High urban NOx triggers a substantial chemical downward flux of ozone

Nitrogen oxides (NOx) play a central role in catalyzing tropospheric ozone formation. Nitrogen dioxide (NO2) has recently reemerged as a key target for air pollution control measures, and observational evidence points toward a limited understanding of ozone in high-NOx environments. A complete understanding of the mechanisms controlling the rapid atmospheric cycling between ozone (O3)-nitric oxide (NO)-NO2 in high-NOx regimes at the surface is therefore paramount but remains challenging because of competing dynamical and chemical effects. Here, we present long-term eddy covariance measurements of O3, NO, and NO2, over an urban area, that allow disentangling important physical and chemical processes. When generalized, our findings suggest that the depositional O3 flux near the surface in urban environments is negligible compared to the flux caused by chemical conversion of O3. This leads to an underestimation of the Leighton ratio and is a key process for modulating urban NO2 mixing ratios. As a consequence, primary NO2 emissions have been significantly overestimated.

Quantitative Kinetics of HO2 Reactions with Aldehydes in the Atmosphere: High-Order Dynamic Correlation, Anharmonicity, and Falloff Effects Are All Important

Kinetics provides the fundamental parameters for elucidating sources and sinks of key atmospheric species and for atmospheric modeling more generally. Obtaining quantitative kinetics in the laboratory for the full range of atmospheric temperatures and pressures is quite difficult. Here, we use computational chemistry to obtain quantitative rate constants for the reactions of HO₂ with HCHO, CH₃CHO, and CF₃CHO. First, we calculate the high-pressure-limit rate constants by using a dual-level strategy that combines conventional transition state theory using a high level of electronic structure wave function theory with canonical variational transition state theory including small-curvature tunneling using density functional theory. The wave-function level is beyond-CCSD(T) for HCHO and CCSD(T)-F12a (Level-A) for XCHO (X = CH₃, CF₃), and the density functional (Level-B) is specifically validated for these reactions. Then, we calculate the pressure-dependent rate constants by using system-specific quantum RRK theory (SS-QRRK) and also by an energy-grained master equation. The two treatments of the pressure dependence agree well. We find that the Level-A//Level-B method gives good agreement with CCSDTQ(P)/CBS. We also find that anharmonicity is an important factor that increases the rate constants of all three reactions. We find that the HO₂ + HCHO reaction has a significant dependence on pressure, but the HO₂ + CF₃CHO reaction is almost independent of pressure. Our findings show that the HO₂ + HCHO reaction makes important contribution to the sink for HCHO, and the HO₂ + CF₃CHO reaction is the dominant sink for CF₃CHO in the atmosphere.

Traffic, transport, and vegetation drive VOC concentrations in a major urban area in Texas

The population of Texas has increased rapidly in the past decade. The San Antonio Field Study (SAFS) was designed to investigate ozone (O₃) production and precursors in this rapidly changing, sprawling metropolitan area. There are still many questions regarding the sources and chemistry of volatile organic compounds (VOCs) in urban areas like San Antonio which are affected by a complex mixture of industry, traffic, biogenic sources and transported pollutants. The goal of the SAFS campaign in May 2017 was to measure inorganic trace gases, VOCs, methane (CH₄), and ethane (C₂H₆). The SAFS field design included two sites to better assess air quality across the metro area: an urban site (Traveler's World; TW) and a downwind/suburban site (University of Texas at San Antonio; UTSA). The results indicated that acetone (2.52 ± 1.17 and 2.39 ± 1.27 ppbv), acetaldehyde (1.45 ± 1.02 and 0.93 ± 0.45 ppbv) and isoprene (0.64 ± 0.49 and 1.21 ± 0.85 ppbv; TW and UTSA, respectively) were the VOCs with the highest concentrations. Additionally, positive matrix factorization showed three dominant factors of VOC emissions: biogenic, aged urban mixed source, and acetone. Methyl vinyl ketone and methacrolein (MVK + MACR) exhibited contributions from both secondary photooxidation of isoprene and direct emissions from traffic. The C₂H₆:CH₄ demonstrated potential influence of oil and gas activities in San Antonio. Moreover, the high O₃ days during the campaign were in the NO_x-limited O₃ formation regime and were preceded by evening peaks in select VOCs, NO_x and CO. Overall, quantification of the concentration and trends of VOCs and trace gases in a major city in Texas offers vital information for general air quality management and supports strategies for reducing O₃ pollution. The SAFS campaign VOC results will also add to the growing body of literature on urban sources and concentrations of VOCs in major urban areas.

Formaldehyde column density measurements as a suitable pathway to estimate near-surface ozone tendencies from space

In support of future satellite missions that aim to address the current shortcomings in measuring air quality from space, NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign was designed to enable exploration of relationships between column measurements of trace species relevant to air quality at high spatial and temporal resolution. In the DISCOVER-AQ data set, a modest correlation (r 2 = 0.45) between ozone (O3) and formaldehyde (CH2O) column densities was observed. Further analysis revealed regional variability in the O3-CH2O relationship, with Maryland having a strong relationship when data were viewed temporally and Houston having a strong relationship when data were viewed spatially. These differences in regional behavior are attributed to differences in volatile organic compound (VOC) emissions. In Maryland, biogenic VOCs were responsible for ~28% of CH2O formation within the boundary layer column, causing CH2O to, in general, increase monotonically throughout the day. In Houston, persistent anthropogenic emissions dominated the local hydrocarbon environment, and no discernable diurnal trend in CH2O was observed. Box model simulations suggested that ambient CH2O mixing ratios have a weak diurnal trend (±20% throughout the day) due to photochemical effects, and that larger diurnal trends are associated with changes in hydrocarbon precursors. Finally, mathematical relationships were developed from first principles and were able to replicate the different behaviors seen in Maryland and Houston. While studies would be necessary to validate these results and determine the regional applicability of the O3-CH2O relationship, the results presented here provide compelling insight into the ability of future satellite missions to aid in monitoring near-surface air quality.

