The 2005 Study of Organic Aerosols at Riverside (SOAR-1): instrumental intercomparisons and fine particle composition

Abstract

Abstract. Multiple state-of-the-art instruments sampled ambient aerosol in Riverside, California during the 2005 Study of Organic Aerosols at Riverside (SOAR) to investigate sources and chemical composition of fine particles (PMf) in the inland region of Southern California. This paper briefly summarizes the spatial, meteorological and gas-phase conditions during SOAR-1 (15 July–15 August) and provides detailed intercomparisons of complementary measurements and average PMf composition during this period. Daily meteorology and gas-phase species concentrations were highly repetitive with meteorological and gas-phase species concentrations displaying clear diurnal cycles and weekday/weekend contrast, with organic aerosol (OA) being the single largest component contributing approximately one-third of PMf mass. In contrast with historical characterizations of OA in the region, several independent source apportionment efforts attributed the vast majority (~80%) of OA mass during SOAR-1 to secondary organic aerosol (SOA). Given the collocation of complementary aerosol measurements combined with a dominance of SOA during SOAR-1, this paper presents new results on intercomparisons among several complementary measurements and on PMf composition during this period. Total non-refractory submicron (NR-PM1) measurements from a high-resolution aerosol mass spectrometer (HR-AMS) are compared with measurements by tapered element oscillating microbalances (TEOM) including a filter dynamics measurement system (TEOMFDMS). NR-PM1 is highly correlated with PM2.5 TEOMFDMS measurements and accounts for the bulk of PM2.5 mass with the remainder contributed primarily by refractory material. In contrast, measurements from a heated TEOM show substantial losses of semi-volatile material, including ammonium nitrate and semi-volatile organic material. Speciated HR-AMS measurements are also consistent and highly correlated with several complementary measurements, including those of a collocated compact AMS (C-AMS). Finally, elemental analysis (EA) of HR-AMS OA spectra allows direct comparison of HR-AMS organic carbon (OC) with measurements from two collocated Sunset thermal-optical semi-continuous monitors, and investigation of the elemental composition of OA in Riverside. While HR-AMS and base OC measurements from both Sunset instruments are similar within the combined uncertainties, a correction intended to account for the loss of semivolatile OC from the Sunset yields OC measurements ~30% higher than either HR-AMS or base Sunset measurements. Oxygen is the main heteroatom of ambient OA during SOAR-1 with a minimum atomic O/C of 0.28 during the morning rush hour and maximum of 0.42 during the afternoon. H/C is broadly anti-correlated with O/C, while N/C and S/C (excluding organonitrate (ON) and organosulfate (OS) functionalities) are far lower than O/C at about 0.015 and ~0.001, respectively. O/C, N/C, and S/C increase by 21%, a factor of 2, and a factor of 30, respectively, while H/C changes little when ON and OS estimates are included. This implies that ON account for ~1/2 of the organic nitrogen while OS dominate organic sulfur at this location. Accounting for the estimated ON and OS also improves the agreement between anions and cations measured by HR-AMS by ~8%, while amines have a very small impact (1%) on this balance.