Fine Particle pH and Sensitivity to NH and HNO over South Korea During KORUS-AQ.

Using a new approach that constrains thermodynamic modeling of aerosol composition with measured gas-to-particle partitioning of inorganic nitrate, we estimate the acidity levels for aerosol sampled in the South Korean planetary boundary layer during the NASA/NIER KORUS-AQ field campaign. The pH (mean ± 1σ = 2.43±0.

Carbon emissions from the 2023 Canadian wildfires.

The 2023 Canadian forest fires have been extreme in scale and intensity with more than seven times the average annual area burned compared to the previous four decades. Here, we quantify the carbon emissions from these fires from May to September 2023 on the basis of inverse modelling of satellite carbon monoxide observations. We find that the magnitude of the carbon emissions is 647 TgC (570-727 TgC), comparable to the annual fossil fuel emissions of large nations, with only India, China and the USA releasing more carbon per year.

An emergent constraint on the thermal sensitivity of photosynthesis and greenness in the high latitude northern forests.

Despite the general consensus that the warming over the high latitudes northern forests (HLNF) has led to enhanced photosynthetic activity and contributed to the greening trend, isolating the impact of temperature increase on photosynthesis and greenness has been difficult due to the concurring influence of the CO fertilization effect. Here, using an ensemble of simulations from biogeochemical models that have contributed to the Trends in Net Land Atmosphere Carbon Exchange project (TRENDY), we identify an emergent relationship between the simulation of the climate-driven temporal changes in both gross primary productivity (GPP) and greenness (Leaf Area Index, LAI) and the model's spatial sensitivity of these quantities to growing-season (GS) temperature. Combined with spatially-resolved observations of LAI and GPP, we estimate that GS-LAI and GS-GPP increase by 17.

Methane Emissions from Dairy Operations in California's San Joaquin Valley Evaluated Using Airborne Flux Measurements.

State inventories indicate that dairy operations account for nearly half of California's methane budget. Recent analyses suggest, however, that these emissions may be underestimated, complicating efforts to develop emission reduction strategies. Here, we report estimates of dairy methane emissions in the southern San Joaquin Valley (SJV) of California in June 2021 using airborne flux measurements.

Particle-phase accretion forms dimer esters in pinene secondary organic aerosol.

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized.

Atmospheric Photo-Oxidation of 2-Ethoxyethanol: Autoxidation Chemistry of Glycol Ethers.

We investigate the gas-phase photo-oxidation of 2-ethoxyethanol (2-EE) initiated by the OH radical with a focus on its autoxidation pathways. Gas-phase autoxidation─intramolecular H-shifts followed by O addition─has recently been recognized as a major atmospheric chemical pathway that leads to the formation of highly oxygenated organic molecules (HOMs), which are important precursors for secondary organic aerosols (SOAs). Here, we examine the gas-phase oxidation pathways of 2-EE, a model compound for glycol ethers, an important class of volatile organic compounds (VOCs) used in volatile chemical products (VCPs).

Elucidating climatic drivers of photosynthesis by tropical forests.

Tropical forests play a pivotal role in regulating the global carbon cycle. However, the response of these forests to changes in absorbed solar energy and water supply under the changing climate is highly uncertain. Three-year (2018-2021) spaceborne high-resolution measurements of solar-induced chlorophyll fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI) provide a new opportunity to study the response of gross primary production (GPP) and more broadly tropical forest carbon dynamics to differences in climate.

Unequal exposure to heatwaves in Los Angeles: Impact of uneven green spaces.

Cities worldwide are experiencing record-breaking summer temperatures. Urban environments exacerbate extreme heat, resulting in not only the urban heat island but also intracity variations in heat exposure. Understanding these disparities is crucial to support equitable climate mitigation and adaptation efforts.

Hydrocarbon Tracers Suggest Methane Emissions from Fossil Sources Occur Predominately Before Gas Processing and That Petroleum Plays Are a Significant Source.

