Towards closing the total OH reactivity budget over the Amazon rainforest

Abstract:

Tropical forests are the 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, with large fractions of total OH reactivity remaining unattributed. We present the first total OH reactivity and VOCs measurements made at the Amazon Tall Tower Observatory (ATTO, www.attoproject.org) at 80, 150, and 320 m above ground level covering two dry seasons, one wet and one transition season in 2018–2019. By considering a wide range of previously unaccounted for VOCs, which we identified by PTR-ToF-MS, we propose that the OH reactivity budget above the Amazon forest can be closed within the measurement uncertainty of ~35 %. In terms of average daytime OH reactivity, isoprene generally accounted for less than 50 % of the total, oxygenated VOCs (OVOCs) for 21–39 %, while monoterpenes, sesquiterpenes, and green leaf volatiles combined were responsible for 9–14 %.

We discuss vertically resolved seasonal and diel variations of the OH sink, as well as how it is affected by different environmental parameters, such as precipitation and biomass burning. Furthermore, we find that OVOCs were until now an underestimated contributor to the OH sink above the Amazon forest. Finally, 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.