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

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 (PM<sub>f</sub>) 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 PM<sub>f</sub> 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 PM<sub>f</sub> 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 PM<sub>f</sub> composition during this period. Total non-refractory submicron (NR-PM<sub>1</sub>) 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 (TEOM<sub>FDMS</sub>). NR-PM<sub>1</sub> is highly correlated with PM<sub>2.5</sub> TEOM<sub>FDMS</sub> measurements and accounts for the bulk of PM<sub>2.5</sub> 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) functionaliti...

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

“…[31] Based on the diurnal cycles of HOA for the two sites, it seems that that the Pasadena HOA is more oxidized because it has undergone more photochemical aging relative to the Riverside HOA. In Riverside, the HOA concentration peaks in the morning as expected for fresh emissions from the local morning rush hour traffic [Docherty et al, 2011], consistent with the location of the Riverside site downwind and nearby large highways. In Pasadena, the HOA concentration peaks around 13:30, however, due to advection over several hours from the west and south.…”

“…An alternative explanation then is the presence of sodium cations that are not accounted for in Figures A2a and A2b. Typically when using AMS data to evaluate particle acidity, it must be assumed that ammonium is the only significant cation, and cations from sources such as sea salt are ignored [Aiken et al, 2009;Docherty et al, 2011;Zhang et al, 2007a]. For Pasadena, however, sodium concentration data can be estimated from PALMS measurements providing an opportunity to evaluate the role of sea salt in particle neutralization.…”

“…[67] Previously, Docherty et al [2011] demonstrated that for Riverside during the SOAR-1 campaign, organonitrates and organosulfates impacted the charge balance calculation. These species add to the nitrate and sulfate mass measured by the AMS due to fragmentation of the molecular parent, but do not contribute the expected amount of ionic charge, which leads to apparent particle acidity.…”

“…For HOA, O : C is relatively high (0.14) but not out of the range of previously reported values. For instance, O : C values as low as 0.02 (Riverside, CA) and 0.03 (Barcelona) [Docherty et al, 2011;Mohr et al, 2011] and as high as 0.16 (Mexico City) and 0.17 (Beijing) have been reported for HOA components [Aiken et al, 2009;Huang et al, 2010]. The diurnal cycle of HOA is similar to that of EC (and rBC) with a peak between 13:00 and 14:00 PDT (local time), and HOA and EC (and rBC) show high correlation (see Table D-1 in the supporting information and section 3.3 below).…”

“…Particulate matter concentrations in LA are among the highest in the United States (American Lung Association, State of the air, 2011, http://www.stateoftheair.org/), and multiple previous measurement campaigns have aimed to characterize aerosols in this region. These campaigns include the 1987 Southern California Air Quality Study [Lawson, 1990], the 1997 Southern California Ozone Study (SCOS97-NARSTO) [Croes and Fujita, 2003], the Study of Organic Aerosol at Riverside, which took place in the summer (SOAR-1) and fall (SOAR-2) of 2005 [Docherty et al, 2011], and the 2009 Pasadena Aerosol Characterization Observatory (PACO) . In addition, several research flights were performed over California and LA during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites Campaign (ARCTAS-CARB) [Jacob et al, 2010].…”