Browsing by Author "Guo, Fangzhou"

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    Seasonal differences in formation processes of oxidized organic aerosol near Houston, TX
    (Copernicus Publications, 2019) Dai, Qili; Schulze, Benjamin C.; Bi, Xiaohui; Bui, Alexander A.T.; Guo, Fangzhou; Wallace, Henry W.; Sanchez, Nancy P.; Flynn, James H.; Lefer, Barry L.; Feng, Yinchang; Griffin, Robert J.
    Submicron aerosol was measured to the southwest of Houston, Texas, during winter and summer 2014 to investigate its seasonal variability. Data from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) indicated that organic aerosol (OA) was the largest component of nonrefractory submicron particulate matter (NR-PM1) (on average, 38 % ± 13 % and 47 % ± 18 % of the NR-PM1 mass loading in winter and summer, respectively). Positive matrix factorization (PMF) analysis of the OA mass spectra demonstrated that two classes of oxygenated OA (less- and more-oxidized OOA, LO and MO) together dominated OA mass in summer (77 %) and accounted for 39 % of OA mass in winter. The fraction of LO-OOA (out of total OOA) is higher in summer (70 %) than in winter (44 %). Secondary aerosols (sulfate + nitrate + ammonium + OOA) accounted for ∼76 % and 88 % of NR-PM1 mass in winter and summer, respectively, indicating NR-PM1 mass was driven mostly by secondary aerosol formation regardless of the season. The mass loadings and diurnal patterns of these secondary aerosols show a clear winter–summer contrast. Organic nitrate (ON) concentrations were estimated using the NO+x ratio method, with contributions of 31 %–66 % and 9 %–17 % to OA during winter and summer, respectively. The estimated ON in summer strongly correlated with LO-OOA (r=0.73) and was enhanced at nighttime. The relative importance of aqueous-phase chemistry and photochemistry in processing OOA was investigated by examining the relationship of aerosol liquid water content (LWC) and the sum of ozone (O3) and nitrogen dioxide (NO2) (Ox = O3+NO2) with LO-OOA and MO-OOA. The processing mechanism of LO-OOA apparently was related to relative humidity (RH). In periods of RH < 80 %, aqueous-phase chemistry likely played an important role in the formation of wintertime LO-OOA, whereas photochemistry promoted the formation of summertime LO-OOA. For periods of high RH > 80 %, these effects were opposite those of low-RH periods. Both photochemistry and aqueous-phase processing appear to facilitate increases in MO-OOA concentration except during periods of high LWC, which is likely a result of wet removal during periods of light rain or a negative impact on its formation rate. The nighttime increases in MO-OOA during winter and summer were 0.013 and 0.01 µg MO-OOA per µg of LWC, respectively. The increase in LO-OOA was larger than that for MO-OOA, with increase rates of 0.033 and 0.055 µg LO-OOA per µg of LWC at night during winter and summer, respectively. On average, the mass concentration of LO-OOA in summer was elevated by nearly 1.2 µg m−3 for a ∼20 µg change in LWC, which was accompanied by a 40 ppb change in Ox.
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    Sources and Characterization of Ozone and Submicron Aerosol in San Antonio, Texas
    (2021-12-02) Guo, Fangzhou; Griffin, Robert J.
