Abstract. The accumulation of secondary acids and ammonium on individual mineral dust particles during ACEAsia has been measured with an online single-particle mass spectrometer, the ATOFMS. Changes in the amounts of sulphate, nitrate, and chloride mixed with dust particles correlate with air masses from different source regions. The uptake of secondary acids depended on the individual dust particle mineralogy; high amounts of nitrate accumulated on calcium-rich dust while high amounts of sulphate accumulated on aluminosilicate-rich dust. Oxidation of S(IV) to S(VI) by iron in the aluminosilicate dust is a possible explanation for this enrichment of sulphate, which has important consequences for the fertilization of remote oceans by soluble iron. This study shows the segregation of sulphate from nitrate and chloride in individual aged dust particles for the first time. A transport and aging timeline provides an explanation for the observed segregation. Our data suggests that sulphate became mixed with the dust first. This implies that the transport pathway is more important than the reaction kinetics in determining which species accumulate on mineral dust. Early in the study, dust particles in volcanically influenced air masses were mixed predominately with sulphate. Dust mixed with chloride then dominated over sulphate and nitrate when a major dust front reached the R. V. Ronald Brown. We hypothesize that the rapid increase in chloride on dust was due to mixing with HCl(g) released from acidified sea salt particles induced by heterogeneous reaction with volcanic SO 2 (g), prior to the arrival of the dust front. The amount of ammonium mixed with dust correlated strongly with the total amount of secondary acid reaction products in the dust. Submicron dust and ammonium sulphate were internally mixed, contrary to frequent reports that they exist as external mixtures. The size distribution of the mixing Correspondence to: K. Prather (kprather@ucsd.edu) state of dust with these secondary species validates previous mechanisms of the atmospheric processing of dust and generally agrees with simulated aerosol chemistry from the STEM-2K3 model. This series of novel results has important implications for improving the treatment of dust in global chemistry models and highlights a number of key processes that merit further investigation through laboratory and field studies.

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The aerodynamic size and chemical composition of individual ultrafine and accumulation mode particle emissions (Da = 50-300 nm) were characterized to determine mass spectral signatures for heavy duty diesel vehicle (HDDV) emissions that can be used for atmospheric source apportionment. As part of this study, six in-use HDDVs were operated on a chassis dynamometer using the heavy heavy-duty diesel truck (HHDDT) five-cycle driving schedule under different simulated weight loads. The exhaust emissions were passed through a dilution/residence system to simulate atmospheric dilution conditions, after which an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS) was used to sample and characterize the HDDV exhaust particles in real-time. This represents the first study where refractory species including elemental carbon and metals are characterized directly in HDDV emissions using on-line mass spectrometry. The top three particle classes observed with the UF-ATOFMS comprise 91% of the total particles sampled and show signatures indicative of a combination of elemental carbon (EC) and engine lubricating oil. In addition to the vehicle make/year, the effects of driving cycle and simulated weight load on exhaust particle size and composition were investigated.