Aircraft And Satellites Research Articles

An improved representation of fire non-methane organic gases (NMOGs) in models: emissions to reactivity

Abstract. Fires emit a substantial amount of non-methane organic gases (NMOGs), the atmospheric oxidation of which can contribute to ozone and secondary particulate matter formation. However, the abundance and reactivity of these fire NMOGs are uncertain and historically not well constrained. In this work, we expand the representation of fire NMOGs in a global chemical transport model, GEOS-Chem. We update emission factors to Andreae (2019) and the chemical mechanism to include recent aromatic and ethene and ethyne model improvements (Bates et al., 2021; Kwon et al., 2021). We expand the representation of NMOGs by adding lumped furans to the model (including their fire emission and oxidation chemistry) and by adding fire emissions of nine species already included in the model, prioritized for their reactivity using data from the Fire Influence on Regional to Global Environments (FIREX) laboratory studies. Based on quantified emissions factors, we estimate that our improved representation captures 72 % of emitted, identified NMOG carbon mass and 49 % of OH reactivity from savanna and temperate forest fires, a substantial increase from the standard model (49 % of mass, 28 % of OH reactivity). We evaluate fire NMOGs in our model with observations from the Amazon Tall Tower Observatory (ATTO) in Brazil, Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) and DC3 in the US, and Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in boreal Canada. We show that NMOGs, including furan, are well simulated in the eastern US with some underestimates in the western US and that adding fire emissions improves our ability to simulate ethene in boreal Canada. We estimate that fires provide 15 % of annual mean simulated surface OH reactivity globally, as well as more than 75 % over fire source regions. Over continental regions about half of this simulated fire reactivity comes from NMOG species. We find that furans and ethene are important globally for reactivity, while phenol is more important at a local level in the boreal regions. This is the first global estimate of the impact of fire on atmospheric reactivity.

Emissions relationships in western forest fire plumes – Part 1: Reducing the effect of mixing errors on emission factors

Abstract. Studies of emission factors from biomass burning using aircraft data complement the results of lab studies and extend them to conditions of immense hot conflagrations. A new theoretical development of plume theory for multiple tracers is developed after examining aircraft samples. We illustrate and discuss emissions relationships for 422 individual samples from many forest fire plumes in the Western USA. Samples are from two NASA investigations: ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) and SEAC4RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys). This work provides sample-by-sample enhancement ratios (EnRs) for 23 gases and particulate properties. Many EnRs provide candidates for emission ratios (ERs, corresponding to the EnR at the source) when the origin and degree of transformation is understood. From these, emission factors (EFs) can be estimated, provided the fuel dry mass consumed is known or can be estimated using the carbon mass budget approach. This analysis requires understanding the interplay of mixing of the plume with surrounding air. Some initial examples emphasize that measured Ctot=CO2+CO in a fire plume does not necessarily describe the emissions of the total carbon liberated in the flames, Cburn. Rather, it represents Ctot=Cburn+Cbkgd, which includes possibly varying background concentrations for entrained air. Consequently, we present a simple theoretical description for plume entrainment for multiple tracers from the flame tops to hundreds of kilometers downwind and illustrate some intrinsic linear behaviors. The analysis suggests a mixed-effects regression emission technique (MERET), which can eliminate occasional strong biases associated with the commonly used normalized excess mixing ratio (NEMR) method. MERET splits Ctot to reveal Cburn by exploiting the fact that Cburn and all tracers respond linearly to dilution, while each tracer has consistent EnR behavior (slope of tracer concentration with respect to Cburn). The two effects are separable. Two or three or preferably more emission indicators are required as a minimum; here we used eight. In summary, MERET allows a fine spatial resolution (EnRs for individual observations) and comparison of similar plumes that are distant in time and space. Alkene ratios provide us with an approximate photochemical timescale. This allows discrimination and definition, by fire situation, of ERs, allowing us to estimate emission factors.

