Research Highlights
Chemical Evolution in Wildfire Plumes
Wildfires affect the weather, climate, and air quality. Despite decades of research, wildfires are still challenging to represent in air quality models, because key physical and chemical processes in the early stage cannot be explicitly modeled.
We use a high resolution, turbulence-resolving Large Eddy Simulation (LES) coupled with chemistry to study the chemical evolution in wildfire plume. We use airborne measurements from the NOAA/NASA FIREX-AQ field campaign to evaluate the model.
Airborne measurements reveal remarkable chemical heterogeneity. Our LES-chemistry model shows promising potential. The observed cross-transect variations of a number of primary and secondary pollutants (CO, O3, PAN, ...) are well captured by the model.
This exercise has broader implications for large-scale air quality models with coarse resolutions: the chemistry is highly non-linear, the numerical dilution (caused by the coarse grid resolution) shifts the chemical regime, leading to bias in ozone chemistry. It is likely that the impact of small fires (most of them really) on ozone is largely underestimated in currently air quality models.
Also check out a few cool animations created using the same model!
Ocean biogeochemistry control on the atmosphere
Ocean emits a wide range of trace gases; many have profound impacts on the atmospheric chemistry and the climate system. Ocean biogeochemistry plays a key role in the synthesis of these compounds in the sea water, yet many of these processes remain poorly understand and underrepresented in many state-of-the-art chemistry-climate models.
Currently, most models use prescribed emissions (mostly top-down, i.e. driven by observations). This is simple and straightforward, but isn't entirely skillful.
An online air-sea exchange framework (link: OASISS) is developed for CESM2, which calculates the bi-directional oceanic fluxes for trace gases, considering the local states of the ocean (temperature, salinity, waves & bubbles) and the atmosphere (meteorology, chemistry). See Wang et al GRL 2019, and a later section for a cool demo!
A novel machine-learning framework is developed, coupling the ocean biogeochemistry with the air-sea exchange. This novel model framework provides improved skillfulness and predictability. And it is computationally efficient.
This model framework is used to develop new bottom-up oceanic emission inventories of brominated very short-lived substance (CHBr3 and CH2Br2, Wang et al. JGR 2019), acetone (Wang et al. JGR 2020), and DMS (working in progress). See more details below.
Online Air-Sea Interface for Soluble Species (OASISS)
The video shows a demo of the newly developed marine emission model for CESM2 CAM-chem. This is the air-sea exchange of an imaginary soluble gas, driven by constant surface seawater concentration (left panel).
As seen this marine-emitted compound is dispersed (middle panel). The ocean is both emitting and uptaking this compound (right panel): in the regions with high seawater concentration, the surface seawater is supersaturated and the ocean is net emitting; in the region where seawater concentration is low, the surface seawater is undersaturated and the ocean is net uptaking! The intensity of both the upward and the downward fluxes are affected by the ocean physics and the meteorology.
This module is quite flexible and user friendly (easy setup in the user_nl_cam namelist file). See CAM-chem wiki for more details. This module has been used in our recent acetaldehyde analysis (Wang et al., GRL 2019) for the NASA ATom project.
DispersionOfOASISSFullDemo_v0.mp4New bottom-up oceanic Br-VSLS inventory using machine-learning
In this work, we represent a new bottom-up oceanic emission inventory of brominated very short-lived ozone-depleting substances (bromoform and dibromomethane), with a novel machine-learning approach to couple the ocean biogeochemistry with the air-sea exchange.
The HalOcAt dataset (Ziska et al ACP 2013) is used to train the machine-learning.
The modeled surface atmospheric bromoform and dibromomethane are evaluated with the measurements from the NOAA/ESRL global monitoring network (PI: Stephen Montzka).
The vertical distributions of these compounds are evaluated with the airborne observations using three separate techniques: NCAR TOGA (PI: Eric Apel), UCI WAS (PI: Donald Blake), and NOAA PFP (PI: Stephen Montzka), from the recent global scale, multi-seasonal field campaign, NASA ATom.
This model framework takes advantage of the recently developed Online Air-Sea Interface for Soluble Species (OASISS, see Wang et al GRL 2019), and can certainly be used for other compounds of interest!
Acetone: global budget & air-sea exchange
Our paper on global atmospheric acetone budget is accepted for publication! Wang et al. JGR (2020)
Acetone (CH3COCH3) is one of the most abundant volatile organic compound in the atmosphere. It's emitted from biogenic and anthropogenic sources, also produced from photochemical degradation from other compounds in the atmosphere. Acetone is also naturally produced in the seawater! Ocean is both emitting and uptaking acetone depending on season and region.
We develop a new bottom-up oceanic emission inventory of acetone, with using an observationally trained machine-learning algorithm, coupled to the online air-sea exchange module, to better constrain the air-sea exchange of acetone.
We use two global models, CESM (with a new air-sea exchange module, OASISS) and GEOS-Chem to study the global atmospheric budget of acetone, taking advantage of the rich dataset collected during NASA ATOM.
