Importance of biogenic volatile organic compounds to acyl peroxy nitrates (APN) production in the southeastern US during SOAS 2013

Gas-phase atmospheric concentrations of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were measured on the ground using a gas chromatograph electron capture detector (GC-ECD) during the Southern Oxidants and Aerosols Study (SOAS) 2013 campaign (1 June to 15 July 2013) in Centreville, Alabama, in order to study biosphere&ndash;atmosphere interactions. Average levels of PAN, PPN, and MPAN were 169, 5, and 9&thinsp;pptv, respectively, and the sum accounts for an average of 16&thinsp;% of NOy during the daytime (10:00 to 16:00 local time). Higher concentrations were seen on average in air that came to the site from the urban NOx sources to the north. PAN levels were the lowest observed in ground measurements over the past two decades in the southeastern US. A multiple regression analysis indicates that biogenic volatile organic compounds (VOCs) account for 66&thinsp;% of PAN formation during this study. Comparison of this value with a 0-D model simulation of peroxyacetyl radical production indicates that at least 50&thinsp;% of PAN formation is due to isoprene oxidation. MPAN has a statistical correlation with isoprene hydroxynitrates (IN). Organic aerosol mass increases with gas-phase MPAN and IN concentrations, but the mass of organic nitrates in particles is largely unrelated to MPAN.</p

Organic nitrate chemistry and its implications for nitrogen budgets in an isoprene- and monoterpene-rich atmosphere: constraints from aircraft (SEAC4RS) and ground-based (SOAS) observations in the Southeast US

Formation of organic nitrates (RONO2) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NOx), but the chemistry of RONO2 formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO2) in the GEOS-Chem global chemical transport model with  ∼  25  x  25 km2 resolution over North America. We evaluate the model using aircraft (SEAC4RS) and ground-based (SOAS) observations of NOx, BVOCs, and RONO2 from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO2 species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50 % of observed RONO2 in surface air, and we find that another 10 % is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10 % of observed boundary layer RONO2 were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO3 accounts for 60 % of simulated gas-phase RONO2 loss in the boundary layer. Other losses are 20 % by photolysis to recycle NOx and 15 % by dry deposition. RONO2 production accounts for 20 % of the net regional NOx sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NOx emissions. This segregation implies that RONO2 production will remain a minor sink for NOx in the Southeast US in the future even as NOx emissions continue to decline

Production and degradation of isoprene-derived organic nitrates in the atmosphere

Ground-level ozone is a regulated pollutant due to its adverse effects on human health and crop yields. This pollutant is catalytically produced in the photochemical reactions that involve volatile organic compounds and nitrogen oxides. Nitrogen oxides can facilitate the formation of ozone. However, when nitric oxide is sequestered by an organic compound to form an organic nitrate, the nitric oxide is removed from the catalytic cycle that forms ozone. Hence, formation of organic nitrates can reduce the ozone production rate near the ground. However, when organic nitrates decompose and release nitrogen oxides back into the atmosphere, ozone formation is enhanced again. Therefore, detailed understanding of the production and degradation processes of organic nitrates is important to controlling ground-level ozone to a safe level. Among all the volatile organic compounds emitted into the atmosphere, isoprene is the most important non-methane organic compounds, marked by its high emission rate and high photochemical reactivity. Therefore, this work is focused on the chemical processes surrounding isoprene-derived organic nitrates. Chapter two describes the sensitivity tests on the ionization efficiency using iodide as the reagent ion to detect multi-functional organic nitrates via mass spectrometry, such as hydroxynitrate. Different organic molecules were tested using an iodide-based chemical ionization mass spectrometer. The results suggested that iodide ion is not sensitive to alkyl alcohol or alkyl nitrate, but the combination of these two functionalities, such as those in α,β-hydroxynitrates, can increase the sensitivity by five orders of magnitude, making iodide-based chemical ionization a very powerful tool to study organic nitrates. Chapter three describes the laboratory study to quantify the first-generation isoprene hydroxynitrates with the iodide-based chemical ionization mass spectrometer. Two authentic standards were synthesized for instrument calibration. For the important tertiary nitrate that cannot be synthesized, a GC-ECD/MS interface system was developed to calibrate the relative sensitivity for the nitrate isomers generated in the gas phase. A series of chamber experiments was conducted to derive the yield of the first-generation hydroxynitrates in the OH-initiated oxidation of isoprene under high NO condition. Chapter four describes a field study conducted in the summer of 2013 in rural Alabama that was focused on ambient measurements of isoprene hydroxynitrates. A distinctive diurnal profile for the concentrations of isoprene hydroxynitrates was observed. By calculating the production and loss rates of isoprene hydroxynitrates, it is inferred that the isoprene hydroxynitrates had fast production in the morning, but in the afternoon, their production was overshadowed by the oxidative loss, due to the limited availability of NO during this study. Therefore, the isoprene oxidation chemistry undergoes a transition from the high NO to the low NO regime through the course of one day, and highly oxidized products such as dihydroxy hydroxyperoxy nitrate can be expected. Chapter five describes a zero-dimensional photochemical kinetics modeling study to simulate the formation and degradation of isoprene hydroxynitrates in rural Alabama. The model includes the hydroxynitrate yield obtained in Chapter 3, as well as the most recent update in the isoprene oxidation mechanism in literature. Since the zero-dimensional model does not include factors such as transport and mixing that can significantly affect the observed concentrations of species in the atmosphere, a relative concentration, instead of the measured absolute concentration, was simulated. The modeled result was able to capture the general profile of the field observation, lending support to the hydroxynitrate yield obtained in the laboratory. However, the model-observation discrepancy suggested that vertical transport in the morning can also have ~27% influence on the observed hydroxynitrate concentrations. Chapter six describes a focused kinetics study on a synthesized carbonyl nitrate, which is a product of the NO3-initiated isoprene oxidation reaction. The NMR, IR and UV spectra of the molecule were reported for structural characterization. The rate constants for the nitrate to react with OH or to react with O3 were obtained using the relative rate method. The photolysis frequency of the nitrate was also obtained in chamber experiments and extrapolated to the ambient environment. It is estimated that for this unsaturated carbonyl nitrate, photolysis is its dominant atmospheric degradation pathway, instead of reaction with OH, which is the most important loss mechanism for most organic compounds in the atmosphere. Two nitrate products from the OH oxidation reactions were observed and quantified. Chapter seven describes future studies of organic nitrates relevant to this thesis work. More effort should be focused on understanding the isomeric nitrates produced from conjugated biogenic volatile organic compounds and performing chamber experiments in conditions that resemble the ambient environment

