Robert W. Pinder

Model Representation of Secondary Organic Aerosol in CMAQv4.7

Numerous scientific upgrades to the representation of secondary organic aerosol (SOA) are incorporated into the Community Multiscale Air Quality (CMAQ) modeling system. Additions include several recently identified SOA precursors: benzene, isoprene, and sesquiterpenes; and pathways: in-cloud oxidation of glyoxal and methylglyoxal, particle-phase oligomerization, and acid enhancement of isoprene SOA. NO(x)-dependent aromatic SOA yields are also added along with new empirical measurements of the enthalpies of vaporization and organic mass-to-carbon ratios. For the first time, these SOA precursors, pathways and empirical parameters are included simultaneously in an air quality model for an annual simulation spanning the continental U.S. Comparisons of CMAQ-modeled secondary organic carbon (OC(sec)) with semiempirical estimates screened from 165 routine monitoring sites across the U.S. indicate the new SOA module substantially improves model performance. The most notable improvement occurs in the central and southeastern U.S. where the regionally averaged temporal correlations (r) between modeled and semiempirical OC(sec) increase from 0.5 to 0.8 and 0.3 to 0.8, respectively, when the new SOA module is employed. Wintertime OC(sec) results improve in all regions of the continental U.S. and the seasonal and regional patterns of biogenic SOA are better represented.

To what extent can biogenic SOA be controlled

The implicit assumption that biogenic secondary organic aerosol (SOA) is natural and can not be controlled hinders effective air quality management. Anthropogenic pollution facilitates transformation of naturally emitted volatile organic compounds (VOCs) to the particle phase, enhancing the ambient concentrations of biogenic secondary organic aerosol (SOA). It is therefore conceivable that some portion of ambient biogenic SOA can be removed by controlling emissions of anthropogenic pollutants. Direct measurement of the controllable fraction of biogenic SOA is not possible, but can be estimated through 3-dimensional photochemical air quality modeling. To examine this in detail, 22 CMAQ model simulations were conducted over the continental U.S. (August 15 to September 4, 2003). The relative contributions of five emitted pollution classes (i.e., NO(x), NH(3), SO(x), reactive non methane carbon (RNMC) and primary carbonaceous particulate matter (PCM)) on biogenic SOA were estimated by removing anthropogenic emissions of these pollutants, one at a time and all together. Model results demonstrate a strong influence of anthropogenic emissions on predicted biogenic SOA concentrations, suggesting more than 50% of biogenic SOA in the eastern U.S. can be controlled. Because biogenic SOA is substantially enhanced by controllable emissions, classification of SOA as biogenic or anthropogenic based solely on VOC origin is not sufficient to describe the controllable fraction.

Towards a climate-dependent paradigm of ammonia emission and deposition

Existing descriptions of bi-directional ammonia (NH3) land–atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated the main spatial patterns on local to global scales, it fails to address the environment- and climate-dependence of emissions. To handle these issues, we outline the basis for a new modelling paradigm where both NH3 emissions and deposition are calculated online according to diurnal, seasonal and spatial differences in meteorology. We show how measurements reveal a strong, but complex pattern of climatic dependence, which is increasingly being characterized using ground-based NH3 monitoring and satellite observations, while advances in process-based modelling are illustrated for agricultural and natural sources, including a global application for seabird colonies. A future architecture for NH3 emission–deposition modelling is proposed that integrates the spatio-temporal interactions, and provides the necessary foundation to assess the consequences of climate change. Based on available measurements, a first empirical estimate suggests that 5°C warming would increase emissions by 42 per cent (28–67%). Together with increased anthropogenic activity, global NH3 emissions may increase from 65 (45–85) Tg N in 2008 to reach 132 (89–179) Tg by 2100.

Ammonia emissions in the United States, European Union, and China derived by high‐resolution inversion of ammonium wet deposition data: Interpretation with a new agricultural emissions inventory (MASAGE_NH3)

We use the adjoint of a global 3-D chemical transport model (GEOS-Chem) to optimize ammonia (NH3) emissions in the U.S., European Union, and China by inversion of 2005–2008 network data for NH4+ wet deposition fluxes. Optimized emissions are derived on a 2° × 2.5° grid for individual months and years. Error characterization in the optimization includes model errors in precipitation. Annual optimized emissions are 2.8 Tg NH3−N a−1 for the contiguous U.S., 3.1 Tg NH3−N a−1 for the European Union, and 8.4 Tg NH3−N a−1 for China. Comparisons to previous inventories for the U.S. and European Union show consistency (∼±15%) in annual totals but some large spatial and seasonal differences. We develop a new global bottom-up inventory of NH3 emissions (Magnitude And Seasonality of Agricultural Emissions model for NH3 (MASAGE_NH3)) to interpret the results of the adjoint optimization. MASAGE_NH3 provides information on the magnitude and seasonality of NH3 emissions from individual crop and livestock sources on a 0.5° × 0.5° grid. We find that U.S. emissions peak in the spring in the Midwest due to corn fertilization and in the summer elsewhere due to manure. The seasonality of European emissions is more homogeneous with a well-defined maximum in spring associated with manure and mineral fertilizer application. There is some evidence for the effect of European regulations of NH3 emissions, notably a large fall decrease in northern Europe. Emissions in China peak in summer because of the summertime application of fertilizer for double cropping.

Epoxide Pathways Improve Model Predictions of Isoprene Markers and Reveal Key Role of Acidity in Aerosol Formation

Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.

