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NCAR GV taking off from Guam airport, 19 Jan 2014, for Research Flight 4 of the NSF CONTRAST field campaign

Our research focuses on quantification of the effect of human activity on atmospheric composition, with a focus on climate change, tropospheric chemistry, air quality, the carbon cycle, and stratospheric ozone depletion and recovery.  These efforts are motivated by the need to quantify how atmospheric composition is being altered by emissions of greenhouse gases and pollutants that drive global warming and degrade the environment.

Climate Change: Surface temperature responds to a variety of natural and anthropogenic forcings including warming due to rising levels of greenhouse gases (GHGs). We have developed a model that tracks the influence on global temperature of GHGs, volcanic and industrial aerosol particles, the 11 year variation in total solar irradiance, the temporary heat exchange between the ocean and atmosphere due to phenomena known as the El Niño Southern Oscillation (ENSO), the Atlantic Meridional Overturning Circulation, as well as long-term export of atmospheric heat to the world’s oceans (Canty et al., ACP, 2013).

Recently, we used this model to show that if all of the pledges of the Paris Climate Agreement are followed, and if the carbon intensity of the world’s economies can be improved such that at least 50% of global energy can be obtained from renewables by year 2060, climate catastrophe will likely be averted (Salawitch et al., Springer Climate, 2017). The basis of the 50% renewables by 2060 target is the finding that, for an empirical model trained by data, the RCP 4.5 pathway for GHG emissions is the actual 2°C warming pathway.  This suggestion is consistent with our best understanding of historical variations in global warming and the radiative forcing (RF) of climate, which reveal the association of ~1.0°C with about 2.25 Watts per squared meter (W m-2) of prior forcing:

Time series of  global mean surface temperature anomaly, ΔT, and the RF of climate.   Simply put, 1.0°C warming is to 2.25  W m-2 RF of climate as 2.0°C warming is to 4.5 W m-2 RF.  Our actual approach for projecting global warming is much more sophisticated, taking into account uncertainties in RF due to anthropogenic aerosols, climate feedback, and ocean heat uptake.  Nonetheless, the image above is based on IPCC (2013) best estimates and provides support for the notion that RCP 4.5 is the true 2°C warming pathway, since RCP 4.5 implies 4.5 W m-2 RF of climate by end century.  Can learn more about our group's work on climate at this link.

We are now assessing the impact on global warming forecasts of the recent U.S. withdrawal from the Paris Climate Agreement agreement, integrating emissions from the Shared Socioeconomic Pathways (SSP) database into our model framework, and converting material from Chapters 2, 3, and 4 of our book into journal articles.  The journal article presentation of our global warming projections will include a more realistic depiction of ocean heat uptake than was used in the book. While some of the details will change, the overall message is the same: adherence to RCP 4.5 GHG emissions would place the world on a 2°C warming pathway.


Tropospheric Chemistry: Hydroxyl radical (OH) is the primary oxidant in Earth's atmosphere and determines the lifetime with respect to removal of many pollutants, such as the methane (CH4), a potent greenhouse gas. Our past research has focused on improving our knowledge of two quantitative aspects of the abundance of OH in Earth's troposphere:

1) the level of OH implied by detailed observations of O3, H2O, NO, and other species in the Tropical Western Pacific (TWP) during the CONTRAST campaign (Nicely et al., JGR, 2016)

2)  the reason why levels of OH found by various global models of atmospheric chemistry differ by such a large extent (Nicely et al., JGR, 2017)

The first study revealed that the concentration of OH in the middle and upper troposphere of the TWP, illustrated below, is likely larger than previously assumed due to prior underestimates of O3 (see also Newton et al., ACP, 2016) and nitrogen monoxide (NO) (see also Anderson et al., Nat. Comm., 2016) as well as a source of OH supplied by oceanic release of of acetaldehyde (CH3CHO) that others had not considered.  As such, gases such as CH2Br2 will be oxidized in the mid-troposphere: compounds such as CH2Br2 can reach the stratosphere, but only if lofted from the marine boundary layer to the upper troposphere by energetic convection that bypasses middle troposphere.

