Abstract
Abstract. Measurements of aerosol composition were made with an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) on board the NSF/NCAR C-130 aircraft as part of the Intercontinental Chemical Transport Experiment Phase B (INTEX-B) field campaign over the Eastern Pacific Ocean. The HR-ToF-AMS measurements of non-refractory submicron aerosol mass are shown to compare well with other aerosol instrumentation in the INTEX-B field study. Two case studies are described for pollution layers transported across the Pacific from the Asian continent, intercepted 3–4 days and 7–10 days downwind of Asia, respectively. Aerosol chemistry is shown to be a robust tracer for air masses originating in Asia, specifically the presence of sulfate dominated aerosol is a distinguishing feature of Asian pollution layers that have been transported to the Eastern Pacific. We examine the time scales of processing for sulfate and organic aerosol in the atmosphere and show that our observations confirm a conceptual model for transpacific transport from Asia proposed by Brock et al. (2004). Our observations of both sulfate and organic aerosol in aged Asian pollution layers are consistent with fast formation near the Asian continent, followed by washout during lofting and subsequent transformation during transport across the Pacific. Our observations are the first atmospheric measurements to indicate that although secondary organic aerosol (SOA) formation from pollution happens on the timescale of one day, the oxidation of organic aerosol continues at longer timescales in the atmosphere. Comparisons with chemical transport models of data from the entire campaign reveal an under-prediction of organic aerosol mass in the MOZART model, but much smaller discrepancies with the GEOS-Chem model than found in previous studies over the Western Pacific. No evidence is found to support a previous hypothesis for significant secondary organic aerosol formation in the free troposphere.
Highlights
Aerosols play important roles in atmospheric processes that affect global climate change (IPCC, 2007), stratospheric ozone depletion (Solomon, 1999), regional visibility (Watson, 2002), human health (Pope et al, 2002; Pope, 2000; Schwartz, 1994), and ecological integrity via acid, toxic, and nutrient deposition (Bytnerowicz et al, 1996; Schindler, 1988, 1999)
This paper describes the deployment of the HR-ToF-AMS on board the C-130 (Sect. 2), the observations of aerosol during the Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) campaign (Sect. 3) and the chemical transformation of sulfate and organic aerosols during the transport of Asian aerosol across the Pacific, as well as comparisons of observations with chemical transport models (Sect. 4)
We compare observed tracer ratios to chemical transport model (CTM) predictions over the domain of the C-130 flight tracks. (Comparisons of observations with the Sulfur Transport and dEposition Model (STEM) model are presented elsewhere (Adhikary et al, 2009).) We focus on ensemble characteristics rather than individual plumes, which are not well resolved at the coarse (∼200 km) CTM resolution and which may be displaced geographically in the model owing to uncertainty in the meteorological fields
Summary
Aerosols play important roles in atmospheric processes that affect global climate change (IPCC, 2007), stratospheric ozone depletion (Solomon, 1999), regional visibility (Watson, 2002), human health (Pope et al, 2002; Pope, 2000; Schwartz, 1994), and ecological integrity via acid, toxic, and nutrient deposition (Bytnerowicz et al, 1996; Schindler, 1988, 1999). Long range transport of aerosols is an important issue for climate, and the International Global Atmospheric Chemistry (IGAC) Intercontinental Transport and Chemical Transformation (ITCT) series of campaigns was designed to study long-range transport in various regions of the world (http: //www.igac.noaa.gov/ITCT.php). As part of IGAC-ITCT, the Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) field campaign focused on in-situ measurements from aircraft and ground sites of transported Asian pollution over the Eastern Pacific and west coast of North America (Singh et al, 2009). The Pacific portion of INTEX-B took place from 17 April to 15 May 2006, with the intention of capturing the spring time maximum in transpacific transport from Asia to North America (Singh et al, 2009)
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