Nucleation in the free troposphere (FT) and subsequent growth of new particles represent a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been ...shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes. In addition, the impact of urban emissions and biomass burning on the NPF in tropical marine FT remains poorly understood. In this study, we examine NPF in the lower and mid-troposphere (3–8.5 km) over the tropical ocean and coastal region using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). NPF was mostly observed above 5.5 km and coincided with elevated relative humidity (RH) and reduced condensation sink (CS), suggesting that NPF occurs in convective cloud outflow. The frequency of NPF increases with altitude, reaching ∼ 50 % above 8 km. An abrupt decrease in NPF frequency coincides with early monsoon transition and is attributed to increased CS resulting from reduced convective activity and more frequent transport of aged urban plumes. Surprisingly, a large fraction of NPF events in background air were observed in the early morning, and the NPF is likely made possible by very low CS despite low actinic flux. Convectively detrained biomass-burning plumes and fresh urban emissions enhance NPF as a result of elevated precursor concentrations and scavenging of pre-existing particles. In contrast, NPF is suppressed in aged urban plumes where the reactive precursors are mostly consumed, while CS remains relatively high. This study shows a strong impact of urban and biomass-burning emissions on the NPF in tropical marine FT. The results also illustrate the competing influences of different variables and interactions among anthropogenic emissions, convective clouds, and meteorology, which lead to NPF under a variety of conditions in tropical marine environments.
Mesoscale organization of marine convective clouds into linear or clustered states is prevalent across the tropical and subtropical oceans, and its investigation served as a guiding focus for a ...series of process study flights conducted as part of the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) during summer 2020, 2021, and 2022. These select ACTIVATE flights involved a novel strategy for coordinating two aircraft, with respective remote sensing and in situ sampling payloads, to probe regions of organized shallow convection for several hours. The main purpose of this measurement report is to summarize the aircraft sampling approach, describe the characteristics and evolution of the cases, and provide an overview of the datasets that can serve as a starting point for more detailed modeling and analysis studies.
Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire ...emissions: a satellite-based top-down approach and a fuels-based bottom-up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high-resolution using three separate approaches: top-down, bottom-up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high-resolution estimates and compare with estimates from lower resolution, global top-down and bottom-up inventories. We uncover strong, linear relationships between all of the high-resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high-resolution approaches and displayed evidence of systematic bias. The disparity between the low resolution global inventories and the high-resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom-up inventories and imperfect assumptions in top-down inventories.
Plain Language Summary
Smoke emitted by wildland fires is dangerous to human health and contributes to climate change.To predict and evaluate the impacts of fires, we need to know how much smoke is emitted into the atmosphere. There are two state-of-the-art methods used to estimate the mass of smoke emitted by fires, but they often disagree. In this study, we use unusually detailed measurements collected using an aircraft that flew within wildland fire smoke plumes to calculate the amount ofsmoke emitted from fires in the Western United States. We compare emission rates derived from the exceptionally high spatial and temporal resolution approach to the two traditional, lower resolution approaches to understand why they sometimes diverge
KORUS-AQ was an international cooperative air quality field study in South Korea that measured local and remote sources of air pollution affecting the Korean Peninsula during May–June 2016. Some of ...the largest aerosol mass concentrations were measured during a Chinese haze transport event (24 May). Air quality forecasts using the WRF-Chem model with aerosol optical depth (AOD) data assimilation captured AOD during this pollution episode but overpredicted surface particulate matter concentrations in South Korea, especially PM2.5, often by a factor of 2 or larger. Analysis revealed multiple sources of model deficiency related to the calculation of optical properties from aerosol mass that explain these discrepancies. Using in situ observations of aerosol size and composition as inputs to the optical properties calculations showed that using a low-resolution size bin representation (four bins) underestimates the efficiency with which aerosols scatter and absorb light (mass extinction efficiency). Besides using finer-resolution size bins (8–16 bins), it was also necessary to increase the refractive indices and hygroscopicity of select aerosol species within the range of values reported in the literature to achieve better consistency with measured values of the mass extinction efficiency (6.7 m2 g−1 observed average) and light-scattering enhancement factor (f(RH)) due to aerosol hygroscopic growth (2.2 observed average). Furthermore, an evaluation of the optical properties obtained using modeled aerosol properties revealed the inability of sectional and modal aerosol representations in WRF-Chem to properly reproduce the observed size distribution, with the models displaying a much wider accumulation mode. Other model deficiencies included an underestimate of organic aerosol density (1.0 g cm−3 in the model vs. observed average of 1.5 g cm−3) and an overprediction of the fractional contribution of submicron inorganic aerosols other than sulfate, ammonium, nitrate, chloride, and sodium corresponding to mostly dust (17 %–28 % modeled vs. 12 % estimated from observations). These results illustrate the complexity of achieving an accurate model representation of optical properties and provide potential solutions that are relevant to multiple disciplines and applications such as air quality forecasts, health impact assessments, climate projections, solar power forecasts, and aerosol data assimilation.
