Biomass burning nitrogen dioxide emissions derived from space with TROPOMI: methodology and validation

Smoke from wildfires is a significant source of air pollution, which can adversely impact air quality and ecosystems downwind. With the recently increasing intensity and severity of wildfires, the threat to air quality is expected to increase. Satellite-derived biomass burning emissions can fill in gaps in the absence of aircraft or ground-based measurement campaigns and can help improve the online calculation of biomass burning emissions as well as the biomass burning emissions inventories that feed air quality models. This study focuses on satellite-derived NOx emissions using the high-spatial-resolution TROPOspheric Monitoring Instrument (TROPOMI) NO2 dataset. Advancements and improvements to the satellite-based determination of forest fire NOx emissions are discussed, including information on plume height and effects of aerosol scattering and absorption on the satellite-retrieved vertical column densities. Two common top-down emission estimation methods, (1) an exponentially modified Gaussian (EMG) and (2) a flux method, are applied to synthetic data to determine the accuracy and the sensitivity to different parameters, including wind fields, satellite sampling, noise, lifetime, and plume spread. These tests show that emissions can be accurately estimated from single TROPOMI overpasses. The effect of smoke aerosols on TROPOMI NO2 columns (via air mass factors, AMFs) is estimated, and these satellite columns and emission estimates are compared to aircraft observations from four different aircraft campaigns measuring biomass burning plumes in 2018 and 2019 in North America. Our results indicate that applying an explicit aerosol correction to the TROPOMI NO2 columns improves the agreement with the aircraft observations (by about 10&thinsp;%&ndash;25&thinsp;%). The aircraft- and satellite-derived emissions are in good agreement within the uncertainties. Both top-down emissions methods work well; however, the EMG method seems to output more consistent results and has better agreement with the aircraft-derived emissions. Assuming a Gaussian plume shape for various biomass burning plumes, we estimate an average NOx e-folding time of 2&thinsp;&plusmn;1&thinsp;h from TROPOMI observations. Based on chemistry transport model simulations and aircraft observations, the net emissions of NOx are 1.3 to 1.5 times greater than the satellite-derived NO2 emissions. A correction factor of 1.3 to 1.5 should thus be used to infer net NOx emissions from the satellite retrievals of NO2. </div

Heterogeneity and chemical reactivity of the remote troposphere defined by aircraft measurements

The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10&thinsp;s (2&thinsp;km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O3 and CH4 (O3, CH4, CO, H2O, HCHO, H2O2, CH3OOH, C2H6, higher alkanes, alkenes, aromatics, NOx, HNO3, HNO4, peroxyacetyl nitrate, other organic nitrates), consisting of 146&thinsp;494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O3 and CH4 photochemical tendencies from this modeling data stream for ATom 1. We find that 80&thinsp;%&ndash;90&thinsp;% of the total reactivity lies in the top 50&thinsp;% of the parcels and 25&thinsp;%&ndash;35&thinsp;% in the top 10&thinsp;%, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. In other words, accurate simulation of the least reactive 50&thinsp;% of the troposphere is unimportant for global budgets. Surprisingly, the probability densities of species and reactivities averaged on a model scale (100&thinsp;km) differ only slightly from the 2&thinsp;km ATom data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find excellent agreement with the loss of O3 and CH4 but sharp disagreement with production of O3. The models sharply underestimate O3 production below 4&thinsp;km in both Pacific and Atlantic basins, and this can be traced to lower NOx levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation. &nbsp;</p

The Fires, Asian, and Stratospheric Transport-Las Vegas Ozone Study (FAST-LVOS)

The Fires, Asian, and Stratospheric Transport&ndash;Las Vegas Ozone Study (FAST-LVOS) was conducted in May and June of 2017 to study the transport of ozone (O3) to Clark County, Nevada, a marginal non-attainment area in the southwestern United States (SWUS). This 6-week (20&nbsp;May&ndash;30&nbsp;June 2017) field campaign used lidar, ozonesonde, aircraft, and in situ measurements in conjunction with a variety of models to characterize the distribution of O3 and related species above southern Nevada and neighboring California and to probe the influence of stratospheric intrusions and wildfires as well as local, regional, and Asian pollution on surface O3 concentrations in the Las Vegas Valley (&asymp;&thinsp;900&thinsp;m above sea level, a.s.l.). In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 in Clark County and southern Nevada. The companion paper by Zhang et al.&nbsp;(2020) describes the use of the AM4 and GEOS-Chem global models to simulate the measurements and estimate the impacts of transported O3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O3 layers above Las Vegas on more than 75&thinsp;% (35 of 45) of the sample days and show that entrainment of these layers contributed to mean 8&thinsp;h average regional background O3 concentrations of 50&ndash;55 parts per billion by volume (ppbv), or about 85&ndash;95&thinsp;&micro;g&thinsp;m&minus;3. These high background concentrations constitute 70&thinsp;%&ndash;80&thinsp;% of the current US National Ambient Air Quality Standard (NAAQS) of 70&thinsp;ppbv (&asymp;&thinsp;120&thinsp;&micro;g&thinsp;m&minus;3 at 900&thinsp;m&thinsp;a.s.l.) for the daily maximum 8&thinsp;h average (MDA8) and will make attainment of the more stringent standards of 60 or 65&thinsp;ppbv currently being considered extremely difficult in the interior SWUS. &nbsp;</p