A linear model to derive melt pond depth on Arctic sea ice from hyperspectral data

Abstract. Melt ponds are key elements in the energy balance of Arctic sea ice. Observing their temporal evolution is crucial for understanding melt processes and predicting sea ice evolution. Remote sensing is the only technique that enables large-scale observations of Arctic sea ice. However, monitoring melt pond deepening in this way is challenging because most of the optical signal reflected by a pond is defined by the scattering characteristics of the underlying ice. Without knowing the influence of meltwater on the reflected signal, the water depth cannot be determined. To solve the problem, we simulated the way meltwater changes the reflected spectra of bare ice. We developed a model based on the slope of the log-scaled remote sensing reflectance at 710 nm as a function of depth that is widely independent from the bottom albedo and accounts for the influence of varying solar zenith angles. We validated the model using 49 in situ melt pond spectra and corresponding depths from shallow ponds on dark and bright ice. Retrieved pond depths are accurate (root mean square error, RMSE=2.81 cm; nRMSE=16 %) and highly correlated with in situ measurements (r=0.89; p=4.34×10-17). The model further explains a large portion of the variation in pond depth (R2=0.74). Our results indicate that our model enables the accurate retrieval of pond depth on Arctic sea ice from optical data under clear sky conditions without having to consider pond bottom albedo. This technique is potentially transferrable to hyperspectral remote sensors on unmanned aerial vehicles, aircraft and satellites.

Development and implementation of a new biomass burning emissions injection height scheme (BBEIH v1.0) for the GEOS-Chem model (v9-01-01)

Abstract. Biomass burning is a significant source of trace gases and aerosols to the atmosphere, and the evolution of these species depends acutely on where they are injected into the atmosphere. GEOS-Chem is a chemical transport model driven by assimilated meteorological data that is used to probe a variety of scientific questions related to atmospheric composition, including the role of biomass burning. This paper presents the development and implementation of a new global biomass burning emissions injection scheme in the GEOS-Chem model. The new injection scheme is based on monthly gridded Multi-angle Imaging SpectroRadiometer (MISR) global plume-height stereoscopic observations in 2008. To provide specific examples of the impact of the model updates, we compare the output from simulations with and without the new MISR-based injection height scheme to several sets of observations from regions with active fires. Our comparisons with Arctic Research on the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) aircraft observations show that the updated injection height scheme can improve the ability of the model to simulate the vertical distribution of peroxyacetyl nitrate (PAN) and carbon monoxide (CO) over North American boreal regions in summer. We also compare a simulation for October 2010 and 2011 to vertical profiles of CO over the Amazon Basin. When coupled with larger emission factors for CO, a simulation that includes the new injection scheme also better matches selected observations in this region. Finally, the improved injection height improves the simulation of monthly mean surface CO over California during July 2008, a period with large fires.

Global sensitivity analysis of the GEOS-Chem chemical transport model: ozone and hydrogen oxides during ARCTAS (2008)

Abstract. Developing predictive capability for future atmospheric oxidation capacity requires a detailed analysis of model uncertainties and sensitivity of the modeled oxidation capacity to model input variables. Using oxidant mixing ratios modeled by the GEOS-Chem chemical transport model and measured on the NASA DC-8 aircraft, uncertainty and global sensitivity analyses were performed on the GEOS-Chem chemical transport model for the modeled oxidants hydroxyl (OH), hydroperoxyl (HO2), and ozone (O3). The sensitivity of modeled OH, HO2, and ozone to model inputs perturbed simultaneously within their respective uncertainties were found for the flight tracks of NASA's Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) A and B campaigns (2008) in the North American Arctic. For the spring deployment (ARCTAS-A), ozone was most sensitive to the photolysis rate of NO2, the NO2 + OH reaction rate, and various emissions, including methyl bromoform (CHBr3). OH and HO2 were overwhelmingly sensitive to aerosol particle uptake of HO2 with this one factor contributing upwards of 75 % of the uncertainty in HO2. For the summer deployment (ARCTAS-B), ozone was most sensitive to emission factors, such as soil NOx and isoprene. OH and HO2 were most sensitive to biomass emissions and aerosol particle uptake of HO2. With modeled HO2 showing a factor of 2 underestimation compared to measurements in the lowest 2 km of the troposphere, lower uptake rates (γHO2 < 0. 055), regardless of whether or not the product of the uptake is H2O or H2O2, produced better agreement between modeled and measured HO2.