Glyoxal (CHOCHO), the simplest dicarbonyl in the
troposphere, is a potential precursor for secondary organic aerosol (SOA)
and brown carbon (BrC) affecting air quality and climate. The airborne
...measurement of CHOCHO concentrations during the KORUS-AQ (KORea–US Air
Quality study) campaign in 2016 enables detailed quantification of loss
mechanisms pertaining to SOA formation in the real atmosphere. The
production of this molecule was mainly from oxidation of aromatics (59 %)
initiated by hydroxyl radical (OH). CHOCHO loss to aerosol was found to be
the most important removal path (69 %) and contributed to roughly
∼ 20 % (3.7 µg sm−3 ppmv−1 h−1,
normalized with excess CO) of SOA growth in the first 6 h in Seoul
Metropolitan Area. A reactive uptake coefficient (γ) of
∼ 0.008 best represents the loss of CHOCHO by surface uptake
during the campaign. To our knowledge, we show the first field observation
of aerosol surface-area-dependent (Asurf) CHOCHO uptake, which diverges
from the simple surface uptake assumption as Asurf increases in ambient
condition. Specifically, under the low (high) aerosol loading, the CHOCHO
effective uptake rate coefficient, keff,uptake, linearly increases
(levels off) with Asurf; thus, the irreversible surface uptake is a
reasonable (unreasonable) approximation for simulating CHOCHO loss to
aerosol. Dependence on photochemical impact and changes in the chemical and
physical aerosol properties “free water”, as well as aerosol viscosity,
are discussed as other possible factors influencing CHOCHO uptake rate. Our
inferred Henry's law coefficient of CHOCHO, 7.0×108 M atm−1, is ∼ 2 orders of magnitude higher than those
estimated from salting-in effects constrained by inorganic salts only
consistent with laboratory findings that show similar high partitioning into
water-soluble organics, which urges more understanding on CHOCHO solubility
under real atmospheric conditions.
Earth's atmosphere oxidizes the greenhouse gas methane and other gases, thus determining their lifetimes and oxidation products. Much of this oxidation occurs in the remote, relatively clean free ...troposphere above the planetary boundary layer, where the oxidation chemistry is thought to be much simpler and better understood than it is in urban regions or forests. The NASA airborne Atmospheric Tomography study (ATom) was designed to produce cross sections of the detailed atmospheric composition in the remote atmosphere over the Pacific and Atlantic Oceans during four seasons. As part of the extensive ATom data set, measurements of the atmosphere's primary oxidant, hydroxyl (OH), and hydroperoxyl (HO2) are compared to a photochemical box model to test the oxidation chemistry. Generally, observed and modeled median OH and HO2 agree to within combined uncertainties at the 2σ confidence level, which is ~±40%. For some seasons, this agreement is within ~±20% below 6-km altitude. While this test finds no significant differences, OH observations increasingly exceeded modeled values at altitudes above 8 km, becoming ~35% greater, which is near the combined uncertainties. Measurement uncertainty and possible unknown measurement errors complicate tests for unknown chemistry or incorrect reaction rate coefficients that would substantially affect the OH and HO2 abundances. Future analysis of detailed comparisons may yield additional discrepancies that are masked in the median values.