The second study utilitized a neural network to simulate the chemistry within 8 global models based on an analysis of archived output.  The goals of this study were to quantify why the lifetime of methane (CH4) for removal by reaction with tropospheric OH differs so much between the models (see below) as well as with a value for the lifetime of CH4 inferred from the decay of CH3CCl3.

We concluded that the largest differences for the tropospheric abundance of OH between these models were, in order of importance: a) chemical mechanism; b) the photolysis frequency of O3 leading to the production of vibrationally excited oxygen [O(1D)]; c) the abundance of O3; d) the abundance of carbon monoxide (CO).

We are now focusing efforts in this area of research to analysis of data collected during the NASA ATom mission as well as archived results from global model output provided by groups that participate in the Climate-Chemistry Model Initiative.  Our efforts are conducted in close collaboration with Dr. Julie Nicely of NASA Goddard, who laid the ground-work for our work in this area.  The overall goal of these endeavors is to provide better theoretical underpinnings for representation of the loss of CH4 by reaction with OH in global models.


Air Quality. Elevated levels of tropospheric ozone cause respiratory problems linked to increased morbidity and mortality in humans as well as significant damage to crops and plants. High levels of surface ozone are caused by nitrogen oxides and hydrocarbons released in the exhaust of power plants, factories, and vehicles. Our research effort is focused on the analysis of NASA satellite and  aircraft observations of atmospheric composition, using regional air quality models such as CMAQ and CAMx as well as data collected by aircraft deployed as part of the UMD Regional Measurement, Modeling and Prediction Program (RAMMPP), to provide the scientific basis for policy decisions focused on achieving stringent, future air quality standards.

We recently have shown that elevated ozone on hot summer days in the mid-Atlantic is caused, in part, by pollution from power station peaking units utilized to meet unusually high demand for electricity during the warmest days of summer (He et al., GRL, 2013).   We have also shown, based on analysis of data collected during the DISCOVER-AQ field campaign, that the emission of nitrogen oxides from automobiles within inventories used by EPA air quality models is likely high, by nearly a factor of two, compared to the actual emissions (Anderson et al., Atmos. Envir., 2014).  Canty et al. (ACP, 2015) showed observations of the rural to urban ratio of tropospheric column NO2 observed by the NASA OMI instrument are simulated more accurately upon use of the factor of 2 reduction in mobile NOx emissions as well as faster deposition of  a class of compounds known as alkyl nitrates.  Finally, we assessed the implications of all of the above work for future regulatory actions needs to attain the National Ambient Air Quality Standard (NAAQS) for surface O3 in the mid-Atlantic: simply put, more benefit will likely result from power plant emission reductions than commonly thought (Goldberg et al., GRL, 2016).

NASA P3 aircraft during DISCOVER-AQ as seen from the UMD RAMMPP aircraft.

Our current air quality research efforts are focused on:

1)  continuing to improve the representation of the photochemical mechanism within air quality models, including a suggestion for a modification of the HCHO to C5H8 ratio within the CB6r2 mechanism we termed CB6r2-UMD (Marvin et al., Atmos. Envir., 2017)

2) improving the representation of the emission of pollutants by marine vessels within air quality models (Ring et al., submitted, 2017)

3) conducting a retrospective analysis of surface measurements from various air quality sites since the early 1970s, as well as satellite measurements of column NO2 and HCHO available for the past several decades, to assess the relative contributions of emission reductions of NOx, VOCs, and CO on improvements in air quality (Roberts et al., manuscript in preparation)

4) Supporting the Maryland Department of the Environment (MDE) in their preparation of  documents that must be submitted to the U.S. Environmental Protection Agency in order for Maryland to achieve the NAAQS for surface O3

The group's efforts on air quality have a strong bearing on policy, as manifest by my service on the MDE Air Quality Control Advisory Council.


Carbon Cycle. Carbon dioxide (CO2) is the most important anthropogenic GHG and, quite literally, the single greatest waste product of modern society. About half of the CO2 released by human activity is taken up by the world’s oceans and terrestrial biosphere. The precise location and magnitude of these carbon sinks is unknown, yet of enormous importance for defining interactions within the global carbon cycle that might be altered by climate change.   Quantification of these carbon sinks is vital for future management of the global carbon cycle. We are part of the NASA Orbiting Carbon Observatory (OCO-2) science teamOCO-2 launched on 2 July 2014, ascended to it's lead spot on the NASA A-train in early August 2014, and is now producing observations of atmospheric CO2 that are improving our understanding of the global carbon cycle.

