Evolution of Asian aerosols during transpacific transport in INTEX-B

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.

Organic aerosol composition and sources in Pasadena, California, during the 2010 CalNex campaign

“…When comparing the time series of total OOA (SV-OOA + LV-OOA) versus O x in Figure 9, similar temporal changes are observed (R 2 = 0.53), but the correlation is stronger during the more polluted periods of high OOA concentrations that occurred in June (R 2 = 0.72 for the 2-6 June high pollution period). It has also been observed that for long-range transported air, the correlation vanishes due to differing losses and ongoing chemistry of SOA and O x [Dunlea et al, 2009]. In Pasadena, the regression slope for OOA versus O x is 0.146(±0.001) μg sm À3 ppbv À1 (Figure 9 inset).…”

Aircraft measurements of polar organic tracer compounds in tropospheric particles (PM<sub>10</sub>) over central China

“…Thus, cloud processing may enhance the formation of highergeneration SOA tracers at an altitude around 2 km over central China, a point that warrants further study. The observed decreasing trend in the ratios of organic species to sulfate at elevation above 2 km may be explained by less efficient wet removal of SO 2 than those of preexisting aerosols and oxidized VOCs during air mass lifting (Dunlea et al, 2009). The further processing of the resulting air mass may thus lead to the formation of particles relatively enriched with sulfate compared to organics.…”

Aged organic aerosol in the Eastern Mediterranean: the Finokalia Aerosol Measurement Experiment – 2008

“…Factor analysis is not always possible on highly aged organic aerosol datasets. For example, Dunlea et al (2009) found that the limited spectral variability in their measured organic aerosol precluded them from separating it into physically meaningful factors. Here we explore the possible physical meanings of the two OOA factors found for FAME-2008.…”