Wet scavenging of aerosols by continental deep convective clouds is studied for a supercell storm complex observed over Oklahoma during the Deep Convective Clouds and Chemistry campaign. A new ...passive‐tracer‐based transport analysis framework is developed to characterize convective transport using vertical profiles of several passive trace gases. For this case, the analysis estimates that observed passive gas mixing ratios in the upper troposphere convective outflow consist of 47% low level (<3 km) inflow air, 32% entrained midtroposphere air, and 21% upper troposphere air. The new analysis framework is used to estimate aerosol wet scavenging efficiencies. Observations yield high overall scavenging efficiencies of 81% for submicron aerosol mass. Organic, sulfate, and ammonium aerosols have similar wet scavenging efficiencies (80%–84%). The apparent scavenging efficiency for nitrate aerosol is much lower (57%), but the scavenging efficiency for nitrate aerosol plus nitric acid combined (84%) is close to the other species. Scavenging efficiencies for aerosol number are high for larger particles (84% for 0.15–2.5 µm diameter) but are lower for smaller particles (64% for 0.03–0.15 µm). The storm is simulated using the chemistry version of the Weather Research and Forecasting model. Compared to the observation‐based analysis, the standard model strongly underestimates aerosol scavenging efficiencies by 32% and 41% in absolute differences for submicron mass and number. Adding a new treatment of secondary activation significantly improves simulated aerosol scavenging, producing wet scavenging efficiencies that are only 7% and 8% lower than observed efficiencies. This finding emphasizes the importance of secondary activation for aerosol wet removal in deep convective storms.
Key Points
A multipassive tracer method for analyzing convective transport and wet removal
High wet removal rates for aerosol number/mass with little chemical selectivity
Secondary activation is important for aerosol wet removal in convective storms
Accurate and consistent monitoring of anthropogenic combustion is imperative because of its significant health and environmental impacts, especially at city-to-regional scale. Here, we assess the ...performance of the Copernicus Atmosphere Monitoring Service (CAMS) global prediction system using measurements from aircraft, ground sites, and ships during the Korea-United States Air Quality (KORUS-AQ) field study in May to June 2016. Our evaluation focuses on CAMS CO and CO2 analyses as well as two higher-resolution forecasts (16 and 9 km horizontal resolution) to assess their capability in predicting combustion signatures over east Asia. Our results show a slight overestimation of CAMS CO2 with a mean bias against airborne CO2 measurements of 2.2, 0.7, and 0.3 ppmv for 16 and 9 km CO2 forecasts, and analyses, respectively. The positiveCO2 mean bias in the 16 km forecast appears to be consistent across the vertical profile of the measurements. In contrast, we find a moderate underestimation of CAMS CO with an overall bias against airborne CO measurements of -19.2 (16 km), -16.7 (9 km), and -20.7 ppbv (analysis). This negative CO mean bias is mostly seen below 750 hPa for all three forecast/analysis configurations. Despite these biases, CAMS shows a remarkable agreement with observed enhancement ratios of CO withCO2 over the Seoul metropolitan area and over the West (Yellow) Sea, where east Asian outflows were sampled during the study period. More efficient combustion is observed over Seoul (dCO/dCO2=9 ppbv ppmv-1) compared to the West Sea (dCO/dCO2=28 ppbv ppmv-1). This “combustion signature contrast” is consistent with previous studies in these two regions. CAMS captured this difference in enhancement ratios (Seoul: 8–12 ppbv ppmv-1, the West Sea: ∼30 ppbv ppmv-1) regardless of forecast/analysis configurations. The correlation of CAMS CO bias with CO2 bias is relatively high over these two regions (Seoul: 0.64–0.90, the West Sea: ∼0.80) suggesting that the contrast captured by CAMS may be dominated by anthropogenic emission ratios used in CAMS. However, CAMS shows poorer performance in terms of capturing local-to-urban CO and CO2 variability. Along with measurements at ground sites over the Korean Peninsula, CAMS produces too high CO andCO2 concentrations at the surface with steeper vertical gradients (∼0.4 ppmv hPa-1 for CO2 and 3.5 ppbv hPa-1 for CO) in the morning samples than observed (∼0.25 ppmv hPa-1 forCO2 and 1.7 ppbv hPa-1 for CO), suggesting weaker boundary layer mixing in the model. Lastly, we find that the combination of CO analyses (i.e., improved initial condition) and use of finer resolution (9 km vs. 16 km) generally produces better forecasts.
