Chemically-resolved aerosol volatility measurements from two megacity field studies

Abstract

Abstract. The volatilities of different chemical species in ambient aerosols are important but remain poorly characterized. The coupling of a recently developed rapid temperature-stepping thermodenuder (TD, operated in the range 54–230°C) with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) during field studies in two polluted megacities has enabled the first direct characterization of chemically-resolved urban particle volatility. Measurements in Riverside, CA and Mexico City are generally consistent and show ambient nitrate as having the highest volatility of any AMS aerosol species while sulfate showed the lowest volatility. Total organic aerosol (OA) showed volatility intermediate between nitrate and sulfate, with an evaporation rate of 0.6% K−1 near ambient temperature, although OA dominates the residual species at the highest temperatures. Different types of OA were characterized with marker ions, diurnal cycles, and positive matrix factorization (PMF) and show significant differences in volatility. Reduced hydrocarbon-like OA (HOA, a surrogate for primary OA, POA), oxygenated OA (OOA, a surrogate for secondary OA, SOA), and biomass-burning OA (BBOA) separated with PMF were all determined to be semi-volatile. The most aged OOA-1 and its dominant ion, CO2+, consistently exhibited the lowest volatility, with HOA, BBOA, and associated ions for each among the highest. The similar or higher volatility of HOA/POA compared to OOA/SOA contradicts the current representations of OA volatility in most atmospheric models and has important implications for aerosol growth and lifetime. Our results strongly 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 ions indicative of amines (e.g. C5H12N+) with very different volatility behaviors than inorganic-dominated ions. The oxygen-to-carbon ratio of OA in each ambient study was shown to increase both with TD temperature and from morning to afternoon, while the hydrogen-to-carbon ratio showed the opposite trend.

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

“…The average temperatures and RHs for the different sites with similar OOA/O x were as follows: 18°C and 83% (Pasadena), 27°C and 53% (Riverside), and 16°C and 50% (Mexico City). While higher temperatures in Riverside will favor partitioning of OOA to the vapor phase, the measured volatility of OA for Riverside [Huffman et al, 2009] indicates the amount of evaporation will be small (less than 10%) and, thus, will have little effect on OOA/O x . Additionally, the higher RH in Pasadena relative to Riverside and Mexico City may not influence OOA concentrations given that aerosol water was not an important absorbing phase [Zhang et al, 2012] as discussed in section 3.1.…”

“…The five PMF components identified are (1) hydrocarbon-like organic aerosol (HOA), (2) cooking-influenced organic aerosol (CIOA), (3) local organic aerosol (LOA), (4) semivolatile oxygenated organic aerosol (SV-OOA), and (5) low-volatility oxygenated organic aerosol (LV-OOA). The naming based on volatility is adopted according to several studies showing a relationship between high oxygenation and lower volatility for the OOA components [e.g., Cappa and Jimenez, 2010;Huffman et al, 2009;Jimenez et al, 2009;Lanz et al, 2007;Ulbrich et al, 2009]. The HOA component has been previously described as a surrogate for primary combustion OA, and the SV-OOA and LV-OOA components as surrogates for "fresher" and "aged" SOA, respectively.…”

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

“…In contrast, ambient OA is highly oxidized (0.5 ≤ O∶C ≤ 1.0) (1, 15) and not very volatile (16). Ambient SOA is much less volatile than ambient POA (16).…”

“…Ambient SOA is much less volatile than ambient POA (16). Consequently, "chamber" SOA does not represent the atmosphere.…”

Estimating ambient particulate organic carbon concentrations and partitioning using thermal optical measurements and the volatility basis set

“…Based on earlier work (Donahue et al 2011) as well as other ambient samples (Huffman et al 2009), we hypothesize that filters collected close to sources should have larger total OC and thus a larger particle fraction in OC for any given OC stage, while filters in urban background locations should show a smaller particle fraction in each OC stage.…”