Inter-continental Transport of Air Pollutants
Research team:  Denise Mauzerall, Junfeng Liu (post-doc), Larry Horowitz (GFDL) and collaborators.

Mauzerall et al., (2000) first characterized the seasonal cycle of ozone production over Asia and export to the Pacific Ocean.  We found that although ozone production was largest in summer, export was largest in spring.  We next developed the global chemical transport Model of Ozone and Related Tracers, version 2 (MOZART-2) [Horowitz, Walters, Mauzerall et al., 2003].  We designed it to accept a variety of meteorological inputs to facilitate examination of processes controlling inter-annual variability in inter-continental transport of air pollution. 

We analyzed the effect of meteorology on the magnitude of seasonal and inter-annual variability in trans-Pacific transport of pollutants by using MOZART-2 with assimilated meteorology (real winds) for 1991-2001 and uniformly emitted continental tracers with prescribed first-order decay rates.  We found that the average length of time required for transport of continental tracers from East Asia to the US in spring is approximately 3-weeks, significantly longer than the 1-2 week transport time that scientists have observed in rapidly transported pollution plumes [Liu and Mauzerall, 2005].  In addition, because no existing atmospheric indices were able to explain the inter-annual variability in trans-Pacific transport potentials (defined as transport across 130ºW in the eastern Pacific), we developed a new index we call the Eastern Pacific Index (EPI).  The EPI successfully reproduces most of the variability in influx of East Asian tracers to the US between 1991-2001 [Liu, Mauzerall and Horowitz, 2005]. 

We analyzed the effect of varying East Asian (EA) sulfur emissions on sulfate concentrations in the Northern Hemisphere, using the global MOZART-2 model. We found that from west to east across the North Pacific, EA sulfate contributes approximately 80%–20% of sulfate at the surface, but at least 50% at 500 hPa (Liu et al., 2008).  We found that EA sulfate concentrations over most downwind regions respond nearly linearly to changes in EA SO2 emissions, but sulfate concentrations over the EA source region increase more slowly than SO2 emissions, particularly at the surface and in winter, due to limited availability of oxidants (mostly H2O2).  Our findings suggest that future changes in EA sulfur emissions may cause little change in the sulfate induced health impact over downwind continents but SO2 emission reductions may significantly reduce the sulfate related climate cooling over the North Pacific and the United States.

Substantial research has been conducted by the atmospheric science community on the export of pollution from Asia to the Pacific and the import of Asian pollution into the United States.  A follow-on question of direct policy relevance, however, is what the influence the influx of foreign pollutants have on human health and welfare.  To address this question, we evaluated the inter-continental transport of aerosols using the global atmospheric chemistry model MOZART-2 (Liu et al., 2009a) and examined the global health impacts resulting from that transport (Liu et al., 2009b).  We estimated global annual premature mortalities (for adults age 30 and up) due to inter-continental transport of PM2.5 to be nearly 380 thousand (K) in 2000. Approximately half of these deaths occur in the Indian subcontinent (IN), mostly due to aerosols transported from Africa and the Middle East (ME). We found that approximately 90K deaths globally are associated with exposure to foreign (i.e., originating outside a receptor region) non-dust PM2.5. More than half of the premature mortalities associated with foreign non-dust aerosols are due to aerosols originating from Europe (20K), ME (18K) and EA (15K); and nearly 60% of the 90K deaths occur in EA (21K), IN (19K) and Southeast Asia (16K).  The lower and higher bounds of our estimated 95% confidence interval (considering uncertainties from the concentration–response relationship and simulated aerosol concentrations) are 18% and 240% of the estimated deaths, respectively, and could be larger if additional uncertainties were quantified.  We also found that in 2000 nearly 6.6K premature deaths in North America (NA) were associated with foreign PM2.5 exposure (5.5K from dust PM2.5). NA is least impacted by foreign PM2.5 compared to receptors on the Eurasian continent. However, the number of premature mortalities associated with foreign aerosols in NA (mostly occurring in the U.S.) is comparable to the reduction in premature mortalities expected to result from tightening the U.S. 8-h O3 standard from 0.08 ppmv to 0.075 ppmv. International efforts to reduce inter-continental transport of fine aerosol pollution would substantially benefit public health on the Eurasian continent and would also be of benefit in the United States.


Horowitz, L. W., Walters, S., Mauzerall, D. L., Emmons, L. K., Rasch, P. J., Granier, C., Tie, X., Lamarque, J.-F., Schultz, M. G., Tyndall, G. S., Orlando, J. J., and Brasseur, G. P., “A Global Simulation of Tropospheric Ozone and Related Tracers: Description and Evaluation of MOZART, Version 2,” J. of Geophys. Res., 108 (D24), 4784, doi:10.1029/2002JD002853, 2003.

Liu, J., and Mauzerall, D. L., “Estimating the Average Time for Inter-continental Transport of Air Pollutants,” Geophys. Res. Lett., 32, L11814, doi:10.1029/2005GL022619, 2005.

Liu, J., Mauzerall, D. L., Horowitz, L.W., “Analysis of Seasonal and Interannual Variability in Transpacific Transport,” J. Geophys. Res., 110, D04302, doi: 10.1029/2004JD005207, 2005.

Liu, J. and D. L. Mauzerall, Evaluating the potential influence of inter-continental transport of sulfate aerosols on air quality, Environ. Res. Lett. 2 045029, doi:10.1088/1748-9326/2/4/045029, 2007.

Liu, J., Mauzerall, D. L., Horowitz, L.W. Source-Receptor Relationships between East Asian Sulfur Dioxide Emissions and Northern Hemisphere Sulfate Concentrations, Atmos. Chem. Phys., 8, 3721–3733, 2008.

Liu, J, DL Mauzerall, LW Horowitz, P Ginoux, AM Fiore.  Evaluating Inter-continental transport of fine aerosols:  (1) Methodology, global aerosol distribution and optical depth, Atmospheric Environment, doi:10.1016/j.atmosenv.2009.03.054, 2009a. [full text] [Auxiliary]

Liu, J, DL Mauzerall, LW Horowitz.  Evaluating Inter-continental transport of fine aerosols:  (2) Global Health Impacts, Atmospheric Environment, doi:10.1016/j.atmosenv.2009.05.032, 2009b

Mauzerall, D. L., Narita, D., Akimoto H., Horowitz L., Walters S., Hauglustaine D., Brasseur, B., “Seasonal Characteristics of Tropospheric Ozone Production and Mixing Ratios Over East Asia: A Global Three-dimensional Chemical Transport Model Analysis,” J. Geophys. Res., 105, pp. 17895-17910, 2000.