Integrating near-explicit gas- and aqueous-chemistry mechanisms for examining the impact of ethanol (E85) on urban air pollution with and without a fog [electronic resource]

Atmospheric models that solve chemistry in three dimensions (3-D) generally do not explicitly model organic chemistry due to computer time constraints. Organic species are grouped together based on their structures, which can result in inaccuracies for gas- and aqueous-chemistry, gas-to-aqueous transfer, and secondary organic aerosol (SOA) formation because reactivity, diffusion and vapor pressures can differ significantly between species in the same group. Here, we develop an atmospheric box model that uses a near-explicit chemical mechanism, the Master Chemical Mechanism (MCM) version 3.1 (MCM 2002), and an extensive aqueous-phase chemical mechanism, the Chemical Aqueous Phase Reactive Mechanism (CAPRAM 3.0i), in a sparse-matrix Gear-based solver, SMVGEAR II, to solve gas-phase and aqueous-phase tropospheric organic chemistry accurately and quickly enough for 3-D. We first examine the speed and accuracy of solving the MCM v. 3.1 with SMVGEAR II. The MCM has over 13,500 organic reactions and over 4,600 species. SMVGEAR II is a sparse-matrix vectorized Gear solver that reduces computation time significantly on scalar and vector machines, which is necessary for solving such a large mechanism. Although we use a box model for this study, we determine and demonstrate in a separate study that the speed of the MCM with SMVGEAR II allows the MCM to be modeled in 3 dimensions. We validate the MCM by comparing model results with smog chamber data for four organic species -- two alkenes and two aromatics. The model predictions match the smog chamber data very well for all cases except for toluene, where further development of the mechanism is needed. The steps for incorporating the aqueous-phase chemical mechanism and the gas-to-aqueous transfer method into SMVGEAR II are discussed in detail. CAPRAM 3.0i treats aqueous chemistry among 390 species and 829 reactions (including 51 gas-to-aqueous phase reactions). We couple gas- and aqueous-phase species through time-dependent dissolutional growth and dissociation equations. This method is validated with a smaller mechanism against results from a previous model intercomparison. When the smaller mechanism is compared with the full MCM-CAPRAM mechanism, some concentrations are still similar but others differ due to the greater detail in chemistry. We also expand the mechanism to include gas-aqueous transfer of two acids, glycolic acid and glyoxylic acid, and modify the glyoxal Henry's law constant from recent measurements. Glyoxal is important for SOA modeling. The average glyoxal partitioning in the cloud changes from 67% aqueous-phase to 87% aqueous- phase with the modifications. The addition of gas-aqueous transfer reactions increases the average gas-phase percentage of glycolic acid to 19% and of glyoxylic acid to 16%. This gas-phase and aqueous-phase chemistry module is a useful tool for studying detailed air pollution and SOA formation, in clear sky, cloudy, or foggy conditions. The increased use of ethanol in transportation fuels warrants an investigation of its consequences. An important component of such an investigation is the temperature-dependence of ethanol and gasoline exhaust chemistry. We use the model with species-resolved tailpipe emissions data for E85 (15% gasoline, 85% ethanol fuel blend) and gasoline vehicles to compare the impact of each on nitrogen oxides, organic gases, and ozone as a function of ambient temperature and background concentrations with and without a fog, using Los Angeles in 2020 as a base case. We use two different emissions sets -- one a compilation of exhaust and evaporative data taken near 24 ºC and the other from exhaust data taken at -7 ºC -- to determine how atmospheric chemistry and emissions are affected by temperature. We include diurnal effects by examining two day scenarios. We find that, accounting for chemistry and dilution alone without a fog, the average ozone concentrations through the range of temperatures tested are higher with E85 than with gasoline by ~7 part per billion (ppb) at higher temperatures (summer conditions) to ~39 ppb at low temperatures and low sunlight (winter conditions) for an area with a high nitrogen oxide (NOx) to non-methane organic gas (NMOG) ratio. The results suggest that E85's effect on health through ozone formation becomes increasingly more significant relative to gasoline at colder temperatures due to the change in exhaust emission composition at lower temperatures. Although ozone concentrations are not usually a concern for cold climates, the increase in ozone concentrations with E85 may be significant enough that it would exceed 35 ppb, the threshold mixing ratio above which short-term health effects occur. The increased risk of mortality due to short-term exposure to ozone is estimated to be 0.0004 per ppb above the threshold. In some areas, ozone concentrations may even exceed the 8 hr National Ambient Air Quality Standard (NAAQS) for ozone (75 ppb). Acetaldehyde and formaldehyde concentrations are also much higher with E85 at cold temperatures, which is a concern because both are carcinogens. These results could have implications for wintertime use of E85. Peroxy acetyl nitrate (PAN), another air pollutant of concern, increases with E85 by 0.3 to 8 ppbv. The sensitivity of the results to box size, initial background concentrations, background emissions, and water vapor are also examined. We continue this study to investigate the air quality impacts when a morning fog is present under summer and winter conditions. We find that E85 slightly increases ozone compared with gasoline in the presence or absence of a fog under summer conditions but increases ozone significantly relative to gasoline during winter conditions, although winter ozone is always lower than summer ozone. A new finding here is that a fog during summer may increase ozone after the fog disappears, due to chemistry alone. Temperatures are high enough in the summer to increase peroxy radical (RO2) production with the morning fog, which lead to the higher ozone after fog dissipation. A fog on a winter day decreases ozone after the fog. Within a fog, ozone is always lower than if no fog occurs. The sensitivity of the results to fog parameters like droplet size, liquid water content, fog duration, and photolysis are investigated and discussed. The results suggest E85 and gasoline both enhance pollution with E85 enhancing pollution significantly more at low temperatures.

Advisor: Jacobson Mark Z. (Mark Zachary) primary advisor. Thesis advisor
Golden David
Hildemann Lynn M. (Lynn Mary) advisor. Thesis advisor