Sensitive response of atmospheric oxidative capacity to the uncertainty in the emissions of nitric oxide (NO) from soils in Amazonia

Soils are a major source of nitrogen oxides, which in the atmosphere help govern its oxidative capacity. Thus the response of soil nitric oxide (NO) emissions to forcings such as warming or forest loss has a meaningful impact on global atmospheric chemistry. We find that the soil emission rate of NO in Amazonia from a common inventory is biased low by at least an order of magnitude in comparison to tower-based observations. Accounting for this regional bias decreases the modeled global methane lifetime by 1.4% to 2.6%. In comparison, a fully deforested Amazonia, representing a 37% decrease in global emissions of isoprene, decreases methane lifetime by at most 4.6%, highlighting the sensitive response of oxidation rates to changes in emissions of NO compared to those of terpenes. Our results demonstrate that improving our understanding of soil NO emissions will yield a more accurate representation of atmospheric oxidative capacity.

Methods
Flux Data from Tapajos National Forest:

Mixing ratios of NO, nitrogen dioxide (NO2), and O3 were measured at the Tapajos National Forest from January to August of 2015. Ambient air in excess of instrument requirements was drawn in at 4-6 liters per minute (lpm) through inlets located at eight heights off of the tall tower (0.91, 3.05, 10.42, 19.57, 28.71, 39.41, 53.04, 62.24 m above the ground). The NO chemiluminescence analyzer drew ~2 lpm and the O3 analyzer drew ~1 lpm. Excess flow was pulled by a bypass pump to maintain constant pressure measured by a pressure controller. The inlets were sampled in sequence for 4 minutes each. NO and NO2 were measured by an EcoPhysics CLD-780TR analyzer equipped with an external NO2 photolysis cell using a Hamamatsu LED with peak wavelength at 365 nm (Pollack et al., 2010). The photolysis cell was toggled on and off at 60 s intervals to provide both NO and NO + NO2 measurement at each sample height. All instruments were housed in an air conditioned shed near the base of the tower. Instrument background signal was measured by periodically adding O3 generated by a Hg-vapor lamp to the sample stream to convert NO to NO2 before the sample entered the detector. Instrument gain and NO2 conversion efficiency were determined by routinely adding a small flow of NO or NO2 standard to the sample inlet.

Climate model simulation output:

We performed six sets of coupled biosphere-atmospheric chemistry simulations using the CESM2-CAM-Chem global model (Danabasoglu et al., 2020; Emmons et al., 2010) with active biogeochemistry at ~1° spatial resolution. Three separate sets were initialized with forests in Amazonia in a state that is representative of the early 1980s. The other three were initialized with Amazonia that was effectively devoid of trees by changing the plant functional type to a grassland over the region (from 16°S to 8°N and from 48°W to 78°W), as illustrated in Figure S1. Leaf area is calculated prognostically, as are other carbon fluxes and pools, however, atmospheric CO2 concentrations are specified based on observed concentrations for each year. The removal of trees in the model changes physical fluxes of energy and water, as well as carbon and chemical species (e.g. isoprene). The model calculated total leaf area index is about 6.5 m2 m-2 in the forested scenarios and about 1.8 m2 m-2 in the deforested scenario, wherein the emission rates of biogenic terpenes including isoprene and monoterpenes are negligible. For each of the forested and deforested Amazonia scenarios we calculated three sets of simulations representing different soil NO conditions: (i) baseline soil NO emission rate based on the work of Yienger and Levy (1995), and factors of (ii) 10´ that rate, and (ii) 20´ of that rate. All simulation scenarios span from 2001 to 2005, except for the forested baseline soil NO scenario which spans from 1980 to 2015. All aspects of the model except for plant type and soil NO fluxes within the Amazon vary transiently across years based on observed quantities, including greenhouse gas concentrations, sea surface temperatures, and land cover change outside the Amazon basin (“FCfireHIST” compset in CESM2).