Climate change impacts of US reactive nitrogen

Fossil fuel combustion and fertilizer application in the United States have substantially altered the nitrogen cycle, with serious effects on climate change. The climate effects can be short-lived, by impacting the chemistry of the atmosphere, or long-lived, by altering ecosystem greenhouse gas fluxes. Here we develop a coherent framework for assessing the climate change impacts of US reactive nitrogen emissions, including oxides of nitrogen, ammonia, and nitrous oxide (N2O). We use the global temperature potential (GTP), calculated at 20 and 100 y, in units of CO2 equivalents (CO2e), as a common metric. The largest cooling effects are due to combustion sources of oxides of nitrogen altering tropospheric ozone and methane concentrations and enhancing carbon sequestration in forests. The combined cooling effects are estimated at −290 to −510 Tg CO2e on a GTP20 basis. However, these effects are largely short-lived. On a GTP100 basis, combustion contributes just −16 to −95 Tg CO2e. Agriculture contributes to warming on both the 20-y and 100-y timescales, primarily through N2O emissions from soils. Under current conditions, these warming and cooling effects partially offset each other. However, recent trends show decreasing emissions from combustion sources. To prevent warming from US reactive nitrogen, reductions in agricultural N2O emissions are needed. Substantial progress toward this goal is possible using current technology. Without such actions, even greater CO2 emission reductions will be required to avoid dangerous climate change.

Quantifying spatial and seasonal variability in atmospheric ammonia with in situ and space-based observations

Ammonia plays an important role in many biogeochemical processes, yet atmospheric mixing ratios are not well known. Recently, methods have been developed for retrieving NH3 from space-based observations, but they have not been compared to in situ measurements. We have conducted a field campaign combining co-located surface measurements and satellite special observations from the Tropospheric Emission Spectrometer (TES). Our study includes 25 surface monitoring sites spanning 350 km across eastern North Carolina, a region with large seasonal and spatial variability in NH3. From the TES spectra, we retrieve a NH3 representative volume mixing ratio (RVMR), and we restrict our analysis to times when the region of the atmosphere observed by TES is representative of the surface measurement. We find that the TES NH3 RVMR qualitatively captures the seasonal and spatial variability found in eastern North Carolina. Both surface measurements and TES NH3 show a strong correspondence with the number of livestock facilities within 10 km of the observation. Furthermore, we find that TES NH3 RVMR captures the month-to-month variability present in the surface observations. The high correspondence with in situ measurements and vast spatial coverage make TES NH3 RVMR a valuable tool for understanding regional and global NH3 fluxes.

Effects of nitrogen deposition on greenhouse-gas fluxes for forests and grasslands of North America

Human activities have substantially elevated the atmospheric deposition of reactive nitrogen (N) onto terrestrial ecosystems of North America. Some of this N can stimulate carbon (C) storage in terrestrial ecosystems, but the fertilization effect of added N can be diminished by elevated concentrations of tropospheric ozone (O3) and by chronically high N deposition. In this review, we discuss spatial patterns and effects of N deposition and tropospheric O3 on net greenhouse-gas fluxes in North American forest and grassland ecosystems. While projected rates of ammonia emissions and deposition are expected to remain unchanged, projected declines in nitrogen oxide (NOx) emissions within the US will likely lead to reductions in total N deposition and O3. The net effect of these changes in N deposition onto forests and grasslands will likely initiate declines in C sequestration attributed to N deposition over the next 40 years.

Spatially Refined Aerosol Direct Radiative Forcing Efficiencies

Global aerosol direct radiative forcing (DRF) is an important metric for assessing potential climate impacts of future emissions changes. However, the radiative consequences of emissions perturbations are not readily quantified nor well understood at the level of detail necessary to assess realistic policy options. To address this challenge, here we show how adjoint model sensitivities can be used to provide highly spatially resolved estimates of the DRF from emissions of black carbon (BC), primary organic carbon (OC), sulfur dioxide (SO(2)), and ammonia (NH(3)), using the example of emissions from each sector and country following multiple Representative Concentration Pathway (RCPs). The radiative forcing efficiencies of many individual emissions are found to differ considerably from regional or sectoral averages for NH(3), SO(2) from the power sector, and BC from domestic, industrial, transportation and biomass burning sources. Consequently, the amount of emissions controls required to attain a specific DRF varies at intracontinental scales by up to a factor of 4. These results thus demonstrate both a need and means for incorporating spatially refined aerosol DRF into analysis of future emissions scenario and design of air quality and climate change mitigation policies.

GLIMPSE: A Rapid Decision Framework for Energy and Environmental Policy

Over the coming decades, new energy production technologies and the policies that oversee them will affect human health, the vitality of our ecosystems, and the stability of the global climate. The GLIMPSE decision model framework provides insights about the implications of technology and policy decisions on these outcomes. Using GLIMPSE, decision makers can identify alternative techno-policy futures, examining their air quality, health, and short- and long-term climate impacts. Ultimately, GLIMPSE will support the identification of cost-effective strategies for simultaneously achieving performance goals for these metrics. Here, we demonstrate the utility of GLIMPSE by analyzing several future energy scenarios under existing air quality regulations and potential CO2 emission reduction policies. We find opportunities for substantial cobenefits in setting both climate change mitigation and health-benefit based air quality improvement targets. Though current policies which prioritize public health protection increase near-term warming, establishing policies that also reduce greenhouse gas emissions may offset warming in the near-term and lead to significant reductions in long-term warming.