Evaluating model parameterizations of submicron aerosol scattering and absorption with in situ data from ARCTAS 2008

Abstract. Accurate modeling of the scattering and absorption of ultraviolet and visible radiation by aerosols is essential for accurate simulations of atmospheric chemistry and climate. Closure studies using in situ measurements of aerosol scattering and absorption can be used to evaluate and improve models of aerosol optical properties without interference from model errors in aerosol emissions, transport, chemistry, or deposition rates. Here we evaluate the ability of four externally mixed, fixed size distribution parameterizations used in global models to simulate submicron aerosol scattering and absorption at three wavelengths using in situ data gathered during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. The four models are the NASA Global Modeling Initiative (GMI) Combo model, GEOS-Chem v9-02, the baseline configuration of a version of GEOS-Chem with online radiative transfer calculations (called GC-RT), and the Optical Properties of Aerosol and Clouds (OPAC v3.1) package. We also use the ARCTAS data to perform the first evaluation of the ability of the Aerosol Simulation Program (ASP v2.1) to simulate submicron aerosol scattering and absorption when in situ data on the aerosol size distribution are used, and examine the impact of different mixing rules for black carbon (BC) on the results. We find that the GMI model tends to overestimate submicron scattering and absorption at shorter wavelengths by 10–23 %, and that GMI has smaller absolute mean biases for submicron absorption than OPAC v3.1, GEOS-Chem v9-02, or GC-RT. However, the changes to the density and refractive index of BC in GC-RT improve the simulation of submicron aerosol absorption at all wavelengths relative to GEOS-Chem v9-02. Adding a variable size distribution, as in ASP v2.1, improves model performance for scattering but not for absorption, likely due to the assumption in ASP v2.1 that BC is present at a constant mass fraction throughout the aerosol size distribution. Using a core-shell mixing rule in ASP overestimates aerosol absorption, especially for the fresh biomass burning aerosol measured in ARCTAS-B, suggesting the need for modeling the time-varying mixing states of aerosols in future versions of ASP.

Satellite observations of ethylene (C2H4) from the Aura Tropospheric Emission Spectrometer: A scoping study

We present a study focusing on detection and initial quantitative estimates of ethylene (C2H4) in observations from the Tropospheric Emission Spectrometer (TES), a Fourier transform spectrometer aboard the Aura satellite that measures thermal infrared radiances with high spectral resolution (0.1 cm−1). We analyze observations taken in support of the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission and demonstrate the feasibility of future development of C2H4 into a TES standard product. In the Northern Hemisphere, C2H4 is commonly associated with boreal fire plumes, motor vehicle exhaust and petrochemical emissions. It has a short lifetime (∼14–32 h) in the troposphere due to its reaction with OH and O3. Chemical destruction of C2H4 in the atmosphere leads to the production of ozone and other species such as carbon monoxide (CO) and formaldehyde. Results indicate a correlation between C2H4 and CO in boreal fire plumes. Quantitative C2H4 estimates are sensitive to assumptions about the plume height and width. We find that C2H4 greater than 2–3 ppbv can be detected in a single TES observation (for a fire plume at 3 km altitude and 1.5 km width). Spatial averaging will be needed for surface-peaking profiles where TES sensitivity is lower.

Ozone export from East Asia: The role of PAN

AbstractPeroxyacetyl nitrate (PAN) is an important ozone (O3) precursor. The lifetime of PAN is approximately 1 month in the free troposphere, and this allows O3 production to occur in pollution plumes at intercontinental distances from its source. In this study we use the Goddard Earth Observing System (GEOS)‐Chem global chemical transport model, new satellite measurements of PAN from the Aura Tropospheric Emission Spectrometer (TES), and data from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign over North America, to study the role of natural and anthropogenic Asian emissions on free tropospheric (900–400 hPa) PAN distributions and subsequent O3 production. Using the ARCTAS data with GEOS‐Chem, we show that while GEOS‐Chem is unbiased with respect to the aircraft data, TES version 7 PAN data are biased high for regions with surface temperatures colder than 285 K. However, GEOS‐Chem and TES measurements provide a consistent representation (within 15% difference) of PAN abundance over East Asia. Because of the good agreement between model and observations, we use the GEOS‐Chem model to evaluate the sources of PAN precursors and the effect of free tropospheric PAN on the export of O3 from Asia to North America. The GEOS‐Chem model results show that the largest contributors to free tropospheric PAN over Asia and the northern Pacific are anthropogenic and soil NOx emissions. Biomass burning emissions have important contributions to free tropospheric PAN over northern Pacific (25% in April), while the contribution from lightning over northern Pacific is significant in July (40%). Strong springtime transport in April results in more export of free tropospheric PAN and O3 from East Asian emissions. This free tropospheric PAN contributes about 35% to the abundance of free tropospheric O3 over western North America in spring and 25% in summer.

