. ATTO is situated ~150 km northeast of Manaus, Brazil.…”

“…ATTO is situated ~150 km northeast of Manaus, Brazil. We use total OHR 405 measurements taken at 80, 150, and 320m on the tower during two intensive observation periods in October 2018 and September 2019 using the Comparative Reactivity Method (CRM, Sinha et al, 2008), which is described in more detail by Pfannerstill et al, (2021). We confirm that simulated OHR is mostly driven by isoprene during this campaign as Pfannerstill et al (2021) show for the observations and find that the model (in red; median = 21.5 s -1 ) captures the overall 410 observed (in black; median = 22.4 s -1 ) cOHR (Fig.…”