Trans‐Pacific transport of black carbon and fine aerosols (D < 2.5 μm) into North America

Abstract

[1] This study presents estimates of long-range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 mm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130°W, (30°N-60°N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25-32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near-surface overprediction does not impair the major conclusions of this study regarding long-range aerosol and BC transport, as most of the long-range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 mg/m 3 . Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04-2.55 W/m 2 absorbed in the atmosphere and a dimming of À1.45 to À1.47 W/m 2 at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.

Decadal changes in shortwave irradiance at the surface in the period from 1960 to 2000 estimated from Global Energy Balance Archive Data

“…Then it is plausible that, in densely populated regions, decadal trends in aerosol emissions induce decadal trends in SWIR with opposite sign; and, since aerosols are transported over large distances [e.g., Shaw, 1981;Uematsu et al, 1983;Ramanathan et al, 2001b;Prospero et al, 2003;Colarco et al, 2003;Heald et al, 2006;Hadley et al, 2007] these trends may also affect SWIR in adjacent regions and possibly even at remote sites. These speculations are reconcilable with (1) the trends in aerosols as reported by Streets et al [2006], (2) the measurements of atmospheric transmission in Figure 12 of Ohmura [2006] and (3) the increase in SWIR under clearsky conditions at remote BSRN sites in the period from 1992 through to 2002 estimated by Wild et al [2005].…”

Biomass burning contribution to black carbon in the Western United States Mountain Ranges

“…BC in this region is mainly from North American anthropogenic emissions (Park et al, 2003), transpacific transport of Asian emissions especially during spring Hadley et al, 2007;Park et al, 2003), and North American biomass burning emissions during the summer and fall fire season (Spracklen et al, 2009;Park et al, 2003). However, the relative contributions from these sources particularly biomass burning to BC in the WUS are still uncertain.…”

Global source attribution of sulfate concentration and direct and indirect radiative forcing

“…For example, the strong anthropogenic emissions over East Asia have led to an increasing interest in quantifying the impact of aerosols exported from East Asia. Recent studies indicate that the transpacific transport of sulfate from East Asia contributes to 30-50 % of the background (sulfate produced from non-local emissions) surface concentrations in the western US and 10-30 % in the eastern US (Park et al, 2004;Hadley et al, 2007;Liu et al, 2008), which are larger than contributions from all other foreign sources (Liu et al, 2009). In addition, among the major emitting regions assessed for 2001 conditions, European sources were shown to account for 1-5 µg m −3 of surface sulfate concentrations over northern Africa and western Asia, and their contribution to East Asia (0.2-0.5 µg m −3 ) was twice as much as the contribution (0.1-0.2 µg m −3 ) of Asian sources to North America (Chin et al, 2007).…”