Development and application of the WRFPLUS-Chem online chemistry adjoint and WRFDA-Chem assimilation system

Abstract. Here we present the online meteorology and chemistry adjoint and tangent linear model, WRFPLUS-Chem (Weather Research and Forecasting plus chemistry), which incorporates modules to treat boundary layer mixing, emission, aging, dry deposition, and advection of black carbon aerosol. We also develop land surface and surface layer adjoints to account for coupling between radiation and vertical mixing. Model performance is verified against finite difference derivative approximations. A second-order checkpointing scheme is created to reduce computational costs and enable simulations longer than 6 h. The adjoint is coupled to WRFDA-Chem, in order to conduct a sensitivity study of anthropogenic and biomass burning sources throughout California during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign. A cost-function weighting scheme was devised to reduce the impact of statistically insignificant residual errors in future inverse modeling studies. Results of the sensitivity study show that, for this domain and time period, anthropogenic emissions are overpredicted, while wildfire emission error signs vary spatially. We consider the diurnal variation in emission sensitivities to determine at what time sources should be scaled up or down. Also, adjoint sensitivities for two choices of land surface model (LSM) indicate that emission inversion results would be sensitive to forward model configuration. The tools described here are the first step in conducting four-dimensional variational data assimilation in a coupled meteorology–chemistry model, which will potentially provide new constraints on aerosol precursor emissions and their distributions. Such analyses will be invaluable to assessments of particulate matter health and climate impacts.

Space-based retrieval of NO<sub>2</sub> over biomass burning regions: quantifying and reducing uncertainties

Abstract. The accuracy of space-based nitrogen dioxide (NO2) retrievals from solar backscatter radiances critically depends on a priori knowledge of the vertical profiles of NO2 and aerosol optical properties. This information is used to calculate an air mass factor (AMF), which accounts for atmospheric scattering and is used to convert the measured line-of-sight "slant" columns into vertical columns. In this study we investigate the impact of biomass burning emissions on the AMF in order to quantify NO2 retrieval errors in the Ozone Monitoring Instrument (OMI) products over these sources. Sensitivity analyses are conducted using the Linearized Discrete Ordinate Radiative Transfer (LIDORT) model. The NO2 and aerosol profiles are obtained from a 3-D chemistry-transport model (GEOS-Chem), which uses the Fire Locating and Monitoring of Burning Emissions (FLAMBE) daily biomass burning emission inventory. Aircraft in situ data collected during two field campaigns, the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) and the Dust and Biomass-burning Experiment (DABEX), are used to evaluate the modeled aerosol optical properties and NO2 profiles over Canadian boreal fires and West African savanna fires, respectively. Over both domains, the effect of biomass burning emissions on the AMF through the modified NO2 shape factor can be as high as −60%. A sensitivity analysis also revealed that the effect of aerosol and shape factor perturbations on the AMF is very sensitive to surface reflectance and clouds. As an illustration, the aerosol correction can range from −20 to +100% for different surface reflectances, while the shape factor correction varies from −70 to −20%. Although previous studies have shown that in clear-sky conditions the effect of aerosols on the AMF was in part implicitly accounted for by the modified cloud parameters, here it is suggested that when clouds are present above a surface layer of scattering aerosols, an explicit aerosol correction would be beneficial to the NO2 retrieval. Finally, a new method that uses slant column information to correct for shape-factor-related AMF error over NOx emission sources is proposed, with possible application to near-real-time OMI retrievals.

