Abstract
Abstract. A fully coupled meteorology-chemistry model (WRF-Chem, the Weather Research and Forecasting model coupled with chemistry) has been configured to conduct quasi-global simulation for 5 years (2010–2014) and evaluated with multiple observation data sets for the first time. The evaluation focuses on the simulation over the trans-Pacific transport region using various reanalysis and observational data sets for meteorological fields and aerosol properties. The simulation generally captures the overall spatial and seasonal variability of satellite retrieved aerosol optical depth (AOD) and absorbing AOD (AAOD) over the Pacific that is determined by the outflow of pollutants and dust and the emissions of marine aerosols. The assessment of simulated extinction Ångström exponent (EAE) indicates that the model generally reproduces the variability of aerosol size distributions as seen by satellites. In addition, the vertical profile of aerosol extinction and its seasonality over the Pacific are also well simulated. The difference between the simulation and satellite retrievals can be mainly attributed to model biases in estimating marine aerosol emissions as well as the satellite sampling and retrieval uncertainties. Compared with the surface measurements over the western USA, the model reasonably simulates the observed magnitude and seasonality of dust, sulfate, and nitrate surface concentrations, but significantly underestimates the peak surface concentrations of carbonaceous aerosol likely due to model biases in the spatial and temporal variability of biomass burning emissions and secondary organic aerosol (SOA) production. A sensitivity simulation shows that the trans-Pacific transported dust, sulfate, and nitrate can make significant contribution to surface concentrations over the rural areas of the western USA, while the peaks of carbonaceous aerosol surface concentrations are dominated by the North American emissions. Both the retrievals and simulation show small interannual variability of aerosol characteristics for 2010–2014 averaged over three Pacific sub-regions. The evaluation in this study demonstrates that the WRF-Chem quasi-global simulation can be used for investigating trans-Pacific transport of aerosols and providing reasonable inflow chemical boundaries for the western USA, allowing one to further understand the impact of transported pollutants on the regional air quality and climate with high-resolution nested regional modeling.
Highlights
Aerosols, originating from natural and anthropogenic sources in Europe, North Africa, and East Asia, can be transported thousands of miles downwind across the Pacific Ocean to North America and even beyond
The seasonal mean wind fields at 850 hPa averaged for the period 2010–2014 from the Weather Research and Forecasting (WRF)-Chem simulation are compared with the Modern-Era Retrospective www.geosci-model-dev.net/9/1725/2016/
The simulation results are evaluated for the first time with various reanalysis and observational data sets, including precipitation from Global Precipitation Climatology Project (GPCP), wind fields from MERRA, aerosol optical depth (AOD), EAE, and absorbing AOD (AAOD) from Moderate Resolution Imaging SpectroRadiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), Ozone Monitoring Instrument (OMI), and AErosol RObotic NETwork (AERONET), aerosol extinction profiles from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), and aerosol surface mass concentrations from Interagency Monitoring for Protected Visual Environments (IMPROVE)
Summary
Aerosols, originating from natural and anthropogenic sources in Europe, North Africa, and East Asia, can be transported thousands of miles downwind across the Pacific Ocean to North America and even beyond. Z. Hu et al.: Trans-Pacific transport and evolution of aerosols prevailing westerlies across the Pacific Ocean and reach the west coast of North America and beyond, and its efficiency is the largest in spring (e.g., Takemura et al, 2002; Chin et al, 2007; Huang et al, 2008; Yu et al, 2008; Uno et al, 2009, 2011; Alizadeh-Choobari et al, 2014). Chin et al (2007) found that the long-range transported dust brought 3 to 4 times more fine particles than anthropogenic pollutants to the total surface fine particles over the USA on annual average with a maximum influence in spring, and over the northwestern USA Yu et al (2008) estimated that about 25 % of the Asian outflow reaches the west coast of North America, which is about 15 % of the total North American emissions; the transport fluxes are largest in spring and smallest in summer. Uno et al (2011) revealed that the dust trans-Pacific path sometimes could be split into two branches: a southern path to the central USA and a northern path that is trapped and stagnant for a longer time and subsides over the northwestern USA
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