Stationary and portable multipollutant monitors for high-spatiotemporal-resolution air quality studies including online calibration

The distribution and dynamics of atmospheric pollutants are spatiotemporally heterogeneous due to variability in emissions, transport, chemistry, and deposition. To understand these processes at high spatiotemporal resolution and their implications for air quality and personal exposure, we present custom, low-cost air quality monitors that measure concentrations of contaminants relevant to human health and climate, including gases (e.g., O3, NO, NO2, CO, CO2, CH4, and SO2) and size-resolved (0.3–10 μm) particulate matter. The devices transmit sensor data and location via cellular communications and are capable of providing concentration data down to second-level temporal resolution. We produce two models: one designed for stationary (or mobile platform) operation and a wearable, portable model for directly measuring personal exposure in the breathing zone. To address persistent problems with sensor drift and environmental sensitivities (e.g., relative humidity and temperature), we present the first online calibration system designed specifically for low-cost air quality sensors to calibrate zero and span concentrations at hourly to weekly intervals. Monitors are tested and validated in a number of environments across multiple outdoor and indoor sites in New Haven, CT; Baltimore, MD; and New York City. The evaluated pollutants (O3, NO2, NO, CO, CO2, and PM2.5) performed well against reference instrumentation (e.g., r = 0.66–0.98) in urban field evaluations with fast e-folding response times (≤1 min), making them suitable for both large-scale network deployments and smaller-scale targeted experiments at a wide range of temporal resolutions. We also provide a discussion of best practices on monitor design, construction, systematic testing, and deployment

Organic nitrate chemistry and its implications for nitrogen budgets in an isoprene- and monoterpene-rich atmosphere: constraints from aircraft (SEAC&lt;sup&gt;4&lt;/sup&gt;RS) and ground-based (SOAS) observations in the Southeast US

Formation of organic nitrates (RONO2) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NOx), but the chemistry of RONO2 formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO2) in the GEOS-Chem global chemical transport model with  ∼  25  x  25 km2 resolution over North America. We evaluate the model using aircraft (SEAC4RS) and ground-based (SOAS) observations of NOx, BVOCs, and RONO2 from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO2 species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50 % of observed RONO2 in surface air, and we find that another 10 % is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10 % of observed boundary layer RONO2 were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO3 accounts for 60 % of simulated gas-phase RONO2 loss in the boundary layer. Other losses are 20 % by photolysis to recycle NOx and 15 % by dry deposition. RONO2 production accounts for 20 % of the net regional NOx sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NOx emissions. This segregation implies that RONO2 production will remain a minor sink for NOx in the Southeast US in the future even as NOx emissions continue to decline