An airborne assessment of atmospheric particulate emissions from the processing of Athabasca oil sands

Abstract. During the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign, two NASA research aircraft, a DC-8 and a P-3B, were outfitted with extensive trace gas (the DC-8) and aerosol (both aircraft) instrumentation. Each aircraft spent about a half hour sampling air around the oil sands mining and upgrading facilities near Ft. McMurray, Alberta, Canada. The DC-8 circled the area, while the P-3B flew directly over the upgrading plants, sampling close to the exhaust stacks, then headed downwind to monitor the aerosol as it aged. At short range, the plume from the oil sands is a complex mosaic of freshly nucleated ultrafine particles from a SO2- and NO2-rich plume, soot and possibly fly ash from industrial processes, and dust from dirt roads and mining operations. Shortly downwind, organic aerosol appears in quantities that rival SO4, either as volatile organic vapors condense or as they react with the H2SO4. The DC-8 pattern allowed us to integrate total flux from the oil sands facilities within about a factor of 2 uncertainty that spanned values consistent with 2008 estimates from reported SO2 and NO2 emissions, though there is no reason to expect one flyby to represent average conditions. In contrast, CO fluxes exceeded reported regional emissions, due either to variability in production or sources missing from the emissions inventory. The conversion rate of SO2 to aerosol SO4 of ~6% per hour is consistent with earlier reports, though OH concentrations are insufficient to accomplish this. Other oxidation pathways must be active. Altogether, organic aerosol and black carbon emissions from the oil sands operations are small compared with annual forest fire emissions in Canada. The oil sands do contribute significant sulfate and exceed fire production of SO2 by an order of magnitude.

Comparison of MODIS 3 km and 10 km resolution aerosol optical depth retrievals over land with airborne sunphotometer measurements during ARCTAS summer 2008

Abstract. Airborne sunphotometer measurements acquired by the NASA Ames Airborne Tracking Sunphotometer (AATS-14) aboard the NASA P-3 research aircraft are used to evaluate dark-target over-land retrievals of extinction aerosol optical depth (AOD) from spatially and temporally near-coincident measurements by the Moderate Resolution Imaging Spectroradiometer (MODIS) during the summer 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign. The new MODIS Collection 6 aerosol data set includes retrievals of AOD at both 10 km × 10 km and 3 km × 3 km (at nadir) resolution. In this paper we compare MODIS and AATS AOD at 553 nm in 58 10 km and 134 3 km retrieval grid cells. These AOD values were derived from data collected over Canada on four days during short time segments of five (four Aqua and one Terra) satellite overpasses of the P-3 during low-altitude P-3 flight tracks. Three of the five MODIS–AATS coincidence events were dominated by smoke: one included a P-3 transect of a well-defined smoke plume in clear sky, but two were confounded by the presence of scattered clouds above smoke. The clouds limited the number of MODIS retrievals available for comparison, and led to MODIS AOD retrievals that underestimated the corresponding AATS values. This happened because the MODIS aerosol cloud mask selectively removed 0.5 km pixels containing smoke and clouds before the aerosol retrieval. The other two coincidences (one Terra and one Aqua) occurred during one P-3 flight on the same day and in the same general area, in an atmosphere characterized by a relatively low AOD (< 0.3), spatially homogeneous regional haze from smoke outflow with no distinguishable plume. For the ensemble data set for MODIS AOD retrievals with the highest-quality flag, MODIS AOD agrees with AATS AOD within the expected MODIS over-land AOD uncertainty in 60% of the retrieval grid cells at 10 km resolution and 69% at 3 km resolution. These values improve to 65 % and 74%, respectively, when the cloud-affected case with the strongest plume is excluded. We find that the standard MODIS dark-target over-land retrieval algorithm fails to retrieve AOD for thick smoke, not only in cloud-contaminated regions but also in clear sky. We attribute this to deselection, by the cloud and/or bright surface masks, of 0.5 km resolution pixels that contain smoke.

Source attributions of pollution to the Western Arctic during the NASA ARCTAS field campaign

Abstract. We use the NASA GEOS-5 transport model with tagged tracers to investigate the contributions of different regional sources of CO and black carbon (BC) to their concentrations in the Western Arctic (i.e., 50–90° N and 190–320° E) in spring and summer 2008. The model is evaluated by comparing the results with airborne measurements of CO and BC from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaigns to demonstrate the strengths and limitations of our simulations. We also examine the reliability of tagged CO tracers in characterizing air mass origins using the measured fossil fuel tracer of dichloromethane and the biomass burning tracer of acetonitrile. Our tagged CO simulations suggest that most of the enhanced CO concentrations (above background level from CH4 production) observed during April originate from Asian anthropogenic emissions. Boreal biomass burning emissions and Asian anthropogenic emissions are of similar importance in July domain wise, although the biomass burning CO fraction is much larger in the area of the ARCTAS field experiments. The fraction of CO from Asian anthropogenic emissions is larger in spring than in summer. European sources make up no more than 10% of CO levels in the campaign domain during either period. Comparisons of CO concentrations along the flight tracks with regional averages from GEOS-5 show that the along-track measurements are representative of the concentrations within the large domain of the Western Arctic in April but not in July.

Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008

Abstract. The NASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic during the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and anthropogenic industrial pollution were sampled. Observations were spatially extensive (50–85° N and 40–130° W) and exhibit significant variability in aerosol and CCN concentrations. The chemical composition was dominated by highly oxidized organics (66–94% by volume), with a water-soluble mass fraction of more than 50%. The aerosol hygroscopicity parameter, κ, ranged between κ = 0.08–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08 ± 0.01, while the Arctic background had the highest and most variable κ of 0.32 ± 0.21, resulting from a lower and more variable organic fraction. Both fresh and aged (long-range transported) biomass burning air masses exhibited remarkably similar κ (0.18 ± 0.13), consistent with observed rapid chemical and physical aging of smoke emissions in the atmosphere, even in the vicinity of fresh fires. The organic hygroscopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (εWSOM), with a κ = 0.12, such that κorg = 0.12εWSOM. Assuming bulk (size-independent) composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all environments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an external mixture and size-dependent composition was required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predictions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing and therefore may not always be representative of the environments sampled. No correlation was observed between κorg and O : C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concentrations exceeded 1.5×104 cm−3 and introduced supersaturation depletions of ≥25%. Not accounting for supersaturation depletion in these high concentration environments would therefore bias CCN closure up to 25% and inferred κ by up to 50%.

Characteristics of tropospheric ozone depletion events in the Arctic spring: analysis of the ARCTAS, ARCPAC, and ARCIONS measurements and satellite BrO observations

Abstract. Arctic ozone depletion events (ODEs) are caused by halogen catalyzed ozone loss. In situ chemistry, advection of ozone-poor air mass, and vertical mixing in the lower troposphere are important factors affecting ODEs. To better characterize the ODEs, we analyze the combined set of surface, ozonesonde, and aircraft in situ measurements of ozone and bromine compounds during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS), the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC), and the Arctic Intensive Ozonesonde Network Study (ARCIONS) experiments (April 2008). Tropospheric BrO columns retrieved from satellite measurements and back trajectory calculations are also used to investigate the characteristics of observed ODEs. In situ observations from these field experiments are inadequate to validate tropospheric BrO columns derived from satellite measurements. In view of this difficulty, we construct an ensemble of tropospheric column BrO estimates from two satellite (OMI and GOME-2) measurements and with three independent methods of calculating stratospheric BrO columns. Furthermore, we select analysis methods that do not depend on the absolute magnitude of column BrO, such as time-lagged correlation analysis of ozone and tropospheric column BrO, to understand characteristics of ODEs. Time-lagged correlation analysis between in situ (surface and ozonesonde) measurements of ozone and satellite derived tropospheric BrO columns indicates that the ODEs are due to either local halogen-driven ozone loss or short-range (∼1 day) transport from nearby regions with ozone depletion. The effect of in situ ozone loss is also evident in the diurnal variation difference between low (10th and 25th percentiles) and higher percentiles of surface ozone concentrations at Alert, Canada. Aircraft observations indicate low-ozone air mass transported from adjacent high-BrO regions. Correlation analyses of ozone with potential temperature and time-lagged tropospheric BrO column show that the vertical extent of local ozone loss is surprisingly deep (1–2 km) at Resolute and Churchill, Canada. The unstable boundary layer during ODEs at Churchill could potentially provide a source of free-tropospheric BrO through convective transport and explain the significant negative correlation between free-tropospheric ozone and tropospheric BrO column at this site.

