Sensitivity of US Air Quality to Mid-Latitude Cyclone Frequency and Implications of 1980–2006 Climate Change

We show that the frequency of summertime mid-latitude cyclones tracking across eastern North America at 40°–50° N (the southern climatological storm track) is a strong predictor of stagnation and ozone pollution days in the eastern US. The NCEP/NCAR Reanalysis, going back to 1948, shows a significant long-term decline in the number of summertime mid-latitude cyclones in that track starting in 1980 (−0.15 a−1). The more recent but shorter NCEP/DOE Reanalysis (1979–2006) shows similar interannual variability in cyclone frequency but no significant long-term trend. Analysis of NOAA daily weather maps for 1980–2006 supports the trend detected in the NCEP/NCAR Reanalysis 1. A GISS general circulation model (GCM) simulation including historical forcing by greenhouse gases reproduces this decreasing cyclone trend starting in 1980. Such a long-term decrease in mid-latitude cyclone frequency over the 1980–2006 period may have offset by half the ozone air quality gains in the northeastern US from reductions in anthropogenic emissions. We find that if mid-latitude cyclone frequency had not declined, the northeastern US would have been largely compliant with the ozone air quality standard by 2001. Mid-latitude cyclone frequency is expected to decrease further over the coming decades in response to greenhouse warming and this will necessitate deeper emission reductions to achieve a given air quality goal.Engineering and Applied Science

Scientific Studies in Association with the Halogen Occultation Experiment

This work examines measurements of ozone, NO, NO2, and HCl made by the Halogen Occultation Experiment (HALOE) to track chemical change in the stratosphere. In addition, HALOE observations of two long-lived species, HF and CH4, are used as tracers to distinguish between change due to transport processes and change due to chemistry. The first study investigates the response of NO(x), (NO and NO2) and ozone to the presence of large amounts of sulfate aerosol in the stratosphere following the 1991 eruption of Mount Pinatubo. As the Pinatubo aerosol cleared the atmosphere at 17 mb (about 27-28 km), the partitioning of total reactive nitrogen shifted more toward NO(x), and ozone amounts declined. This trend is opposite that observed at lower altitudes. The second study examines the chemical aftermath of severe ozone depletion over Antarctica in spring. When ozone levels drop to a threshold amount (about 1 ppm near 20 km), the partitioning of the total chlorine family shifts rapidly from reactive species to the reservoir molecule HCl. This sudden repartitioning shuts down further ozone loss and may be significant as filaments of air peel off the polar vortex and mix with mid-latitude air

Impact of 2000–2050 climate change on fine particulate matter (PM_2.5) air quality inferred from a multi-model analysis of meteorological modes

Studies of the effect of climate change on fine particulate matter (PM_2.5 air quality using general circulation models (GCMs) show inconsistent results including in the sign of the effect. This reflects uncertainty in the GCM simulations of the regional meteorological variables affecting PM_2.5. Here we use the CMIP3 archive of data from fifteen different IPCC AR4 GCMs to obtain improved statistics of 21st-century trends in the meteorological modes driving PM_2.5 variability over the contiguous US. We analyze 1999–2010 observations to identify the dominant meteorological modes driving interannual PM_2.5 variability and their synoptic periods T. We find robust correlations (<i>r</i> > 0.5) of annual mean PM_2.5 with T, especially in the eastern US where the dominant modes represent frontal passages. The GCMs all have significant skill in reproducing present-day statistics for T and we show that this reflects their ability to simulate atmospheric baroclinicity. We then use the local PM_2.5-to-period sensitivity (dPM_2.5/dT) from the 1999–2010 observations to project PM_2.5 changes from the 2000–2050 changes in T simulated by the 15 GCMs following the SRES A1B greenhouse warming scenario. By weighted-average statistics of GCM results we project a likely 2000–2050 increase of ~ 0.1 μg m⁻³ in annual mean PM_2.5 in the eastern US arising from less frequent frontal ventilation, and a likely decrease albeit with greater inter-GCM variability in the Pacific Northwest due to more frequent maritime inflows. Potentially larger regional effects of 2000–2050 climate change on PM_2.5 may arise from changes in temperature, biogenic emissions, wildfires, and vegetation, but are still unlikely to affect annual PM_2.5 by more than 0.5 μg m⁻³

Impacts of Changes in Land Use and Land Cover on Atmospheric Chemistry and Air Quality over the 21st Century