Detection of iodine monoxide in the tropical free troposphere

Atmospheric iodine monoxide (IO) is a radical that catalytically destroys heat trapping ozone and reacts further to form aerosols. Here, we report the detection of IO in the tropical free troposphere (FT). We present vertical profiles from airborne measurements over the Pacific Ocean that show significant IO up to 9.5 km altitude and locate, on average, two-thirds of the total column above the marine boundary layer. IO was observed in both recent deep convective outflow and aged free tropospheric air, suggesting a widespread abundance in the FT over tropical oceans. Our vertical profile measurements imply that most of the IO signal detected by satellites over tropical oceans could originate in the FT, which has implications for our understanding of iodine sources. Surprisingly, the IO concentration remains elevated in a transition layer that is decoupled from the ocean surface. This elevated concentration aloft is difficult to reconcile with our current understanding of iodine lifetimes and may indicate heterogeneous recycling of iodine from aerosols back to the gas phase. Chemical model simulations reveal that the iodine-induced ozone loss occurs mostly above the marine boundary layer (34%), in the transition layer (40%) and FT (26%) and accounts for up to 20% of the overall tropospheric ozone loss rate in the upper FT. Our results suggest that the halogen-driven ozone loss in the FT is currently underestimated. More research is needed to quantify the widespread impact that iodine species of marine origin have on free tropospheric composition, chemistry, and climate.

A tunable diode laser absorption spectrometer for formaldehyde atmospheric measurements validated by simulation chamber instrumentation

A tunable diode laser absorption spectrometer (TDLAS) for formaldehyde atmospheric measurements has been set up and validated through comparison experiments with a Fourier transform infrared spectrometer (FT-IR) in a simulation chamber. Formaldehyde was generated in situ in the chamber from reaction of ethene with ozone. Three HCHO ro-vibrational line intensities (at 2909.71, 2912.09 and 2914.46 cm(-1)) possibly used by TDLAS were calibrated by FT-IR spectra simultaneously recorded in the 1600-3200 cm(-1) domain during ethene ozonolysis, enabling the on-line deduction of the varying concentration for HCHO in formation. The experimental line intensities values inferred confirmed the calculated ones from the updated HITRAN database. In addition, the feasibility of stratospheric in situ HCHO measurements using the 2912.09 cm(-1) line was demonstrated. The TDLAS performances were also assessed, leading to a 2sigma detection limit of 88 ppt in volume mixing ratio with a response time of 60 sec at 30 Torr and 294 K for 112 m optical path. As part of this work, the room-temperature rate constant of this reaction and the HCHO formation yield were found to be in excellent agreement with the compiled literature data.

Direct Dynamics Study on the Hydrogen Abstraction Reaction CH2O + HO2 → CHO + H2O2

We present a direct ab initio dynamics study on the hydrogen abstraction reaction CH2O + HO2 --> CHO + H2O2, which is predicted to have four possible reaction channels caused by different attacking orientations of HO2 radical to CH2O. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of the four reaction channels are calculated at the B3LYP/cc-pVTZ level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of some single-point multilevel energy calculations (HL). The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that, in the whole temperature range, the more favorable reaction channels are Channels 1 and 3. The total ICVT/SCT rate constants of the four channels at the HL//B3LYP/cc-pVTZ level of theory are in good agreement with the available experiment data over the measured temperature ranges, and the corresponding three-parameter expression is k(ICVT/SCT) = 3.13 x 10(-20) T(2.70) exp(-11.52/RT) cm3 mole(-1) s(-1) in the temperature range of 250-3000 K. Additionally, the flexibility of the dihedral angle of H2O2 is also discussed to explain the different experimental values.

Formation of Hydroxyl Radical from the Photolysis of Frozen Hydrogen Peroxide

Hydrogen peroxide (HOOH) in ice and snow is an important chemical tracer for the oxidative capacities of past atmospheres. However, photolysis in ice and snow will destroy HOOH and form the hydroxyl radical (*OH), which can react with snowpack trace species. Reactions of *OH in snow and ice will affect the composition of both the overlying atmosphere (e.g., by the release of volatile species such as formaldehyde to the boundary layer) and the snow and ice (e.g., by the *OH-mediated destruction of trace organics). To help understand these impacts, we have measured the quantum yield of *OH from the photolysis of HOOH on ice. Our measured quantum yields (Phi(HOOH --> *OH)) are independent of ionic strength, pH, and wavelength, but are dependent upon temperature. This temperature dependence for both solution and ice data is best described by the relationship ln(Phi(HOOH --> *OH)) = -(684 +/- 17)(1/T) + (2.27 +/- 0.064) (where errors represent 1 standard error). The corresponding activation energy (Ea) for HOOH (5.7 kJ mol(-1)) is much smaller than that for nitrate photolysis, indicating that the photochemistry of HOOH is less affected by changes in temperature. Using our measured quantum yields, we calculate that the photolytic lifetimes of HOOH in surface snow grains under midday, summer solstice sunlight are approximately 140 h at representative sites on the Greenland and Antarctic ice sheets. In addition, our calculations reveal that the majority of *OH radicals formed on polar snow grains are from HOOH photolysis, while nitrate photolysis is only a minor contributor. Similarly, HOOH appears to be much more important than nitrate as a photochemical source of *OH on cirrus ice clouds, where reactions of the photochemically formed hydroxyl radical could lead to the release of oxygenated volatile organic compounds to the upper troposphere.