    The work presented here describes the observations of 1) the criteria air pollutant ozone (O3) and its precursors- nitrogen oxides (NOX) and volatile organic compounds (VOCs) and 2) non-refractory submicron aerosol (NR-PM1) at two sites in the city of San Antonio, Texas. Chemical modeling and source apportionment techniques are used to characterize the reaction pathways and sources of the observed pollutants. A customized urban-scale zero-dimensional model is created to calculate the rate of change in concentration of proxies for secondary aerosols across the urban core. Based on the inter-site comparison of ozone modeling results, we discover that San Antonio is mostly in a NOX-sensitive O3 formation regime throughout the daytime during the campaign. In general, O3 destruction rate is one order of magnitude smaller than the O3 formation rate, leading to a large net O3 production. The VOCs exhibiting high reactivity are formaldehyde, isoprene, and alkenes. The existence of an elevated regional O3 level combined with strong solar radiation and local O3 formation could lead to non-attainment or near non-attainment on high O3 days. These results suggest emissions control strategies for NOX to reduce direct local O3 production. Emissions control on VOCs, including formaldehyde precursors, could also help in reducing the regional background O3. These altogether would decrease the O3 level during the late spring peak in San Antonio. A customized urban-scale zero-dimensional model is created to calculate the rate of change in concentration of proxies for secondary aerosols across the urban core of San Antonio. Detailed parameterization and sensitivity tests provide the basis for us to discover that oxygenated organic aerosol (OOA) species experience a reactive chemical loss during the transport. Strong parabolic relationships between the net chemical reaction rate of less oxidized (LO)-OOA and ambient temperature is observed, which is the first field observation of the temperature impact on secondary organic aerosol formation rate, consistent with the relationship in concentrations predicted for earlier chamber simulation studies. More oxidized (MO)-OOA does not show such a relationship likely due to it being more oxidized, less volatile, and less reactive. More active chemical formation occurs if the air gets warm enough and oxidants are at a significant enough level during the transport across the urban core. The modeling results also indicate a very small net loss rate of sulfate aerosol during the transport across the city and negligible sulfate formation from local sources. The backward trajectories cluster analysis categorizes the air inflow during the campaign period into “Oceanic”, “Near Inland”, and “Continental” scenarios. Higher NR-PM1 loadings in Oceanic scenarios are driven mostly by high sulfate and ammonium. The averaged mass loadings of sulfate and ammonium nearly triple from Continental to Oceanic scenarios. Organics are relatively consistent, and nitrate and chloride altogether contribute less than 2.5% of the total NR-PM1 on a mass basis. Aerosol liquid water (ALW) contents and inorganic aerosol pH calculated using a thermodynamic model decrease from Oceanic to Continental scenarios. A positive trend between estimated methanesulfonic acid (MSA) and binned ALW under relatively acidic conditions is observed. MSA also correlates well with sulfate, suggesting shared oxidation pathways of secondary species along the transport pathway. Regional source contribution function analysis reveals major sources of anthropogenic sulfates on the western and central Gulf and a few hot spots on southern and eastern Texas. Biogenic sulfates mostly originate in the coastal and central Gulf.
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    Urban core-downwind differences and relationships related to ozone production in a major urban area in Texas
    (Elsevier, 2021) Guo, Fangzhou; Bui, Alexander A.T.; Schulze, Benjamin C.; Yoon, Subin; Shrestha, Sujan; Wallace, Henry W.; Sakai, Yuta; Actkinson, Blake W.; Erickson, Matthew H.; Alvarez, Sergio; Sheesley, Rebecca; Usenko, Sascha; Flynn, James; Griffin, Robert J.
    San Antonio, the second-most populous city in Texas and the seventh-most populous city in the United States (US), has been designated a marginal non-attainment area by the US Environmental Protection Agency with respect to the 2015 ozone (O3) National Ambient Air Quality Standard. While stationary air quality monitoring sites are operated in the region by the Texas Commission on Environmental Quality (TCEQ), there are limited in situ field measurements for O3 and its precursors in the urban core. To better understand O3 dynamics in San Antonio, a suite of meteorological and gas instruments was deployed during May 2017. We incorporate field measurements from two campaign sites and one TCEQ stationary monitoring site into a zero-dimensional O3 model to characterize the local formation and destruction rates of O3, hydroxyl radical (OH) reactivity of volatile organic compounds (VOCs), O3 production efficiency, and O3 formation regime in the urban core and directly downwind of San Antonio. Upwind/downwind differences indicate the importance of photochemical processing of VOCs with carbon-carbon double bonds. San Antonio was mostly in a nitrogen oxide (NOX)-sensitive regime throughout the daytime during the campaign period, with O3 formation peaking at noon in the city center and early afternoon at the downwind region. Formaldehyde (HCHO), isoprene, and alkenes dominated VOC reactivity, with alkenes and isoprene from San Antonio's core (upwind) likely contributing to the downwind formation of HCHO and enhancing its OH reactivity. However, their direct impact on downwind O3 production was not observed. Model results suggest further strengthening NOX emission controls to decrease O3 formation in San Antonio.