Direct detection of bromine atoms in the atmosphere
Our paper reporting the first direct detection of bromine atoms in the Arctic Boundary layer is now in PNAS (link)!
Story in the news: UMich News; C&EN; PHYS.ORG; ScienceBulletin
Bromine atoms are thought to be the dominant oxidant of atmospheric mercury (toxic pollutant) and directly involved in the ozone depletion. Direct detection has been lacking due to tremendous technological difficulties.
We report the first direct detection of bromine atoms, along with other reactive halogen species, in Utqiaġvik (previously known as Barrow) Alaska, using the University of Michigan chemical ionization mass spectrometry (CIMS; PI: Kerri Pratt).
Ozone and atmospheric mercury depletion events were quantitatively explained by the observed bromine atoms. This is also the first time that the highly uncertain mercury chemistry is directly validated in the field.
The findings in this work may have implications in the upper troposphere/lower stratosphere (UTLS) in the tropical regions where models predict the existence of a "ring of bromine atoms" (e.g., Saiz-Lopez et al GRL 2016).
Bromine chemistry in the free troposphere
See Wang et al PNAS (2015) for details. In brief:
Atmospheric bromine chemistry catalytically destroys ozone, oxidizes atmospheric mercury, and affects the oxidative capacity.
Column observations from ground and satellite (Salawitch et al; Theys et al; etc) consistently point to the existence of a possibly ubiquitous tropospheric BrO background that is unexplained by most models.
During NSF TORERO, the CU AMAX-DOAS (PI: R Volkamer, Univ of Colorado Boulder) onboard the NSF/GV aircraft measured BrO in the pristine air in the tropical and subtropical free troposphere.
The results highlight the importance of heterogeneous chemistry on ice clouds in the upper free troposphere, and has implications for mercury oxidation mechanisms.
Acetaldehyde: ocean emissions and missing source(s)
See Wang et al GRL (2019) and AMS 2019 video for details. In brief:
Acetaldehyde (CH3CHO) is a simple carbonyl compound widely found in the atmosphere, ocean, also in coffee, bread, fruit (even in deep space).
During NASA ATOM, the NCAR TOGA (PI: Eric Apel, NCAR) mapped a number of volatile organic compounds including acetaldehyde in the remote troposphere. Interestingly, the observed acetaldehyde was much higher than predicted based on known chemistry.
A global chemistry-climate model, CESM2 CAM-chem equipped with a new air-sea exchange module (OASISS), indicated that the global ocean is a net source of acetaldehyde; but direct oceanic emissions of acetaldehyde mostly cannot reach deep into the free troposphere due to the short lifetime.
Combined observational and modeling analysis suggest there is a missing acetaldehyde source in the remote atmosphere, and the observed organic aerosols cannot explain the observed acetaldehyde.
The missing acetaldehyde source is a smoking gun of a potentially large reactive carbon source in the atmosphere currently not captured by models or measurement techniques.
Halogen chemistry in the arctic boundary layer
See Wang and Pratt JGR (2017) for details. In brief:
Reactive halogen chemistry in the polar boundary layer leads to episodic ozone depletion, affecting the radiative forcing, oxidative capacity, and the oxidation and deposition of atmospheric mercury.
Elevated levels of molecule halogens (e.g. Br2, Cl2, BrCl) have been reported in the polar boundary layer, yet the production mechanism(s) remain unclear.
During NSF BROMEX near Utqiaġvik (Barrow), Alaska, the CIMS (PI: Kerri Pratt, Univ of Michigan, Ann Arbor) simultaneously measured a number of reactive halogens.
Br + BrNO2/BrONO2 may lead to elevated daytime Br2 under low O3 conditions. This is consistent with duel isotop analysis (Morin et al) indicating that the snowpack emissions of NOx is systematically associated with reactive bromine chemistry.
This paper is submitted in memory of our mentor, colleague, and friend Roland von Glasow (University of East Anglia), who asked about the production and removal mechanisms of Cl2 as indicated by the unique Cl2 diurnal variation in an email in 2016. At least we can now answer part of his question: heterogeneous uptake of Cl2 is an important missing sink of Cl2 leading to the production of BrCl in the Arctic.
Air-Snow Interactions: a 1-d modeling study
See Wang et al. ACS Earth & Space Chem (2020) for details.
N2O5 is an important reservoir of reactive NOx in urban locations. Using a 1-D multiphase photochemistry model, we show that N2O5 deposits strongly onto the salty snowpack.
Depending on ambient/snow conditions, snowpack may be a net source or sink of ClNO2: When the salty snowpack is cold and dry, ClNO2 is produced from the reaction between N2O5 and snowpack chloride. When it's warm and damp, ClNO2 may undergo hydrolysis in snowpack, making snowpack a net sink of ClNO2.
This is a companion paper to McNamara et al. ACS Cent. Sci. (2020): application of roadsalt as de-icing agent in winter urban may affect air quality.
