The Pasadena Aerosol Characterization Observatory (PACO): chemical and physical analysis of the Western Los Angeles Basin aerosol

• Published in: Atmospheric chemistry and physics discussions 2011-02, Vol.11 (2), p.5867-5933
• Main Authors: Hersey, S P, Craven, J S, Schilling, KA, Metcalf, A R, Sorooshian, A, Chan, M N, Flagan, R C, Seinfeld, J H
• Format: Article
• Language: English
• ISSN: 1680-7367; 1680-7375

The Pasadena Aerosol Characterization Observatory (PACO) represents the first major aerosol characterization experiment centered in the Western/Central Los Angeles Basin. The sampling site, located on the campus of the California Institute of Technology in Pasadena, was positioned to sample a continuous afternoon influx of transported urban aerosol with a photochemical age of 1-2 h and generally free from major local contributions. Sampling spanned 5 months during the summer of 2009, which were broken into 3 regimes on the basis of distinct meteorological conditions. Regime I was characterized by a series of low pressure systems, resulting in high humidity and rainy periods with clean conditions. Regime II typified early summer meteorology, with significant morning marine layers and warm, sunny afternoons. Regime III was characterized by hot, dry conditions with little marine layer influence. Organic aerosol (OA) is the most significant constituent of Los Angeles aerosol (42, 43, and 55% of total submicron mass in regimes I, II, and III, respectively), and that the overall oxidation state remains relatively constant on timescales of days to weeks (O:C = 0.44 plus or minus 0.08, 0.55 plus or minus 0.05, and 0.48 plus or minus 0.08 during regimes I, II, and III, respectively), with no difference in O:C between morning and afternoon periods. Periods characterized by significant morning marine layer influence followed by photochemically favorable afternoons displayed significantly higher aerosol mass and O:C ratio, suggesting that aqueous processes may be important in the generation of secondary aerosol and oxidized organic aerosol (OOA) in Los Angeles. Water soluble organic mass (WSOM) reaches maxima near 14:00-15:00 local time (LT), but the percentage of AMS organic mass contributed by WSOM remains relatively constant throughout the day. Sulfate and nitrate reside predominantly in accumulation mode aerosol, while afternoon SOA production coincides with the appearance of a distinct fine mode dominated by organics. Particulate NH sub(4)NO sub(3) and (NH sub(4)) sub(2)SO sub(4) appear to be NH sub(3)-limited in regimes I and II, but a significant excess of particulate NH sub(4) super(+) in the hot, dry regime III suggests less marine SO sub(4) super(2-) and the presence of organic amines. Positive Matrix Factorization (PMF) analysis of C-ToF-AMS data resolved three factors, corresponding to a hydrocarbon-like OA (HOA), semivolatile OOA (SV-OOA), and low-volatil