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Bromine radicals (Br + BrO) are important atmospheric species owing to their ability to catalytically destroy ozone as well as their potential impacts on the oxidative pathways of many trace gases, including dimethylsulfide and mercury. Using space‐based observations of BrO, recent studies have reported rapid enhancements of tropospheric BrO over large areas (so called “BrO explosions”) connected to near‐surface ozone depletion occurring in polar spring. However, the source(s) of reactive bromine and mechanism(s) that initiate these BrO explosions are uncertain. In this study, we investigate the relationships between Arctic BrO explosions and two of the proposed sources of reactive bromine: sea‐salt aerosol (SSA) generated from blowing snow and first‐year (seasonal) sea ice. We use tropospheric column BrO derived from the Ozone Monitoring Instrument (OMI) in conjunction with the Goddard Earth Observing System Version 5 (GEOS‐5) data assimilation system provided by National Aeronautics and Space Administration Global Modeling and Assimilation Office. Case studies demonstrate a strong association between the temporal and spatial extent of OMI‐observed BrO explosions and the GEOS‐5 simulated blowing snow‐generated SSA during Arctic spring. Furthermore, the frequency of BrO explosion events observed over the 11‐year record of OMI exhibits significant correlation with a time series of the simulated SSA emission flux in the Arctic and little to no correlation with a time series of satellite‐based first‐year sea ice area. Therefore, we conclude that SSA generated by blowing snow is an important factor in the formation of the BrO explosion observed from space during Arctic spring. OMI‐observed Arctic tropospheric BrO explosions are associated with GEOS‐5 simulated sea‐salt aerosols (SSAs) from wind‐driven blowing snowThe 11‐year time series analysis shows that the Arctic BrO explosion frequency and simulated blowing snow SSA emission flux are correlatedThe satellite‐based record of first‐year sea ice area does not show significant correlation with the Arctic BrO explosion frequency

A recent airborne field campaign over the remote western Pacific obtained the first intensive in situ ozone sampling over the warm pool region from oceanic surface to 15 km altitude (near 360 K potential temperature level). The new data set quantifies ozone in the tropical tropopause layer under significant influence of convective outflow. The analysis further reveals a bimodal distribution of free tropospheric ozone mixing ratio. A primary mode, narrowly distributed around 20 ppbv, dominates the troposphere from the surface to 15 km. A secondary mode, broadly distributed with a 60 ppbv modal value, is prominent between 3 and 8 km (320 K to 340 K potential temperature levels). The latter mode occurs as persistent layers of ozone‐rich drier air and is characterized by relative humidity under 45%. Possible controlling mechanisms are discussed. These findings provide new insight into the physical interpretation of the “S”‐shaped mean ozone profiles in the tropics. New airborne in situ ozone data over tropical western Pacific from surface to 15 kmData show a dominant feature in ozone distribution near 20 ppbv throughout the troposphereA secondary mode near 60 ppbv is formed by O3‐enhanced and H2O‐reduced air masses

The kinetics of the dimerization of ClO radicals, ClO + ClO + M → Cl2O2 + M (1a), and the 210 nm absorption cross sections of the ClO dimer have been studied using the technique of flash photolysis with UV absorption spectroscopy, over the temperature range 183−245 K and pressure range 25−700 Torr. ClO radicals were generated following the photolysis of Cl2/Cl2O/N2 mixtures and were quantified via their differential absorption between the peak of the (12,0) band of the (Ã ← &Xtilde;) transition at 275.2 nm and the adjacent minimum to higher wavelengths, while Cl2O2 formation was simultaneously monitored at 210 nm. ClO differential absorption cross sections were measured under identical conditions to the kinetic experiments by four separate calibration schemes. The rate coefficient measured at the lower temperatures studied (T < 200 K) was found to be up to 40% faster than extrapolation of previous results would suggest, with the limiting low- and high-pressure rate coefficients for reaction 1 in nitrogen determined to be k0 = (1.59 ± 0.60) × 10^-32 × (T/300)^-4.50±0.98 molecules^-2 cm⁶ s^-1 and k∞ = (1.36 ± 0.22) × 10^-12 × (T/300)^-3.09±0.40 molecules^-1 cm³ s^-1 respectively, obtained with Fc = 0.6. The corresponding value for k0 in air is k0(atm) = (1.49 ± 0.56) × 10^-32 × (T/300)^-4.50±0.98 molecules^-2 cm⁶ s^-1. The 210 nm absorption cross section of Cl2O2 was measured following time-resolved monitoring of its formation via ClO radical association, and a mean value of (2.94 ± 0.86) × 10^-18 molecule^-1 cm² obtained, temperature independent (to within ± 15%) between 183 and 245 K. Errors are combined systematic uncertainties and 2 standard deviations statistical variation.

