Investigation of the Correlation Between Odd Oxygen and Secondary Organic Aerosol in Mexico City and Houston

Atmos. Chem. Phys., 10, 8947–8968, 2010 www.atmos-chem-phys.net/10/8947/2010/ Atmospheric doi:10.5194/acp-10-8947-2010 Chemistry © Author(s) 2010. CC Attribution 3.0 License. and Physics Investigation of the correlation between odd oxygen and secondary organic aerosol in Mexico City and Houston E. C. Wood1, M. R. Canagaratna1, S. C. Herndon1, T. B. Onasch1, C. E. Kolb1, D. R. Worsnop1, J. H. Kroll1,*, W. B. Knighton2, R. Seila3, M. Zavala4, L. T. Molina4, P. F. DeCarlo5,6,**, J. L. Jimenez5,6,7, A. J. Weinheimer8, D. J. Knapp8, B. T. Jobson9, J. Stutz10, W. C. Kuster11, and E. J. Williams11 1Aerodyne Research, Inc., Billerica, Massachusetts, USA 2Department of Chemistry, Montana State University, Bozeman, Montana, USA 3United States Environmental Protection Agency, Research Triangle Park, North Carolina, USA 4Molina Center for Energy and the Environment, La Jolla, California, USA 5Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA 6Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA 7Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA 8National Center for Atmospheric Research, Boulder Colorado, USA 9Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington, USA 10Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA 11NOAA Earth System Research Laboratory, Boulder, Colorado, USA *now at: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA **now at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Switzerland Received: 14 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 9 February 2010 Revised: 10 August 2010 – Accepted: 13 September 2010 – Published: 27 September 2010 Abstract. Many recent models underpredict secondary or- by a factor of ∼2 between 08:00 and 13:00 local time. This ganic aerosol (SOA) particulate matter (PM) concentrations decrease is only partially attributable to evaporation of the in polluted regions, indicating serious deficiencies in the least oxidized and most volatile components of OOA; differ- models’ chemical mechanisms and/or missing SOA precur- ences in the diurnal emission trends and timescales for pho- sors. Since tropospheric photochemical ozone production tochemical processing of SOA precursors compared to ozone is much better understood, we investigate the correlation of precursors also likely contribute to the observed decrease. odd-oxygen ([Ox]≡[O3]+[NO2]) and the oxygenated com- The extent of OOA oxidation increased with photochemi- ponent of organic aerosol (OOA), which is interpreted as a cal aging. Calculations of the ratio of the SOA formation surrogate for SOA. OOA and Ox measured in Mexico City in rate to the Ox production rate using ambient VOC measure- 2006 and Houston in 2000 were well correlated in air masses ments and traditional laboratory SOA yields are lower than where both species were formed on similar timescales (less the observed [OOA]/[Ox] ratios by factors of 5 to 15, con- than 8 h) and not well correlated when their formation sistent with several other models’ underestimates of SOA. timescales or location differed greatly. When correlated, the Calculations of this ratio using emission factors for organic ratio of these two species ranged from 30 µg m−3/ppm (STP) compounds from gasoline and diesel exhaust do not repro- in Houston during time periods affected by large petrochem- duce the observed ratio. Although not succesful in repro- ical plant emissions to as high as 160 µg m−3/ppm in Mexico ducing the atmospheric observations presented, modeling −3 City, where typical values were near 120 µg m /ppm. On P (SOA)/P (Ox) can serve as a useful test of photochemical several days in Mexico City, the [OOA]/[Ox] ratio decreased models using improved formulation mechanisms for SOA. Correspondence to: E. C. Wood ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 8948 E. C. Wood et al.: Correlation between odd oxygen and secondary organic aerosol 1 Introduction (Heald et al., 2005). A recent noteworthy study showed that semivolatile and intermediate volatility organic compounds Numerous cities in the world struggle to attain air quality may play a dominant role in SOA formation in urban loca- standards for ozone and fine particulate matter. The adverse tions (Robinson et al., 2007). Furthermore, there is disagree- impact of elevated concentrations of ozone and particulate ment over the importance of anthropogenic versus biogenic matter on public health has been well documented and in- and biomass burning SOA precursors (de Gouw and Jimenez, cludes excess mortality counts (Izzotti et al., 2000), emer- 2009; de Gouw et al., 2009; Marley et al., 