Spectral aerosol direct radiative forcing from airborne radiative measurements during CalNex and ARCTAS

This study presents the aerosol radiative forcing derived from airborne measurements of shortwave spectral irradiance during the 2010 Research at the Nexus of Air Quality and Climate Change (CalNex). Relative forcing efficiency, the radiative forcing normalized by aerosol optical thickness and incident irradiance, is a means of comparing the aerosol radiative forcing for different conditions. In this study, it is used to put the aerosol radiative effects of an air mass in the Los Angeles basin in context with case studies from three field missions that targeted other regions and aerosol types, including a case study from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS). For CalNex, we relied on irradiance measurements onboard the NOAA P‐3 aircraft during a flight on 19 May 2010 over a ground station. CalNex presented a difficulty for determining forcing efficiency since one of the input parameters, optical thickness, was not available from the same aircraft. However, extinction profiles were available from a nearby aircraft. An existing retrieval algorithm was modified to use those measurements as initial estimate for the missing optical thickness. In addition, single scattering albedo and asymmetry parameter (secondary products of the method), were compared with CalNex in situ measurements. The CalNex relative forcing efficiency spectra agreed with earlier studies that found this parameter to be constrained at each wavelength within 20% per unit of aerosol optical thickness at 500 nm regardless of aerosol type and experiment, except for highly absorbing aerosols sampled near Mexico City. The diurnally averaged below‐layer forcing efficiency integrated over the wavelength range of 350–700 nm for CalNex is estimated to be −58.6 ± 13.8 W/m2, whereas for the ARCTAS case it is −48.7 ± 11.5 W/m2.

Airborne intercomparison of HO<sub>x</sub> measurements using laser-induced fluorescence and chemical ionization mass spectrometry during ARCTAS

Abstract. The hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively called HOx, play central roles in tropospheric chemistry. Accurate measurements of OH and HO2 are critical to examine our understanding of atmospheric chemistry. Intercomparisons of different techniques for detecting OH and HO2 are vital to evaluate their measurement capabilities. Three instruments that measured OH and/or HO2 radicals were deployed on the NASA DC-8 aircraft throughout Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in the spring and summer of 2008. One instrument was the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) for OH and HO2 measurements based on Laser-Induced Fluorescence (LIF) spectroscopy. A second instrument was the NCAR Selected-Ion Chemical Ionization Mass Spectrometer (SI-CIMS) for OH measurement. A third instrument was the NCAR Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS) for HO2 measurement. Formal intercomparison of LIF and CIMS was conducted for the first time on a same aircraft platform. The three instruments were calibrated by quantitative photolysis of water vapor by ultraviolet (UV) light at 184.9 nm with three different calibration systems. The absolute accuracies were ±32% (2σ) for the LIF instrument, ±65% (2σ) for the SI-CIMS instrument, and ±50% (2σ) for the PeRCIMS instrument. In general, good agreement was obtained between the CIMS and LIF measurements of both OH and HO2 measurements. Linear regression of the entire data set yields [OH]CIMS = 0.89 × [OH]LIF + 2.8 × 104 cm−3 with a correlation coefficient r2 = 0.72 for OH, and [HO2]CIMS = 0.86 × [HO2]LIF + 3.9 parts per trillion by volume (pptv, equivalent to pmol mol−1) with a correlation coefficient r2 = 0.72 for HO2. In general, the difference between CIMS and LIF instruments for OH and HO2 measurements can be explained by their combined measurement uncertainties. Comparison with box model results shows some similarities for both the CIMS and LIF measurements. First, the observed-to-modeled HO2 ratio increases greatly for higher NO mixing ratios, indicating that the model may not properly account for HOx sources that correlate with NO. Second, the observed-to-modeled OH ratio increases with increasing isoprene mixing ratios, suggesting either incomplete understanding of isoprene chemistry in the model or interferences in the measurements in environments where biogenic emissions dominate ambient volatile organic compounds.

Taking the pulse of pyrocumulus clouds

Large forest fires are a known natural and dominant disturbance factor in high northern latitudes, and form pyrocumulus (pyroCu), and occasionally pyrocumulonimbus (pyroCb) clouds. These clouds can transport emissions into the upper troposphere/lower stratosphere (UT/LS) and produce significant regional and even global climate effects, as is the case with some volcanoes. However, the lack of observational data within pyroCu or pyroCb complicates our ability to investigate pyro-convection and to understand the vertical and cross-isentropic transport mechanisms responsible for UT/LS injection. Here, we report detailed airborne radiation measurements within strong pyroCu taken over boreal forest fires in Saskatchewan, Canada during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) summer field campaign in 2008. We find a prominent smoke core within the pyroCu, which is defined by strong extinction in the UV, VIS and NIR, and high gas-particle concentrations. We also find that the angular distribution of radiance within the pyroCu is closely related to the diffusion domain in water clouds, which is dominated by multiple scattering processes. The radiation field of pyroCu can be described by diffusion approximations that are comprised of simple cosine functions, which can be used to calculate the spatial and temporal characteristics of the radiance field, and applied in cloud resolving models. We demonstrate with Monte Carlo simulations that radiation transport in pyroCu is inherently a 3D problem and must account for particle absorption.

