Trans-Pacific transport of Asian dust and pollution: Accumulation of biomass burning CO in subtropics and dipole structure of transport Junsang Nam 1, Yuhang Wang 1, Chao Luo 1, Allen Chu 2 1 School of Earth and Atmospheric Sciences Georgia Institute of Technology 2 NASA Goddard Space Flight Center GEOS-Chem meeting

Introduction Trans-Pacific transport of Asian aerosols is evident in in-situ and satellite observations. Model simulations show that It has an impact on regional air quality and radiative forcing in the U.S. However, there still remain large uncertainties in model calculations and retrieval of satellite data. To minimize measurement uncertainty, we focus on strong transport events. We used GEOS-Chem to investigate the main features of trans-Pacific transport events well defined by satellites measurements. 2

Data & Model description MODIS AOD from (Terra & Aqua) – Different orbits (13:30 vs. 10:30, ascending vs. descending node)  Different view and cloud, comparable results MOPITT CO – less uncertain than MODIS AOD GEOS-Chem v7.3.6 (GEOS-4, 2 o x2.5 o for 30 levels) – Spring 2003 (2-month spin-up + 1-month simulation) – Carbonaceous aerosol (Cooke et al., 1999; Yevich and Logan, 2003), Sea salt (Monahan et al., 1986), dust (Ginoux et al., 2001), sulfur (Park et al., 2004) – Biomass burning CO/EC/OC from 8-day GFED (Randerson et al., 2008) 3

Spatial pattern of trans-Pacific transport 5 ~ 8 May ~ 17 May ~ 28 May 2003 MODIS AOD GEOS-Chem AOD GEOS-Chem CO  MODIS AOD overestimation – 30% reduced values are shown (above) for MODIS AOD – Spherical dust assumption (Levy et al., 2003; Chu et al., 2005)  Different transport pathways in three events  Model bias in 1 st event AOD simulation (model CO is rather closer to the MODIS AOD) 4

Accumulation of CO over Central Pacific 5 ~ 8 May ~ 17 May ~ 28 May 2003 MOPITT CO GEOS-Chem CO GEOS-Chem CO diff  Subtropical CO underestimation – Lower latitude transport of biomass burning effluents (Heald et al., 2003) – 8x (shown) BB CO in Burma (8x overestimates and 4x underestimates)  Impact of April CO (8x) on May – long lifetime against oxidation 5

Accumulation of CO over Central Pacific Significant spatiotemporal variability in BB emissions – 67% less in 2006 (in the domain, Apr-May) – 1.3% less in 2006 (global, annually) BB CO underestimation confirmed w/ NOAA ESRL Guam island (shown above) surface measurements 6

Characteristics of transport events 7 Dust AOD Other AOD DustSO4OtherCO  Dust contributes as much as sum of others (SO4+EC+OC+SS) during the 1 st event.  Bias in aerosol transport pathway during the 1 st event is driven by dust transport.  Difference in source location between dust and other aerosol  Different transport pathway  Dust is transported at higher altitudes (~400 hPa)  faster transport & different horizontal pathway

Model bias in dust transport pathway 8  Put CO in dust source region  similar pathway w/ dust, JUST upwind source location (meteorology) is very important in transport pathway.  Dipole structure of transport: Aleutian Low (north) vs. Pacific High (south) (Zhang et al., 2008)  Dust sources: W China, strong upflux (~400 hPa), strong lower latitude Pacific High  CO & other aerosol sources: E/NE China, weak upflux (~700 hPa), weak higher latitude Pacific High DustCO SensitivityStandard

Summary The biomass burning emissions over Indochina are underestimated (by a factor of 4-8) in the GFED in April Dust transport occurs at higher altitude and leads to bias in simulated AOD transport. Transport pathway is very sensitive to changes of upwind source location. CO and the other aerosols are transported mostly over lower altitude and through the Aleutian Low, while dust over higher altitude through the Pacific High in the model. Missing dust sources (due to recent desertification) in the eastern Inner Mongolia (Chin et al., 2003) are likely factors contributing to the model bias. 9