The details of aerosol processes and size distributions
in the stratosphere are important for both heterogeneous chemistry and
aerosol–radiation interactions. Using in situ, global-scale measurements ...of
the size distribution of particles with diameters > 3 nm from the
NASA Atmospheric Tomography Mission (ATom), we identify a mode of aerosol
smaller than 12 nm in the lowermost stratosphere (LMS) at mid- and high
latitudes. This mode is substantial only in the Northern Hemisphere (NH)
and was observed in all four seasons. We also observe elevated SO2, an
important precursor for new particle formation (NPF) and growth, in the NH
LMS. We use box modelling and thermodynamic calculations to show that NPF
can occur in the LMS conditions observed on ATom. Aircraft emissions are
shown as likely sources of this SO2, as well as a potential source of
nucleation mode particles directly emitted by or formed in the plume of the
engines. These nucleation mode particles have the potential to grow to
larger sizes and to coagulate with larger aerosol, affecting heterogeneous
chemistry and aerosol–radiation interactions. Understanding all sources and
characteristics of stratospheric aerosols is important in the context of
anthropogenic climate change as well as proposals for climate intervention
via stratospheric sulfur injection. This analysis not only adds to the,
currently sparse, observations of the global impact of aviation, but also
introduces another aspect of climate influence, namely a size distribution
shift of the background aerosol distribution in the LMS.
As one of the dominant sinks of aerosol particles, wet scavenging greatly influences aerosol lifetime and interactions with clouds, precipitation, and radiation. However, wet scavenging remains ...highly uncertain in models, hindering accurate predictions of aerosol spatiotemporal distributions and downstream interactions. In this study, we present a flexible, computationally inexpensive method to identify meteorological variables relevant for estimating wet scavenging using a combination of aircraft, satellite, and reanalysis data augmented by trajectory modeling to account for air mass history. We assess the capabilities of an array of meteorological variables to predict the transport efficiency of black carbon (TEBC) using a combination of nonlinear regression, curve fitting, and k-fold cross-validation. We find that accumulated precipitation along trajectories (APT) – treated as a wet scavenging indicator across multiple studies – does poorly when predicting TEBC. Among different precipitation characteristics (amount, frequency, intensity), precipitation intensity was the most effective at estimating TEBC but required longer trajectories (>48 h) and including only intensely precipitating grid cells. This points to the contribution of intense precipitation to aerosol scavenging and the importance of accounting for air mass history. Predictors that were most able to predict TEBC were related to the distribution of relative humidity (RH) or the frequency of humid conditions along trajectories, suggesting that RH is a more robust way to estimate TEBC than APT. We recommend the following alternatives to APT when estimating aerosol scavenging: (1) the 90th percentile of RH along trajectories, (2) the fraction of hours along trajectories with either water vapor mixing ratios >15 g kg−1 or RH >95 %, and (3) precipitation intensity along trajectories at least 48 h along and filtered for grid cells with precipitation >0.2 mm h−1. Future scavenging parameterizations should consider these meteorological variables along air mass histories. This method can be repeated for different regions to identify region-specific factors influencing wet scavenging.
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