Investigation of the correlation between odd oxygen and secondary organic aerosol in Mexico City and Houston

Abstract

Many recent models underpredict secondary organic aerosol (SOA) particulate matter (PM) concentrations in polluted regions, indicating serious deficiencies in the models' chemical mechanisms and/or missing SOA precursors. Since tropospheric photochemical ozone production is much better understood, we investigate the correlation of odd-oxygen ([O<sub>x</sub>]≡[O<sub>3</sub>]+[NO<sub>2</sub>]) and the oxygenated component of organic aerosol (OOA), which is interpreted as a surrogate for SOA. OOA and O<sub>x</sub> measured in Mexico City in 2006 and Houston in 2000 were well correlated in air masses where both species were formed on similar timescales (less than 8 h) and not well correlated when their formation timescales or location differed greatly. When correlated, the ratio of these two species ranged from 30 μg m<sup>−3</sup>/ppm (STP) in Houston during time periods affected by large petrochemical plant emissions to as high as 160 μg m<sup>−3</sup>/ppm in Mexico City, where typical values were near 120 μg m<sup>−3</sup>/ppm. On several days in Mexico City, the [OOA]/[O<sub>x</sub>] ratio decreased by a factor of ~2 between 08:00 and 13:00 local time. This decrease is only partially attributable to evaporation of the least oxidized and most volatile components of OOA; differences in the diurnal emission trends and timescales for photochemical processing of SOA precursors compared to ozone precursors also likely contribute to the observed decrease. The extent of OOA oxidation increased with photochemical aging. Calculations of the ratio of the SOA formation rate to the O<sub>x</sub> production rate using ambient VOC measurements and traditional laboratory SOA yields are lower than the observed [OOA]/[O<sub>x</sub>] ratios by factors of 5 to 15, consistent with several other models' underestimates of SOA. Calculations of this ratio using emission factors for organic compounds from gasoline and diesel exhaust do not reproduce the observed ratio. Although not succesful in reproducing the atmospheric observations presented, modeling <i>P</i>(SOA)/<i>P</i>(O<sub>x</sub>) can serve as a useful test of photochemical models using improved formulation mechanisms for SOA

A case study of aerosol processing and evolution in summer in New York City

“…LV-OOA + SV-OOA, a surrogate of SOA, shows tight correlation with O 3 (r 2 = 0.84) but weaker correlation (r 2 = 0.40) with O x (NO 2 + O 3 ), likely due to an important contribution of NO 2 from local emissions. Recent studies found that SOA and O x correlated well in air masses where they formed on similar timescales (less than 8 h), but correlated poorly when their formation timescales or location differed greatly (Herndon et al, 2008;Wood et al, 2010). This is consistent with the tight correlations between SOA and O 3 during this stage of photochemical production (06:00-18:00 on 22 July).…”

“…sulfate and nitrate) are fairly well understood, there is considerable uncertainty on the formation of secondary organic aerosol (SOA) (Hallquist et al, 2009). For example, traditional SOA models only considering SOA formation from volatile organic compounds (VOCs) often greatly underestimate the observed SOA (Heald et al, 2005;Volkamer et al, 2006;Dzepina et al, 2009;Wood et al, 2010). In addition to source emissions and secondary formation, other dominant processes affecting ambient aerosol concentrations include cloud processing, additional OA aging, wet deposition by precipitation, and dry deposition by convective transport and diffusion (Pöschl, 2005).…”

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

“…An important exception is the emission ratios for propane and butane, which were disproportionality high in Mexico City due to liquid petroleum use [Apel et al, 2010]. The contribution of these light VOC compounds was found to be small for O x production and zero for SOA production, however [Wood et al, 2010]. Similar to the OA/ΔCO discussion in the preceding section, the consistency in OOA/O x for multiple field campaigns suggests a relatively constant mix of different O 3 and SOA precursor sources, leading to similar production ratios across multiple urban locations in North America.…”

“…Therefore, the ratios of OOA to odd-oxygen provide another metric for quantifying the dependence of SOA concentration on photochemical oxidation. Following the work of Herndon et al [2008] and Wood et al [2010], we examine the correlations of O x instead of O 3 to account for the titration of O 3 by fresh NO emissions which produces NO 2 . When comparing the time series of total OOA (SV-OOA + LV-OOA) versus O x in Figure 9, similar temporal changes are observed (R 2 = 0.53), but the correlation is stronger during the more polluted periods of high OOA concentrations that occurred in June (R 2 = 0.72 for the 2-6 June high pollution period).…”

Urban impacts on regional carbonaceous aerosols: Case study in central Texas

“…The optical attenuation (Wood et al, 2010) using the 678-nm laser source in the TOT analyzer was used to determine b abs and MAC as described by Ram and Sarin (2009) and Cheng et al (2011). All MAC calculations in this study have been corrected by an empirical correction factor, C ¼ 3.6, used to correct the measured absorption for multiple scattering and shadowing effects (Weingartner et al, 2003;Ram and Sarin, 2009) (Supplemental Materials, section S3.1).…”