A44C-01 INVITED
Long-range transport of air pollution into the Arctic
This paper presents an overview of air pollution transport into the Arctic. The major transport processes will be highlighted, as well as their seasonal, interannual, and spatial variability. The source regions of Arctic air pollution will be discussed, with a focus on black carbon (BC) sources, as BC can produce significant radiative forcing in the Arctic. It is found that Europe is the main source region for BC in winter, whereas boreal forest fires are the strongest source in summer, especially in years of strong burning. Two case studies of recent extreme Arctic air pollution events will be presented. In summer 2004, boreal forest fires in Alaska and Canada caused pan-Arctic enhancements of black carbon. The BC concentrations measured at Barrow (Alaska), Alert (Canada), Summit (Greenland) and Zeppelin (Spitsbergen) were all episodically elevated, as a result of the long-range transport of the biomass burning emissions. Aerosol optical depth was also episodically elevated at these stations, with an almost continuous elevation over more than a month at Summit. During the second episode in spring 2006, new records were set for all measured air pollutant species at the Zeppelin station (Spitsbergen) as well as for ozone in Iceland. At Zeppelin, BC, AOD, aerosol mass, ozone, carbon monoxide and other compounds all reached new record levels, compared to the long-term monitoring record. The episode was caused by transport of polluted air masses from Eastern Europe deep into the Arctic, a consequence of the unusual warmth in the European Arctic during the episode. While fossil fuel combustion sources certainly contributed to this episode, smoke from agricultural fires in Eastern Europe was the dominant pollution component. We also suggest a new revolatilization mechanism for persistent organic pollutants (POPs) stored in soils and vegetation by fires, as POPs were strongly elevated during both episodes. All this suggests a considerable influence of biomass burning on the pollutant concentrations in the Arctic in spring and summer, even for species that are produced exclusively by humans, such as POPs.
A44C-02
Distribution of Aerosols in the Arctic as Observed by CALIOP
The Arctic climate is now recognized to be uniquely sensitive to atmospheric perturbations. Pollution aerosols and smoke from boreal fires have potentially important impacts on Arctic climate but there are many uncertainties. Aerosol in the Arctic, generally referred to as "Arctic haze", has been studied with great interest for over thirty years. Much has been learned about the composition and sources of the haze yet our knowledge is largely based on long term measurements at a very few widely dispersed sites, augmented by modeling activities and occasional field campaigns. Transport pathways from source regions into the Arctic are not well understood. Emission patterns have changed over the last several decades, but the impact of this on concentrations and distribution of Arctic haze are understood only in the crudest sense. Due to poor lighting conditions, extended periods of darkness, and surfaces covered by snow and ice, satellite sensors have been unable to provide much information on Arctic haze to date. The CALIPSO satellite carries CALIOP, a two-wavelength polarization lidar, optimized for profiling clouds and aerosols. CALIOP has been acquiring global observations since June 2006 and provides our first opportunity to observe the distribution and seasonal variation of aerosol in the Arctic. The Arctic is characterized by the prevalence of optically thin ice clouds and clouds composed of supercooled water, often occurring in the same atmospheric column along with aerosol. CALIOP depolarization signals are used to discriminate Arctic haze from optically thin cirrus and diamond dust. Two-wavelength returns aid in the discrimination of aerosol and optically thin water cloud. Results of initial analyses of CALIOP aerosol observations in the Arctic will be presented. This work is a preliminary analysis in support of the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign planned for April 2008. http://www-calipso.larc.nasa.gov
A44C-03
Polar Pollution by Pyroconvection: Assessing the transport of smoke into the Arctic and Antarctic
In northern summer of 2007 boreal forest fires erupted into pyrocumulonimbus (pyroCb) on two continents and injected substantial amounts of smoke into the upper troposphere and lower stratosphere (UTLS). Some of this smoke was transported into high Arctic latitudes where it was observed by ground and space-based instruments. In the Austral summer of 2006/2007 an extremely intense pyroCb in Victoria Australia also polluted the UTLS, from tropical to Antarctic latitudes. This wide-ranging influence of a "small", discrete event such as a fire-aided thunderstorm implies that the pyroCb's weather and climate context needs to be much better understood. Here we will present two case studies from the above-mentioned seasons. The Austral case will include an analysis of the immediate post-blowup smoke plume as observed by MODIS imagery and OMI aerosol index (AI). Then the plume will be followed as it circles the globe and spreads to meridional extremes using AI, CALIPSO backscatter, and geostationary satellite visible imagery. The boreal study will summarize the notable pyroCbs of 2007. One of these was in Mongolia, the first such unambiguously determined Asian pyroCb. The main focus of this analysis will on the observations of stratospheric smoke subsequent to these blowups in midlatitudes observed by CALIPSO, and Arctic smoke observed by both CALIPSO and ground-based lidar at Eureka.