Monitoring methane emissions from oil and gas operations

The atmospheric concentration of methane has more than doubled since the start of the Industrial Revolution. Methane is the second-most-abundant greenhouse gas created by human activities and a major driver of climate change. This APS-Optica report provides a technical assessment of the current state of monitoring U.S. methane emissions from oil and gas operations, which accounts for roughly 30% of U.S. anthropogenic methane emissions. The report identifies current technological and policy gaps and makes recommendations for the federal government in three key areas: methane emissions detection, reliable and systematized data and models to support mitigation measures, and effective regulation. &nbsp;</p

Impact of stratospheric air and surface emissions on tropospheric nitrous oxide during ATom

We measured the global distribution of tropospheric N2O mixing ratios during the NASA airborne Atmospheric Tomography (ATom) mission. ATom measured concentrations of &sim;&thinsp;300&nbsp;gas species and aerosol properties in 647&nbsp;vertical profiles spanning the Pacific, Atlantic, Arctic, and much of the Southern Ocean basins, nearly from pole to pole, over four seasons (2016&ndash;2018). We measured N2O concentrations at 1&thinsp;Hz using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our ATom QCLS N2O measurements by a factor of three (based on the standard deviation of calibration measurements). Our measurements show that most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid- and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere&ndash;troposphere exchange through the tropospheric column down to the surface. The highest N2O mixing ratios occur close to the Equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often associated with indicators of photochemical processing, suggesting possible unexpected source processes. In most cases, the results show how difficult it is to separate the mixture of different sources in the atmosphere, which may contribute to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models. &nbsp;</p

On the sources and sinks of atmospheric VOCs: an integrated analysis of recent aircraft campaigns over North America

We apply a high-resolution chemical transport model (GEOS-Chem CTM) with updated treatment of volatile organic compounds (VOCs) and a comprehensive suite of airborne datasets over North America to (i)&nbsp;characterize the VOC budget and (ii)&nbsp;test the ability of current models to capture the distribution and reactivity of atmospheric VOCs over this region. Biogenic emissions dominate the North American VOC budget in the model, accounting for 70&thinsp;% and 95&thinsp;% of annually emitted VOC carbon and reactivity, respectively. Based on current inventories anthropogenic emissions have declined to the point where biogenic emissions are the dominant summertime source of VOC reactivity even in most major North American cities. Methane oxidation is a 2&times; larger source of nonmethane VOCs (via production of formaldehyde and methyl hydroperoxide) over North America in the model than are anthropogenic emissions. However, anthropogenic VOCs account for over half of the ambient VOC loading over the majority of the region owing to their longer aggregate lifetime. Fires can be a significant VOC source episodically but are small on average. In the planetary boundary layer (PBL), the model exhibits skill in capturing observed variability in total VOC abundance (R2=0.36) and reactivity (R2=0.54). The same is not true in the free troposphere (FT), where skill is low and there is a persistent low model bias (&sim;&thinsp;60&thinsp;%), with most (27 of 34) model VOCs underestimated by more than a factor of 2. A comparison of PBL&thinsp;:&thinsp;FT concentration ratios over the southeastern US points to a misrepresentation of PBL ventilation as a contributor to these model FT biases. We also find that a relatively small number of VOCs (acetone, methanol, ethane, acetaldehyde, formaldehyde, isoprene + oxidation products, methyl hydroperoxide) drive a large fraction of total ambient VOC reactivity and associated model biases; research to improve understanding of their budgets is thus warranted. A source tracer analysis suggests a current overestimate of biogenic sources for hydroxyacetone, methyl ethyl ketone and glyoxal, an underestimate of biogenic formic acid sources, and an underestimate of peroxyacetic acid production across biogenic and anthropogenic precursors. Future work to improve model representations of vertical transport and to address the VOC biases discussed are needed to advance predictions of ozone and SOA formation. </div

Radiative and chemical implications of the size and composition of aerosol particles in the existing or modified global stratosphere