The effects of future land use and land cover change on the chemical composition of the atmosphere and air quality are largely unknown. To investigate the potential effects associated with future changes in vegetation driven by atmospheric CO2 concentrations, climate, and anthropogenic land use over the 21st century, we performed a series of model experiments combining a general circulation model with a dynamic global vegetation model and an atmospheric chemical-transport model. Our results indicate that climate- and CO2-induced changes in vegetation composition and density between 2100 and 2000 could lead to decreases in summer afternoon surface ozone of up to 10 ppb over large areas of the northern mid-latitudes. This is largely driven by the substantial increases in ozone dry deposition associated with increases in vegetation density in a warmer climate with higher atmospheric CO2 abundance. Climate-driven vegetation changes over the period 2000–2100 lead to general increases in isoprene emissions, globally by 15% in 2050 and 36% in 2100. These increases in isoprene emissions result in decreases in surface ozone concentrations where the NOx levels are low, such as in remote tropical rainforests. However, over polluted regions, such as the northeastern United States, ozone concentrations are calculated to increase with higher isoprene emissions in the future. Increases in biogenic emissions also lead to higher concentrations of secondary organic aerosols, which increase globally by 10% in 2050 and 20% in 2100. Summertime surface concentrations of secondary organic aerosols are calculated to increase by up to 1 μg m−3 and double for large areas in Eurasia over the period of 2000–2100. When we use a scenario of future anthropogenic land use change, we find less increase in global isoprene emissions due to replacement of higher-emitting forests by lower-emitting cropland. The global atmospheric burden of secondary organic aerosols changes little by 2100 when we account for future land use change, but both secondary organic aerosols and ozone show large regional changes at the surface.Engineering and Applied Science

Global radiative forcing of coupled tropospheric ozone and aerosols in a unified general circulation model

Global simulations of sea salt and mineral dust aerosols are integrated into a previously developed unified general circulation model (GCM), the Goddard Institute for Space Studies (GISS) GCM II′, that simulates coupled tropospheric ozone-NO_x-hydrocarbon chemistry and sulfate, nitrate, ammonium, black carbon, primary organic carbon, and secondary organic carbon aerosols. The fully coupled gas-aerosol unified GCM allows one to evaluate the extent to which global burdens, radiative forcing, and eventually climate feedbacks of ozone and aerosols are influenced by gas-aerosol chemical interactions. Estimated present-day global burdens of sea salt and mineral dust are 6.93 and 18.1 Tg with lifetimes of 0.4 and 3.9 days, respectively. The GCM is applied to estimate current top of atmosphere (TOA) and surface radiative forcing by tropospheric ozone and all natural and anthropogenic aerosol components. The global annual mean value of the radiative forcing by tropospheric ozone is estimated to be +0.53 W m^(−2) at TOA and +0.07 W m^(−2) at the Earth's surface. Global, annual average TOA and surface radiative forcing by all aerosols are estimated as −0.72 and −4.04 W m^(−2), respectively. While the predicted highest aerosol cooling and heating at TOA are −10 and +12 W m^(−2), respectively, surface forcing can reach values as high as −30 W m^(−2), mainly caused by the absorption by black carbon, mineral dust, and OC. We also estimate the effects of chemistry-aerosol coupling on forcing estimates based on currently available understanding of heterogeneous reactions on aerosols. Through altering the burdens of sulfate, nitrate, and ozone, heterogeneous reactions are predicted to change the global mean TOA forcing of aerosols by 17% and influence global mean TOA forcing of tropospheric ozone by 15%

Climatic effects of 1950-2050 changes in US anthropogenic aerosols - Part 2: Climate response

We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period

Effects of 2000–2050 Changes in Climate and Emissions on Global Tropospheric Ozone and the Policy-Relevant Background Surface Ozone in the United States

We use a global chemical transport model (GEOS-Chem) driven by a general circulation model (NASA Goddard Institute for Space Studies GCM) to investigate the effects of 2000–2050 global change in climate and emissions (the Intergovernmental Panel on Climate Change A1B scenario) on the global tropospheric ozone budget and on the policy-relevant background (PRB) ozone in the United States. The PRB ozone, defined as the ozone that would be present in U.S. surface air in the absence of North American anthropogenic emissions, has important implications for setting national air quality standards. We examine separately and then together the effects of changes in climate and anthropogenic emissions of ozone precursors. We find that the 2000–2050 change in global anthropogenic emissions of ozone precursors increases the global tropospheric ozone burden by 17%. The 2000–2050 climate change increases the tropospheric ozone burden by 1.6%, due mostly to lightning in the upper troposphere, and also increases global tropospheric OH by 12%. In the lower troposphere, by contrast, climate change generally decreases the background ozone. The 2000–2050 increase in global anthropogenic emissions of ozone precursors increases PRB ozone by 2–6 ppb in summer; the maximum effect is found in April (3–7 ppb). The summertime PRB ozone decreases by up to 2 ppb with 2000–2050 climate change, except over the Great Plains, where it increases slightly as a result of increasing soil NOx emission. Climate change cancels out the effect of rising global anthropogenic emissions on the summertime PRB ozone in the eastern United States, but there is still a 2–5 ppb increase in the west.Earth and Planetary SciencesEngineering and Applied Science