Recently, the United States has become heavily dependent on the generation of energy from the combustion of natural gas (NG), which is primarily composed of methane (CH4), due to abundant domestic reserves of NG being extracted by hydraulic fracturing (i.e., fracking).  Ostensibly, the combustion NG is better for Earth's climate than the combustion of coal, because about twice as much energy per CO2 molecule released to the atmosphere is provided by NG compared to coal.  However, since CH4 is a more potent greenhouse gas than CO2, leakage of just a small amount of CH4 to the atmosphere, anywhere from extraction to combustion, tips the scales such that use of NG becomes harmful to climate compared to coal.  The leakage rates that cause use of NG to balance use of coal are 2.3% if one considers climate effects over a 20 yr time horizon and 6.9% over a 100 yr horizon (see footnote 35 of this reference for derivation of these break even points).

We are presently studying two aspects of the carbon cycle.  One area of research involves attempting to understand how the efficiency of carbon uptake by the combined ocean / terrestrial biosphere is changing over time.  The figure below shows a time series of atmospheric CO2 (top), emissions of CO2 (middle green) and growth of atmospheric CO2 (middle blue), together with the Ratio of the height of the blue bars (atmospheric growth) to the height of the green bars (total emissions).  The Ratio, shown in grey on the lower chart, suggests the efficiency of carbon uptake by the combined ocean / terrestrial biosphere sink could be declining over time.  Can learn more about the figure shown below at this link.

However, it is important to realize that the latter years have been influenced by a strong ENSO event.

The second area of carbon cycle research involve analysis of aircraft observations of CO2 and CH4 obtained in the Baltimore Washington region by an aircraft campaign called Fluxes of Atmospheric Greenhouse Gases in Maryland (FLAGG-MD) sponsored by the GHG measurement program of the U.S. National Institute of Standards and Technology.  FLAGG-MD began obtaining aircraft observations in winter 2015 and is modeled after the highly successful INFLUX experiment.  Recently, we published a paper that revealed the occurrence of significant emissions of CH4 from the Marcellus Shale region of southwestern Pennsylvania, and area of active fracking operations (Ren et al., JGR, 2017).  The FLAGG-MD observations analyzed by Ren et al. showed a leakage rate of about 3.9%of the total oil and natural gas production mass in this region, which means that the extraction of CH4 by fracking is harmful to Earth's climate, over a 20 year time horizon for conditions encountered during August and September 2015.

Our current carbon cycle research efforts are focused on:

1) quantification of how the efficiency of the removal of CO2 by the combined ocean / terrestrial biosphere sink is changing over time

2) assessing how aircraft measurements of CO2 and CH4 obtained during FLAGG-MD can be used to improve our knowledge of fluxes of these two GHGs over the densely population  Baltimore Washington urban region, using a mass balance approach tied to air parcel trajectories (see Ren et al., JGR, 2017 for a description of the technique) (Ahn et al., manuscript in preparation)

3) quantification of CH4 fluxes over the Marcellus Shale region based on aircraft flights conducted in 2017

4) comparing profiles of CO2 obtained during FLAGG-MD to retrievals of the dry air, column average mole fraction of CO2 (XCO2) retrieved by the OCO-2 team

5) assessing whether the mass balance approach for quantifying fluxes of CO2 can be applied to observations of XCO2 obtained by OCO-2


Stratosheric Ozone Depletion and Recovery.  The Earth's ozone layer protects humans, plants, and animals from the harmful effects of ultraviolet radiation emitted by the Sun.  Ozone depletion has resulted from the industrial release of a class of compounds called ozone depleting substances (ODSs) that contain chlorine and/or bromine.  An excellent, readily accessible summary of the successful regulation of ODSs is provided by this document that I am proud to state I have helped write over the years, together with many colleagues.  I also recently published an succinct essay on Chlorinated Fluorocarbons and Other Ozone-Destroying Chemicals that can be obtained by clicking here.

