Tropospheric Chemistry

We apply global models to advance our understanding of interactions among climate, atmospheric chemistry, and air quality.

1. CONNECTIONS BETWEEN CLIMATE AND AIR QUALITY

1A. CONTRIBUTION OF SHORT-LIVED SPECIES TO CLIMATE CHANGE

Changes in short-lived air pollutants (ozone, black carbon, organic carbon and sulfate) may contribute a significant portion to future warming of surface air. Under the A1B emissions scenario, the projected 65% decrease in global sulfur dioxide emissions combined with a 100% increase in black carbon aerosol may contribute up to 40% of the total warming of U.S. surface air in summer simulated by the GFDL climate model. The regional patterns of warming from short-lived species follow those of the well-mixed greenhouse gases, rather than the tropospheric loadings or radiative forcing.The figure shows the global summertime (June through August) surface temperature change in the twenty-first century due to changing emissions of short-lived gases and particles, in degrees Celsius. Cross-hatching denotes regions where the temperature change is significant at the 95% level. For more information, please see  Levy et al. [2008].

1B. REDUCING METHANE TO IMPROVE AIR QUALITY AND CLIMATE 

Reducing methane emissions is an attractive option for jointly addressing climate and ozone air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that neither the air quality nor climate benefits depend strongly on the location of the methane emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005-2030) transient simulations, we demonstrate that cost-effective methane controls would offset the positive climate forcing from methane and ozone that would otherwise occur (from increases in nitrogen oxide and methane emissions in the baseline "CLE" scenario) and improve ozone air quality. The figure shows the decrease in tropospheric ozone columns (top panels; DU) and the change in daily maximum 8-hour surface ozone (bottom panels; ppb) in 2030 from cost-effective methane emission controls (left column; Scenario "B") and from zeroing out all anthropogenic methane emissions (setting methane concentrations to pre-industrial levels of 700 ppb). Please see Fiore et al. [2008] for details.

1C. CONSTRAINING CHEMICAL UNCERTAINTIES: ISOPRENE

Uncertainties in isoprene emissions and chemistry confound efforts to quantitatively address the role of isoprene in air quality and climate. We have shown that differences in current isoprene emission inventories and in the representation of organic isoprene nitrates (their yield and fate) in chemical mechanisms substantially influence estimates of surface ozone over the eastern United States, locally by up to 15 and 12 ppbv, respectively. The largest uncertainties occur in the high isoprene-emitting southeastern United States. The figure at left shows that when we treat isoprene nitrates as a sink for nitrogen oxides in the MOZART-2 model, surface ozone decreases by 4-12 ppbv over the eastern United States (assuming a 12% yield). The yield and fate of isoprene nitrates are both highly uncertain. For more details, please see  Fiore et al. [2005].

During the summer 2004 ICARTT aircraft campaign over the eastern United States, isoprene and a suite of its oxidation products were measured, including total alkyl nitrates , which are dominated by isoprene nitrates over the eastern United States. The figure above shows results from sensitivity simulations with the MOZART model with different treatments of isoprene nitrates (the yield and the fraction recycling to NOx). See  Horowitz et al. [2007] for more details.

 

2. INTERCONTINENTAL TRANSPORT OF POLLUTION

In recent years, there has been a rapid growth in the literature documenting evidence for air pollution transport from Asia to North America, North America to Europe, and Europe to Asia, from a variety of observational and modeling techniques.

The image at left shows a plume of carbon monoxide (a good tracer of pollutant outflow) leaving the Asian continent and traveling across the Pacific to western North America as simulated with a global 1x1 version of the GFDL chemistry-climate model. Plume edges delineate concentrations above 120 ppbv within an altitude range of 760-530 hPa, with different colors denoting the location of the plume on successive days. Image c/o Remik Ziemlinski.

The image at right shows transport of oxidized nitrogen (NO_y) through the United States boundaries during the summer of 2004. For more information please see Fang et al. [2008].

The m-group at GFDL is also contributing extensively to the multi-model studies occurring under the Task Force on Hemispheric Transport of Air Pollution. A list of publications including GFDL model simulations can be found here.

3. TRENDS AND VARIABILITY IN ATMOSPHERIC CONSTITUENTS

Historical trends in burdens of climatically relevant species as simulated with the MOZART-2 model and described in detail in   Horowitz [2006]. These burdens were used as forcings in the GFDL IPCC-AR4 models CM2.0 and CM2.1 [Delworth et al., 2006].  The figure at left shows tropospheric ozone columns (DU) in 1860, 2000 and 2100 under the A2 scenario; see Ginoux et al. [2006]   for aerosols. We are also working to understand the processes controlling observed trends over the past decades, e.g., for atmospheric methane. The figure at right shows the global annual mean methane abundances measured at the NOAA cooperative network of surface stations (black circles), and MOZART-2 simulations with constant methane emissions (red crosses), and recently available time-varying anthropogenic (blue triangles) and wetland (green crosses) emissions. We find that the flattening of the methane trends post-1998 can be explained in the model by an increase in lightning NOx, by influencing OH concentrations (reaction with OH is the major sink for methane). For more information, please see Fiore et al. [2006].

4. COMMUNITY ACTIVITIES

Our group is also contributing to community efforts to advance the understanding of hemispheric transport of air pollution through the TF HTAP (see also publications including GFDL model results), as well as interactions between atmospheric chemistry and climate under the AC&C Initiative.