The size of aerosol particles has fundamental effects on their chemistry and radiative effects. We explore those effects using aerosol size and composition data in the lowermost stratosphere along with calculations of light scattering. In the size range between about 0.1 and 1.0&thinsp;&micro;m diameter (accumulation mode), there are at least two modes of particles in the lowermost stratosphere. The larger mode consists mostly of particles produced in the stratosphere, and the smaller mode consists mostly of particles transported from the troposphere. The stratospheric mode is similar in the Northern and Southern Hemisphere, whereas the tropospheric mode is much more abundant in the Northern Hemisphere. The purity of sulfuric acid particles in the stratospheric mode shows that there is limited production of secondary organic aerosol in the stratosphere, especially in the Southern Hemisphere. Out of eight sets of flights sampling the lowermost stratosphere (four seasons and two hemispheres) there were three with large injections of specific materials: volcanic, biomass burning, or dust. The stratospheric and tropospheric modes have very different roles for radiative effects on climate and for heterogeneous chemistry. Because the larger particles are more efficient at scattering light, most of the radiative effect in the lowermost stratosphere is due to stratospheric particles. In contrast, the tropospheric particles can have more surface area, at least in the Northern Hemisphere. The surface area of tropospheric particles could have significant implications for heterogeneous chemistry because these particles, which are partially neutralized and contain organics, do not correspond to the substances used for laboratory studies of stratospheric heterogeneous chemistry. We then extend the analysis of size-dependent properties to particles injected into the stratosphere, either intentionally or from volcanoes. There is no single size that will simultaneously maximize the climate impact relative to the injected mass, infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. In addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified stratosphere. &nbsp;</p

Ambient aerosol properties in the remote atmosphere from global-scale in situ measurements

In situ measurements of aerosol microphysical, chemical, and optical properties were made during global-scale flights from 2016&ndash;2018 as part of the Atmospheric Tomography Mission (ATom). The NASA DC-8 aircraft flew from &sim;&thinsp;84∘&thinsp;N to &sim;&thinsp;86∘&thinsp;S latitude over the Pacific, Atlantic, Arctic, and Southern oceans while profiling nearly continuously between altitudes of &sim;&thinsp;160&thinsp;m and &sim;&thinsp;12&thinsp;km. These global circuits were made once each season. Particle size distributions measured in the aircraft cabin at dry conditions and with an underwing probe at ambient conditions were combined with bulk and single-particle composition observations and measurements of water vapor, pressure, and temperature to estimate aerosol hygroscopicity and hygroscopic growth factors and calculate size distributions at ambient relative humidity. These reconstructed, composition-resolved ambient size distributions were used to estimate intensive and extensive aerosol properties, including single-scatter albedo, the asymmetry parameter, extinction, absorption, &Aring;ngstr&ouml;m exponents, and aerosol optical depth (AOD) at several wavelengths, as well as cloud condensation nuclei (CCN) concentrations at fixed supersaturations and lognormal fits to four modes. Dry extinction and absorption were compared with direct in situ measurements, and AOD derived from the extinction profiles was compared with remotely sensed AOD measurements from the ground-based Aerosol Robotic Network (AERONET); this comparison showed no substantial bias. The purpose of this work is to describe the methodology by which ambient aerosol properties are estimated from the in situ measurements, provide statistical descriptions of the aerosol characteristics of different remote air mass types, examine the contributions to AOD from different aerosol types in different air masses, and provide an entry point to the ATom aerosol database. The contributions of different aerosol types (dust, sea salt, biomass burning, etc.) to AOD generally align with expectations based on location of the profiles relative to continental sources of aerosols, with sea salt and aerosol water dominating the column extinction in most remote environments and dust and biomass burning (BB) particles contributing substantially to AOD, especially downwind of the African continent. Contributions of dust and BB aerosols to AOD were also significant in the free troposphere over the North Pacific. Comparisons of lognormally fitted size distribution parameters to values in the Optical Properties of Aerosols and Clouds (OPAC) database commonly used in global models show significant differences in the mean diameters and standard deviations for accumulation-mode particles and coarse-mode dust. In contrast, comparisons of lognormal parameters derived from the ATom data with previously published shipborne measurements in the remote marine boundary layer show general agreement. The dataset resulting from this work can be used to improve global-scale representation of climate-relevant aerosol properties in remote air masses through comparison with output from global models and assumptions used in retrievals of aerosol properties from both ground-based and satellite remote sensing. </div

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data

Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides () and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1&ndash;1.5&thinsp;ppbv&thinsp;h&minus;1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1&thinsp;%, 87.7&thinsp;%, and 99.6&thinsp;%, respectively. Initial NO3 reactivity is 10&ndash;104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for &gt;97&thinsp;% of NO3 loss in sunlit plumes (jNO2 up to ), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11&thinsp;%&ndash;43&thinsp;%, 54&thinsp;%&ndash;88&thinsp;% for alkenes; 18&thinsp;%&ndash;55&thinsp;%, 39&thinsp;%&ndash;76&thinsp;%, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26&thinsp;%&ndash;52&thinsp;%, 22&thinsp;%&ndash;43&thinsp;%, 16&thinsp;%&ndash;33&thinsp;%, respectively). Nitrate radical oxidation accounts for 26&thinsp;%&ndash;52&thinsp;% of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33&thinsp;% and 45&thinsp;%, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72&thinsp;%&ndash;92&thinsp;% (84&thinsp;% in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56&thinsp;%&plusmn;2&thinsp;% of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13&thinsp;%&ndash;57&thinsp;%) and BBVOCs (8&thinsp;%&ndash;72&thinsp;%) at sunrise. &nbsp;</p