Characterization of soluble bromide measurements and a case study of BrO observations during ARCTAS

Abstract. A focus of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission was examination of bromine photochemistry in the spring time high latitude troposphere based on aircraft and satellite measurements of bromine oxide (BrO) and related species. The NASA DC-8 aircraft utilized a chemical ionization mass spectrometer (CIMS) to measure BrO and a mist chamber (MC) to measure soluble bromide. We have determined that the MC detection efficiency to molecular bromine (Br2), hypobromous acid (HOBr), bromine oxide (BrO), and hydrogen bromide (HBr) as soluble bromide (Br−) was 0.9±0.1, 1.06+0.30/−0.35, 0.4±0.1, and 0.95±0.1, respectively. These efficiency factors were used to estimate soluble bromide levels along the DC-8 flight track of 17 April 2008 from photochemical calculations constrained to in situ BrO measured by CIMS. During this flight, the highest levels of soluble bromide and BrO were observed and atmospheric conditions were ideal for the space-borne observation of BrO. The good agreement (R2 = 0.76; slope = 0.95; intercept = −3.4 pmol mol−1) between modeled and observed soluble bromide, when BrO was above detection limit (>2 pmol mol−1) under unpolluted conditions (NO<10 pmol mol−1), indicates that the CIMS BrO measurements were consistent with the MC soluble bromide and that a well characterized MC can be used to derive mixing ratios of some reactive bromine compounds. Tropospheric BrO vertical column densities (BrOVCD) derived from CIMS BrO observations compare well with BrOTROPVCD from OMI on 17 April 2008.

Reactive nitrogen, ozone and ozone production in the Arctic troposphere and the impact of stratosphere-troposphere exchange

Abstract. We use aircraft observations obtained during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission to examine the distributions and source attributions of O3 and NOy in the Arctic and sub-Arctic region. Using a number of marker tracers, we distinguish various air masses from the background troposphere and examine their contributions to NOx, O3, and O3 production in the Arctic troposphere. The background Arctic troposphere has a mean O3 of ~60 ppbv and NOx of ~25 pptv throughout spring and summer with CO decreasing from ~145 ppbv in spring to ~100 ppbv in summer. These observed mixing ratios are not notably different from the values measured during the 1988 ABLE-3A and the 2002 TOPSE field campaigns despite the significant changes in emissions and stratospheric ozone layer in the past two decades that influence Arctic tropospheric composition. Air masses associated with stratosphere-troposphere exchange are present throughout the mid and upper troposphere during spring and summer. These air masses, with mean O3 concentrations of 140–160 ppbv, are significant direct sources of O3 in the Arctic troposphere. In addition, air of stratospheric origin displays net O3 formation in the Arctic due to its sustainable, high NOx (75 pptv in spring and 110 pptv in summer) and NOy (~800 pptv in spring and ~1100 pptv in summer). The air masses influenced by the stratosphere sampled during ARCTAS-B also show conversion of HNO3 to PAN. This active production of PAN is the result of increased degradation of ethane in the stratosphere-troposphere mixed air mass to form CH3CHO, followed by subsequent formation of PAN under high NOx conditions. These findings imply that an adequate representation of stratospheric NOy input, in addition to stratospheric O3 influx, is essential to accurately simulate tropospheric Arctic O3, NOx and PAN in chemistry transport models. Plumes influenced by recent anthropogenic and biomass burning emissions observed during ARCTAS show highly elevated levels of hydrocarbons and NOy (mostly in the form of NOx and PAN), but do not contain O3 higher than that in the Arctic tropospheric background except some aged biomass burning plumes sampled during spring. Convection and/or lightning influences are negligible sources of O3 in the Arctic troposphere but can have significant impacts in the upper troposphere in the continental sub-Arctic during summer.