2009). gency room visits (WHO, 2003), exacerbation of asthma The apportionment of measured particulate organic car- (McCreanor et al., 2007), and other negative health effects bon or OA mass into primary and secondary contributions (Pope and Dockery, 2006). The outflow of elevated levels has not been straightforward, with POA often being overes- of ozone and PM from megacities impacts climate change as timated by the widely used EC-tracer method (Docherty et well. Ozone is a greenhouse gas and exerts a positive radia- al., 2008; Zhang et al., 2005b), which is based on the ra- tive forcing (i.e., warming effect). The uncertainties associ- tio of organic aerosol to elemental carbon. Recent advances ated with the radiative forcings of aerosol (both direct and in organic aerosol analysis with rapid water-soluble organic indirect) continue to comprise the largest source of uncer- carbon (WSOC) analysis and factor analysis of aerosol mass tainty in the radiative forcings in climate models (Forster et spectrometer (AMS) data has, however, provided new in- al., 2007). sights into the partitioning of OA into several components The science of tropospheric ozone formation was first in- in diverse locations (Docherty et al., 2008; Hagino et al., vestigated following the discovery of photochemical smog in 2007; Lanz et al., 2007; Sullivan et al., 2004; Zhang et al., LA in the late 1940s, and has matured greatly in the last 60 2005a). In particular, these analyses allow for the OA mass years. The rate of ozone production can be quantified by var- observed in ambient locations to be separated into contribu- ious methods (Cazorla and Brune, 2009; Kanaya et al., 2008; tions from a few key components. Globally, the oxygenated Thornton et al., 2002; Wood et al., 2009), and its non-linear organic aerosol component (OOA) has been found to ac- dependence on its chemical precursors – volatile organic count for 64% to 95% of observed OA, with higher percent- compounds (VOCs) and nitrogen oxides (NOx) – has been ages observed at more rural sites (Zhang et al., 2007). Two examined in several studies (Jaegle, 1999; Kleinman, 2005; broad subtypes of OOA, “low-volatility OOA” (LV-OOA) Thornton et al., 2002). Although not complete, the chemical and “semi-volatile OOA” (SV-OOA), which differ from each mechanisms through which VOC oxidation leads to ozone other according to their volatility and extent of oxidation production have been elucidated through well-established have also been identified (Jimenez et al., 2009). At most laboratory techniques (Atkinson, 2000). Results from ozone sites, the time trends, mass spectra, correlation of tracers, and photochemical models are used by policymakers to design comparison with SOA estimates from other techniques indi- air pollution mitigation measures such as state implementa- cate that most or all the OOA component corresponds to SOA tion plans (SIPs) in the U.S. Although some predictive mod- (Docherty et al., 2008; Herndon et al., 2008; Volkamer et al., els are accurate to within 30% (e.g., Lei et al., 2007), serious 2006). Another common component of OA is hydrocarbon- knowledge gaps do remain (Hofzumahaus et al., 2009). Ad- like organic aerosol (HOA). The time trends and spectra of ditionally, air quality models are often limited by the quality the HOA component, on the other hand, closely resemble of emissions and meteorological input data. primary organic aerosol (POA), likely from multiple com- In contrast, the underlying science of SOA formation is bustion sources that cannot be separated due to the similarity less mature. The chemical mechanisms that control SOA of their mass spectra (Mohr et al., 2009; Ulbrich et al., 2009). formation often involve larger organic molecules and more Different types of OOA that correlate with inorganic tracers complex pathways and are far from fully understood (Hal- of different volatilities have been identified from AMS mea- lquist et al., 2009). SOA formation is often studied under surements (Lanz et al., 2007; Ulbrich et al., 2009). Biomass controlled conditions in smog chambers. These studies pro- burning organic aerosol (BBOA) components have also been vide valuable insights into SOA formation mechanisms and observed and characterized (Aiken et al., 2009). PM yields. However, the conditions under which these ex- In this paper, we extend the previously reported observa- periments are conducted do not fully simulate atmospheric tions of OOA and Ox in Mexico City (Herndon et al., 2008) conditions. Since most chamber studies are limited to small during the MILAGRO 2006 campaign (Megacity Initiative: numbers of

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