A44C-04
Evidence for a recurring eastern North America upper tropospheric ozone maximum during summer
Daily ozonesondes were launched from 14 N. American sites during August 2006, providing the best set of free tropospheric ozone measurements ever gathered across the continent in a single season. The data reveal that the strongest regional-scale ozone enhancement is not in the lower troposphere, but in the upper troposphere above eastern North America, and centered over the southeastern USA. Recurring each year, the location and strength of the ozone maximum is influenced by the summertime upper tropospheric anticyclone that traps convectively lofted ozone and ozone precursors from anthropogenic sources, as well as ozone produced from lightning NOx, above the southeastern USA. The North American summer monsoon that flows northwards along the Rocky Mountains is embedded within the western side of the anticyclone and also marks the westernmost extent of the ozone maximum. Removing the influence from stratospheric intrusions, median ozone mixing ratios (78 ppbv) in the upper troposphere (> 6 km) above Alabama, near the center of the anticyclone, were nearly twice the level above the US west coast. Model simulations indicate lightning NOx emissions led to the production of 25-30 ppbv of ozone at 250 hPa above the southern USA during the study period.
A44C-05
Studying the upper tropospheric ozone enhancements over North America: Analysis with TES observations and FLEXPART
The Tropospheric Emission Spectrometer on the EOS Aura satellite provides global measurements of vertically resolved ozone. During the summer of 2006, an observation campaign was conducted by TES to make 25,000 profile measurements over North America and the Atlantic. This dataset is used with the FLEXPART Lagrangian particle dispersion model to explore the features of upper tropospheric ozone over North America, to study the influence of long-range transport and in-situ ozone formation, and to compare to similar analysis that have utilized summer ozonesonde data. Specifically, this presentation will focus analysis of the ozone over the southeastern US during August 2006, and compare the analysis using TES measurements to the conclusions reached with the IONS ozonesondes.
A44C-06
Using GOES Water Vapor to Define the Tropopause Break and Detect Stratospheric Ozone in the Upper Troposphere
As part of INTEX-B, we report on the value of a satellite based method for estimating the variable location of the tropopause break. Previous work showed tropopause folding, as evidenced in lidar curtains of ozone, was consistently observed in the vicinity of strong gradients in GOES layer average specific humidity in the upper troposphere. Fold openings were found to correspond to the peaks in gradient magnitude, and the folded features penetrated (over 200km on average) beneath the moist edge that was found to define the tropopause break. Here we further this work using flights made by the DC-8 during INTEX-B, a NASA program focused on AURA satellite validation and characterization of Asian outflow of airborne pollutants crossing the North Pacific and reaching North America during the spring of 2006. Presenting data from a two flight case study (May 1, and May 10, 2006), we have clear evidence of folding observed in ozone lidar curtains corresponding with water vapor gradients. In this work, we characterize the multivariate relationship of ozone (measured on the aircraft) with satellite observations of layer average specific humidity in the upper troposphere from GOES, and with layer average potential vorticity in the upper troposphere from the Global Forecast System (GFS) model. We compare these correlations with recent estimates of the multivariate relationship between TES derived ozone in the upper troposphere with the same GOES and GFS dynamical tracers (Felker et al., this meeting). In addition, the location of satellite derived fold-openings will be compared with independent model analyses of ozone and water vapor from the RAQMS model. Our goal is to illustrate the value of relating observed chemical variations to GOES gradients, and then using the broader availability of the GOES imagery to effectively derive empirical maps illustrating the dynamical influence of stratospheric exchange on ozone in the upper troposphere.
A44C-07
Seasonal Variation of Nitrogen Oxides in the Central North Atlantic Lower Free Troposphere: Influence of Boreal Wildfires and North American Urban Pollution
Measurements of NO, NO2 and NOy (total reactive nitrogen oxides) made at the Pico Mountain station (38.78° N 28.67° W, 2.3~km asl) from July 2002 to August 2005 are analyzed to characterize the seasonal variation and the speciation of nitrogen oxides in the background lower free troposphere (FT) over the central North Atlantic region. These observations reveal a well-defined seasonal cycle of nitrogen oxides (NOx = NO+NO2 and NOy), with higher mixing ratios during the summertime. Observed NOx and NOy levels are consistent with long-range transport of emissions, but with significant removal en-route to the measurement site. Larger summertime nitrogen oxides levels are attributed to impacts of boreal wildfire emissions and more efficient export of NOy from eastern North America during this season. Reactive nitrogen over the central North Atlantic lower FT largely exists in the form of PAN and HNO3 (~80--90% of NOy) year-round. A shift in the composition of NOy from dominance of PAN to dominance of HNO3 occurred from winter--spring to summer--fall, as a result of changes in temperature and photochemistry over the region. A further comparison of the nitrogen oxides measurements with results from the global chemical transport model GEOS-Chem identifies differences between the observations and the model, with simulated nitrogen oxides significantly larger than the observations.