Browsing by Author "Griffin, R.J."
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Item Airmass aging metrics derived from particle and other measurements near Fort Worth(Elsevier, 2016) Cevik, B. Karakurt; Rutter, A.P.; Gong, L.; Griffin, R.J.; Flynn, J.H.; Lefer, B.L.; Kim, S.The composition, concentration, and size of submicron particulate matter (PM1) were measured at five-minute resolution by an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at a semi-rural location northwest of the Dallas-Fort Worth, TX, area during June 2011. Because of increased organic aerosol (OA) levels, focus here is placed on the period from June 17–30. The total measured PM1 mass concentration ranged between 1.1 and 16.5 μg m−3, with a mean of 4.4 ± 2.6 (one s.d.) μg m−3. Significant variability is observed in the time series of total PM1 and of four individual HR-ToF-AMS species, particularly between June 21 and 25. The average PM1mass composition was dominated by OA (55.0 ± 14.8%) and sulfate (30.7 ± 12.3%). Organic aerosol concentrations were correlated positively with carbon monoxide (CO) (R = 0.81). This study uses a variety of aging metrics and their relations to OA/ΔCO to characterize secondary organic aerosol. Photochemical age is estimated by using the toluene to benzene ratio. The average photochemical age was 26.7 ± 5.3 h. Other metrics of age used in this work include the ratio of sulfate to total sulfur and the ratio of nitrogen oxides to total reactive nitrogen. The correlations between the OA/ΔCO and nitrogen aging metrics indicate consistent aging, and a weak relationship is observed between OA/ΔCO and sulfur aging. However, the relationship between photochemical age and OA/ΔCO does not show a statistically significant correlation.Item FORest Canopy Atmosphere Transfer (FORCAsT) 1.0: a 1-D model of biosphere–atmosphere chemical exchange(European Geosciences Union, 2015) Ashworth, K.; Chung, S.H.; Griffin, R.J.; Chen, J.; Forkel, R.; Bryan, A.M.; Steiner, A.L.Biosphere–atmosphere interactions play a critical role in governing atmospheric composition, mediating the concentrations of key species such as ozone and aerosol, thereby influencing air quality and climate. The exchange of reactive trace gases and their oxidation products (both gas and particle phase) is of particular importance in this process. The FORCAsT (FORest Canopy Atmosphere Transfer) 1-D model is developed to study the emission, deposition, chemistry and transport of volatile organic compounds (VOCs) and their oxidation products in the atmosphere within and above the forest canopy. We include an equilibrium partitioning scheme, making FORCAsT one of the few canopy models currently capable of simulating the formation of secondary organic aerosols (SOAs) from VOC oxidation in a forest environment. We evaluate the capability of FORCAsT to reproduce observed concentrations of key gas-phase species and report modeled SOA concentrations within and above a mixed forest at the University of Michigan Biological Station (UMBS) during the Community Atmosphere-Biosphere Interactions Experiment (CABINEX) field campaign in the summer of 2009. We examine the impact of two different gas-phase chemical mechanisms on modelled concentrations of short-lived primary emissions, such as isoprene and monoterpenes, and their oxidation products. While the two chemistry schemes perform similarly under high-NOx conditions, they diverge at the low levels of NOx at UMBS. We identify peroxy radical and alkyl nitrate chemistry as the key causes of the differences, highlighting the importance of this chemistry in understanding the fate of biogenic VOCs (bVOCs) for both the modelling and measurement communities.Item Impact of Environmental Variables on the Reduction of Nitric Acid by Proxies for Volatile Organic Compounds Emitted by Motor Vehicles(Elsevier, 2016) Leong, Y.J.; Rutter, A.P.; Wong, H.Y.; Gutierrez, C.V.; Junaid, M.; Scheuer, E.; Gong, L.; Lewicki, R.; Dibb, J.E.; Tittel, F.K.; Griffin, R.J.Recent work has identified nitric acid (HNO3) as a potential precursor of nitrous acid (HONO), which is an important source of oxidants that regulate ozone and particulate pollution. Recent work in our laboratory has indicated that the reduction of HNO3 to HONO can occur homogeneously in the presence of surrogates for volatile organic compounds (VOCs) emitted by motor vehicles. This study focuses on the impact of environmental variables on the rate of formation of HONO in this process. The observed base case (25.0 °C and ∼20.0% relative humidity (RH)) HONO formation rate was 0.54 ± 0.09 ppb h−1, values comparable to enhancements observed in HONO during morning rush hour in Houston, TX. The rate was enhanced at lower temperatures of ∼20.0 °C, but the rate remained statistically similar (1σ) for experiments conducted at temperatures of 25 °C, 30 °C, and 35 °C. The assumption that multiple reactive components of the VOC mixture react with HNO3 is supported by this observation, and the relative importance of each reactive species in the reaction may vary with temperature. The enhanced rate at lower temperatures could make the proposed reaction mechanism more important at night. The formation rate of HONO does not change substantially when initial HNO3 concentration is varied between 400 and 4600 ppt, suggesting that the concentration of reactive VOCs was the limiting factor. The reduction of HNO3 to HONO appears not to occur heterogeneously on the aerosol surfaces tested. The presence of ∼120 ppb of ammonia has no observable impact on the reaction. However, it is likely that UV irradiation (λ = 350 nm) decreases the formation rate of HONO either by consuming the reactive VOCs involved or by directly interfering with the reaction. The “renoxification” of less reactive HNO3 to more reactive HONO has significant implications for daytime ozone and particulate pollution.Item Overview of surface measurements and spatial characterization of submicrometer particulate matter during the DISCOVER-AQ 2013 campaign in Houston, TX(Taylor & Francis, 2017) Leong, Y.J.; Sanchez, N.P.; Wallace, H.W.; Karakurt Cevik, B.; Hernandez, C.S.; Han, Y.; Flynn, J.H.; Massoli, P.; Floerchinger, C.; Fortner, E.C.; Herndon, S.; Bean, J.K.; Hildebrandt Ruiz, L.; Jeon, W.; Choi, Y.; Lefer, B.; Griffin, R.J.The sources of submicrometer particulate matter (PM1) remain poorly characterized in the industrialized city of Houston, TX. A mobile sampling approach was used to characterize PM1 composition and concentration across Houston based on high-time-resolution measurements of nonrefractory PM1 and trace gases during the DISCOVER-AQ Texas 2013 campaign. Two pollution zones with marked differences in PM1 levels, character, and dynamics were established based on cluster analysis of organic aerosol mass loadings sampled at 16 sites. The highest PM1 mass concentrations (average 11.6 ± 5.7 µg/m3) were observed to the northwest of Houston (zone 1), dominated by secondary organic aerosol (SOA) mass likely driven by nighttime biogenic organonitrate formation. Zone 2, an industrial/urban area south/east of Houston, exhibited lower concentrations of PM1 (average 4.4 ± 3.3 µg/m3), significant organic aerosol (OA) aging, and evidence of primary sulfate emissions. Diurnal patterns and backward-trajectory analyses enable the classification of airmass clusters characterized by distinct PM sources: biogenic SOA, photochemical aged SOA, and primary sulfate emissions from the Houston Ship Channel. Principal component analysis (PCA) indicates that secondary biogenic organonitrates primarily related with monoterpenes are predominant in zone 1 (accounting for 34% of the variability in the data set). The relevance of photochemical processes and industrial and traffic emission sources in zone 2 also is highlighted by PCA, which identifies three factors related with these processes/sources (~50% of the aerosol/trace gas concentration variability). PCA reveals a relatively minor contribution of isoprene to SOA formation in zone 1 and the absence of isoprene-derived aerosol in zone 2. The relevance of industrial amine emissions and the likely contribution of chloride-displaced sea salt aerosol to the observed variability in pollution levels in zone 2 also are captured by PCA. Implications: This article describes an urban-scale mobile study to characterize spatial variations in submicrometer particulate matter (PM1) in greater Houston. The data set indicates substantial spatial variations in PM1 sources/chemistry and elucidates the importance of photochemistry and nighttime oxidant chemistry in producing secondary PM1. These results emphasize the potential benefits of effective control strategies throughout the region, not only to reduce primary emissions of PM1 from automobiles and industry but also to reduce the emissions of important secondary PM1 precursors, including sulfur oxides, nitrogen oxides, ammonia, and volatile organic compounds. Such efforts also could aid in efforts to reduce mixing ratios of ozone.