Chemically-resolved aerosol volatility measurements from two megacity field studies

摘要

The volatilities of different chemical species in ambient aerosols areimportant but remain poorly characterized. The coupling of a recentlydeveloped rapid temperature-stepping thermodenuder (TD, operated in the range54–230°C) with a High-Resolution Time-of-Flight Aerosol MassSpectrometer (HR-ToF-AMS) during field studies in two polluted megacities hasenabled the first direct characterization of chemically-resolved urbanparticle volatility. Measurements in Riverside, CA and Mexico City aregenerally consistent and show ambient nitrate as having the highestvolatility of any AMS standard aerosol species while sulfate showed thelowest volatility. Total organic aerosol (OA) showed volatility intermediatebetween nitrate and sulfate, with an evaporation rate of 0.6%·K−1near ambient temperature, although OA dominates the residual species at thehighest temperatures. Different types of OA were characterized with markerions, diurnal cycles, and positive matrix factorization (PMF) and showsignificant differences in volatility. Reduced hydrocarbon-like OA (HOA, asurrogate for primary OA, POA), oxygenated OA (OOA, a surrogate for secondaryOA, SOA), and biomass-burning OA (BBOA) separated with PMF were alldetermined to be semi-volatile. The most aged OOA-1 and its dominant ion,CO₂+, consistently exhibited the lowest volatility, with HOA, BBOA, andassociated ions for each among the highest. The similar or higher volatilityof HOA/POA compared to OOA/SOA contradicts the current representations of OAvolatility in most atmospheric models and has important implications foraerosol growth and lifetime. A new technique using the AMS background signalwas demonstrated to quantify the fraction of species up to fourorders-of-magnitude less volatile than those detectable in the MS mode, whichfor OA represent ~5% of the non-refractory (NR) OA signal. Our resultsstrongly imply that all OA types should be considered semivolatile in models.The study in Riverside identified organosulfur species (e.g. CH₃HSO₃+ion, likely from methanesulfonic acid), while both studies identified ionsindicative of amines (e.g. C₅H₁₂N+) with very different volatilitybehaviors than inorganic-dominated ions. The oxygen-to-carbon ratio of OA ineach ambient study was shown to increase both with TD temperature and frommorning to afternoon, while the hydrogen-to-carbon ratio showed the oppositetrend.