Our efforts on stratospheric ozone and recovery are focused on quantifying the role of very short-lived (VSL) naturally produced biogenic bromocarbons on the stratospheric bromine loading (Salawitch, Nature, 2006). Here, VSL refers to an atmospheric compound with a photochemical lifetime with respect to removal of less than 6 months. The supply of of stratospheric bromine from VSL sources has been difficult to quantify because:

a) the souce gases have an atmospheric abundance that is quite variable in space and time

b) significant injection could occur via cross-tropopause transport of inorganic gases (so-called products) produced upon tropospheric decomposition of source species.

Nonetheless, it has been established that if significant amounts of bromine enter the stratospheric from this natural source, the ozone layer is more vulnerable to depletion following volcanic eruptions (Salawitch et al., GRL, 2005) and also if society ever decided to attempt to mitigate global warming via the  injection of sulfate aerosols to the stratosphere (Tilmes et al., Science, 2008; Tilmes et al., ACP, 2012).  The abundance of VSL bromine has also been shown to have a significant effect on the depth and perhaps timing of the recovery of the Antarctic ozone hole (Oman et al., GRL, 2016; Fernandez et al., ACP, 2017). Finally, the inference of tropospheric column BrO from a measurement of the total column BrO obtained by satellite instruments is extremely sensitive to the stratospheric burden of bromine supplied by VSL compounds (Salawitch et al., GRL, 2010).

The CONTRAST (Pan et al., BAMS, 2017), ATTREX (Jensen et al., BAMS, 2017) and CAST (Harris et al., BAMS, 2017) aircraft campaigns conducted during the winter of 2014 in Guam were designed, in part, to provide observations that would allow the stratospheric supply of bromine from VSL compounds to be precisely quantified.  This joint effort represents the first comprehensive sampling of VSL bromocarbons, together with BrO, from altitudes ranging from the marine boundary layer to the lower stratosphere.  The measurements revealed the presence of about 3 parts per trillion (pptv) (bromine content) of organic VSL compounds in the tropical tropopause transition layer, as shown below.

As noted above, the quantification of stratospheric supply of bromine from VSL compounds requires quantification of both organic (source gases) and inorganic (product gases).  The NCAR GV aircraft used for CONTRAST contained two instruments that measured BrO, from which the total inorganic bromine burden can be found. One instrument relies on chemical ionization mass spectrometry (CIMS) via in situ  sampling (Chen et al., JGR, 2016).  The other instrument utilizes remote sensing via Differential Optical Absorption Spectrometry (DOAS) (Koenig et al., ACPD, 2017).  Chen et al. (2016) reported product gas injection of Bry to be about 2 pptv which would lead to a total VSL bromine source of about 5 ppt.  Koenig et al. (2017) focused mainly on profiles of BrO in the tropical troposphere, arguing for the presences of a significant source from sea salt aerosol.

Our current stratospheric ozone depletion and recovery research efforts are focused on:

1) quantification of stratospheric injection of bromine from VSL sources by combining observations of organic compounds with the CIMS and DOAS observations of BrO, in a common analysis framework (Wales et al., manuscript in preparation)

2) assessing the impact of future volcanic eruptions on the thickness of the ozone layer by considering, within the AER-2D model, various scenarios for the future evolution of GHGs as well as stratospheric halogen burdens (Klobas et al., GRL, 2017)

3) evaluation of various possible sources of the springtime, annual release of bromine to the Arctic troposphere (the so-called bromine explosion) (Choi et al., JGR, submitted, 2017)

4) reconciliation of the CONTRAST/ATTREX estimate of VSL bromine with total column BrO measured by the NASA OMI instrument, taking into consideration correlative ground-based observations of total column BrO by a suite of instruments deployed in Fairbanks, Alaska (Wales and Spinei, research in progress)

Department of Atmospheric and Oceanic Science                                           College of Computer, Mathematical, and Natural Sciences

Department of Chemistry and Biochemistry                                                                                  The University of Maryland Newsdesk

Earth System Science Interdisciplinary Center                                                                                             The University of Maryland

This page last updated on Monday, 31 July 2017