During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NOx and HOx radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NOx−HNO3 exchange and to understand their control over the partitioning between NO2 and HNO3 in regions of continuous sunlight. We calculate NOx production (PNOx) and loss (LNOx) in a manner directly constrained by the in situ measurements and current rate constant recommendations, using approaches for representing albedo, overhead O3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% between modeled rates of NOx production and loss (LNOx = 1.18PNOx) which is within the measurement uncertainty of ±27%. The partitioning between NOx production processes is [HNO3 + OH (41 ± 2)%; HNO3 + hν (59 ± 2)%] and between NOx loss processes is [NO2 + OH, 90% to >97%; BrONO2 + H2O, 10% to <3%]. The steady-state description of NOx−HNO3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (1) the steady-state value of [NO2]24h-avg/[HNO3] in the continuously sunlit, lower stratosphere is a function only of temperature and number density, and (2) the partitioning of NOx production between HNO3 + OH and HNO3 + hν is nearly constant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately predicting the steady-state value of [NO2]24h-avg/[HNO3] and the partitioning of NOx production within these regions. The results establish a metric to compare observations of [NO2]24h-avg/[HNO3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NOx−HOx photochemistry in the lower stratosphere.

Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HOx (OH + HO2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HOx system. Measurements in the lower stratosphere show a remarkably tight correlation of OH concentration with the solar zenith angle (SZA). This correlation is nearly invariant over latitudes ranging from 70° S to 90° N and all seasons. An analysis of the production and loss of HOx in terms of the rate determining steps of reaction sequences developed by Johnston and Podolske and Johnston and Kinnison is used to clarify the behavior of the system and to directly test our understanding of the system with observations. Calculations using in situ measurements show that the production rate of HOx is proportional to O3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NOy (reactive nitrogen) and the concentration of HO2. In the absence of heterogeneous reactions, the partitioning of NOy is controlled by O3 and HOx and the concentration of HO2 is controlled by NOy and O3, so that the removal rate of OH is buffered against changes in the correlation of O3 and NOy. The heterogeneous conversion of NO2 to HNO3 is not an important net source of HOx because production and removal sequences are nearly balanced. Changes in NOy partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HOx, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO2/HNO3 are offset in the data set examined here by enhanced production rates resulting from increased photolysis rates resulting from the decreased O3 column above the ER-2.

To study emissions of CO2in the Baltimore, MD‐Washington, D.C. (Balt‐Wash) area, an aircraft campaign was conducted in February 2015, as part of the Fluxes of Atmospheric Greenhouse‐Gases in Maryland (FLAGG‐MD) project. During the campaign, elevated mole fractions of CO2were observed downwind of the urban center and local power plants. Upwind flight data and Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model analyses help account for the impact of emissions outside the Balt‐Wash area. The accuracy, precision, and sensitivity of CO2emissions estimates based on the mass balance approach were assessed for both power plants and cities. Our estimates of CO2emissions from two local power plants agree well with their Continuous Emissions Monitoring Systems (CEMS) records. For the 16 power plant plumes captured by the aircraft, the mean percentage difference of CO2emissions was −0.3%. For the Balt‐Wash area as a whole, the 1𝜎 CO2emission rate uncertainty for any individual aircraft‐based mass balance approach experiment was ±38%. Treating the mass balance experiments, which were repeated seven times within 9 days, as individual quantifications of the Balt‐Wash CO2emissions, the estimation uncertainty was ±16% (standard error of the mean at 95% CL). Our aircraft‐based estimate was compared to various bottom‐up fossil fuel CO2(FFCO2) emission inventories. Based on the FLAGG‐MD aircraft observations, we estimate 1.9 ± 0.3 MtC of FFCO2from the Balt‐Wash area during the month of February 2015. The mean estimate of FFCO2from the four bottom‐up models was 2.2 ± 0.3 MtC. 1.9 ± 0.3 MtC of fossil fuel CO2was emitted in Baltimore‐Washington during February 2015 based on data collected during seven aircraft flightsFour bottom‐up inventories indicate 2.2 ± 0.3 MtC of fossil fuel CO2was emitted, in good agreement with our top‐down estimateThe uncertainty from a single flight segment was ±38% (1𝜎); data from seven flights yielded a precision of 16% at the 95% confidence level

Precision requirements are determined for space‐based column‐averaged CO2dry air mole fraction data. These requirements result from an assessment of spatial and temporal gradients in the relationship between precision and surface CO2flux uncertainties inferred from inversions of the data, and the effects of biases on the fidelity of CO2flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these data precision requirements.