Post on 07-Sep-2020
Oxidation of Isoprene and Monoterpenes as a Function of Nitrogen Oxides in the Amazon Rain Forest
Zachary Moon1
Dandan Wei1,2, Jose D Fuentes1, Marcelo Chamecki3, Gabriel G Katul4, William H Brune1, John J Orlando5
1. Penn State University2. University of Michigan3. University of California, Los Angeles4. Duke University5. NCAR
Session: Session 8B Boundary Layer Processes and Biogeochemistry in AmazoniaProgram: 22nd Conference on Atmospheric Chemistry
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Goal of the work: to better simulate BVOC photochemistry in the Amazon rainforest by
- improving in-canopy gas-phase chemistry by better simulating in-canopy light and photolysis frequencies
- utilizing in-canopy wind data to improve modeled eddy diffusivity
Research objectives- Canopy export fractions of terpene BVOC- NOx sensitivity of terpene-OH interactions
Motivation• In the dense, tall canopies of the Amazon, in-canopy processes are especially important yet
poorly understood• The Amazon rainforest also experiences occasionally elevated concentrations of nitrogen
oxides due to regional biomass burning or long-range transport of urban emissions
GoAmazon field campaign April 2014 to January 20152Intro
Canopy height: 25-35 m Leaf area index: 5-7 m2 m-2
Moist -> high OH
Major BVOC source- due to high T, radiation, biomass- secondary OH production
Mostly clean- low NOx, NOx-limited regime
Turbulence data (nine levels)
O3, NOx
VOC data(isoprene, monoterpenes,methanol, acetone)
Forest-atmosphere exchange: BVOC-relevant processes
: We are mostly focusing on improving these
3Methods
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Canopy radiative transferspectrally resolved4-stream solver
ABL turbulent mixing
In-canopy turbulent mixingModeled K(z) with 1st-order closure
Emissions
Soil exchange
ABL depthFree tropospheric exchange
Background mixing
PhotochemistryBVOCs + {OH, O3, NO3} products
Deposition
+ {OH, O3, NO3}in-canopy BVOC processing
OVOC(secondary)
Gas-particle partitioning
Aerosol chemistry
A 1-D modeling system for forest-atmos photochemistry 4Methods
ABL model
Surface layer subcomponent
Canopy model
Soil subcomponent
Model top (e.g., 3–5 km)
Model level as box (0-D) model
(not to scale)
heig
ht
Emissions
Advection Advection
Vertical transport
Vertical transport
Deposition
Chemistry
Chemistry (+ SOA)• Gas-phase chemical reactions• EQ. gas-particle partitioning• Aq. aerosol-phase reactions
Canopy height (e.g., 10–40 m)
<- Including canopy radiative transfer and vertical mixing schemes
Improving photochemical reaction rates
The new model will include the option of explicit photolysis calculations
Methods
𝐽𝐽𝐴𝐴(𝑧𝑧) = �𝜆𝜆
𝐹𝐹 𝑧𝑧, 𝜆𝜆 𝜎𝜎𝐴𝐴 𝜆𝜆 𝜙𝜙𝐴𝐴 𝜆𝜆 𝑑𝑑𝜆𝜆𝐽𝐽𝐴𝐴 𝑧𝑧 =𝐸𝐸𝑃𝑃𝐴𝐴𝑃𝑃 𝑧𝑧
𝐸𝐸𝑃𝑃𝐴𝐴𝑃𝑃 𝑧𝑧 = ℎ𝑐𝑐⋅ 𝐽𝐽𝐴𝐴(𝑧𝑧 = ℎ𝑐𝑐)
PAR weighting method Spectral approach
From measurements or atmosphere model
^ Traditional approach uses a relative irradiance profile to weight a reference value
Our recent work: Moon et al (2020), in revision, AFMdemonstrates the importance of the spectral approach for in-canopy photolysis
^ Spectral approach explicitly calculates photolysis frequency using the spectral irradiance profile
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Observation-based in-canopy eddy diffusivityFreire (2017); approach outlined by Poggi et al. (2008)
Note: far-field formulation⇒ Opportunity to improve
by adding near-field corrections
6Methods
“observed” eddy diffusivity
measured variance of vertical wind
Lagrangian time scale for turbulent momentum transport~ ℎ𝑐𝑐/𝑢𝑢∗0.3 ℎ𝑐𝑐/𝑢𝑢∗ often used within canopy
Eulerian integral time scalefound by numerically integrating w’s autocorrelation function to its first zero-crossing
C2 = 2.8, C0 = 5.5 : “universal constants”
mean horizontal wind
Sensitivity to eddy diffusivity formulation
Traditional K-theory parameterization underpredicts Amazon in-canopy mixing during the day
7Preliminary results
Figures from Dandan Wei’s dissertation (2019)
Underprediction is most prominent (percentage-wise) in the lower third of canopy
Forest BVOC-NOx interactions
With sufficient NOx (= NO + NO2), peroxy radicals (HO2 and RO2) control ozone formation and modulate the atmospheric oxidative capacity,but there can be too much.
8Background
Fitzky et al. (2019)
NOx level Primary HOx loss pathway
low 10–100 pptv HO2 + HO2 → H2O2 + O2
moderate 1 ppbv OH + HO2 → H2O + O2
high 5–20 ppbv NO2 + OH +𝑀𝑀 → HNO3 (+𝑀𝑀∗)
NOx influence on HOxVOC-NOx-O3
BVOC + OH -> RO2
NOx sensitivity
Up to 1–3 ppbv,↑ NOx → ↑ OH → ↓ terpenesDue to increased ozone photolysis facilitated by NO2
Below 0.1 ppbv, less sensitive to percentage changes in NOx(log scale)
Beyond 3 ppbv NOx,↑ NOx → ↓ OH → ↑ terpenesHigh-NOx regime: OH+NO2 reaction dominates, OH loss
9Preliminary results
Average over midday (10:00-14:00) on 2nd day of simulations
from Dandan Wei’s dissertation (for Amazon)
NOx sensitivity – in-canopy terpene processing
Increasing NOx increases PF up to about 4 ppbv. • Monoterpenes react appreciably
with O3 so the subsequent decrease is less than that for isoprene
10Preliminary results
Estimated in-canopy processed fraction (PF)
* Note the different y-axis limits
NOx sensitivity – terpene-OH 11Preliminary results
Monoterpenes can be net OH sources but there is height and NOx dependenceIsoprene always a net sink
Estimated in-canopy terpene OH source/sink
source
sink
OH source/sink ratio
GoAmazon NO in ABL measured by aircraft
Summary
Improving:- modeling of in-canopy light- eddy diffusivity→ more daytime in-canopy mixing ⇒ decreased in-canopy terpene
concentrations and concentration gradients
In the Amazon, monoterpenes act as net OH sources, even under the influence of Manaus pollution.
- Strength depends on NOx
In-canopy terpene processing also depends on NOx
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Acknowledgements
ZM
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NOAA EPP / NCAS-Mthe National Oceanic and Atmospheric Administration, Educational Partnership Program, U.S. Department of Commerce, under Agreement No. NA16SEC4810006-NCAS-M