We use global airborne observations of propane (CH) and ethane (CH) from the Atmospheric Tomography (ATom) and HIAPER Pole-to-Pole Observations (HIPPO), as well as U.S.-based aircraft and tower observations by NOAA and from the NCAR FRAPPE campaign as tracers for emissions from oil and gas operations.

Hydrotrioxide (ROOOH) formation in the atmosphere.

Organic hydrotrioxides (ROOOH) are known to be strong oxidants used in organic synthesis. Previously, it has been speculated that they are formed in the atmosphere through the gas-phase reaction of organic peroxy radicals (RO) with hydroxyl radicals (OH). Here, we report direct observation of ROOOH formation from several atmospherically relevant RO radicals.

Boundary layer versus free tropospheric submicron particle formation: A case study from NASA DC-8 observations in the Asian continental outflow during the KORUS-AQ campaign.

In this study, we contrasted major secondary inorganic species and processes responsible for submicron particle formation (SPF) events in the boundary layer (BL) and free troposphere (FT) over the Korean Peninsula during Korea-United States Air Quality (KORUS-AQ) campaign (May-June, 2016) using aircraft observations. The number concentration of ultrafine particles with diameters between 3 nm and 10 nm (N) during the entire KORUS-AQ period reached a peak (7,606 ± 12,003 cm ) at below 1 km altitude, implying that the particle formation around the Korean Peninsula primarily occurred in the daytime BL. During the BL SPF case (7 May, 2016), the SPF over Seoul metropolitan area was more attributable to oxidation of NO rather than SO-to-sulfate conversion.

Ozone chemistry in western U.S. wildfire plumes.

Wildfires are a substantial but poorly quantified source of tropospheric ozone (O). Here, to investigate the highly variable O chemistry in wildfire plumes, we exploit the in situ chemical characterization of western wildfires during the FIREX-AQ flight campaign and show that O production can be predicted as a function of experimentally constrained OH exposure, volatile organic compound (VOC) reactivity, and the fate of peroxy radicals. The O chemistry exhibits rapid transition in chemical regimes.

Unimolecular Reactions Following Indoor and Outdoor Limonene Ozonolysis.

Limonene is one of the monoterpenes with the largest biogenic emissions and is also widely used as an additive in cleaning products, leading to significant indoor emissions. Studies have found that the formation of secondary organic aerosols (SOAs) from limonene oxidation has important implications for indoor air quality. Although ozonolysis is considered the major limonene oxidation pathway under most indoor conditions, little is known about the mechanisms for SOA formation from limonene ozonolysis.

Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NO from the atmosphere.

The formation of a suite of isoprene-derived hydroxy nitrate (IHN) isomers during the OH-initiated oxidation of isoprene affects both the concentration and distribution of nitrogen oxide free radicals (NO). Experiments performed in an atmospheric simulation chamber suggest that the lifetime of the most abundant isomer, 1,2-IHN, is shortened significantly by a water-mediated process (leading to nitric acid formation), while the lifetime of a similar isomer, 4,3-IHN, is not. Consistent with these chamber studies, NMR kinetic experiments constrain the 1,2-IHN hydrolysis lifetime to less than 10 s in deuterium oxide (DO) at 298 K, whereas the 4,3-IHN isomer has been observed to hydrolyze much less efficiently.

New Insights into the Radical Chemistry and Product Distribution in the OH-Initiated Oxidation of Benzene.

Emissions of aromatic compounds cause air pollution and detrimental health effects. Here, we explore the reaction kinetics and products of key radicals in benzene photo-oxidation. After initial OH addition and reaction with O, the effective production rates of phenol and bicyclic peroxy radical (BCP-peroxy) are experimentally constrained at 295 K to be 420 ± 80 and 370 ± 70 s, respectively.

Stereoselectivity in Atmospheric Autoxidation.

We show that the diastereomers of hydroxy peroxy radicals formed from OH and O addition to C2 and C3, respectively, of crotonaldehyde (CHCHCHCHO) undergo gas-phase unimolecular aldehydic hydrogen shift (H-shift) chemistry with rate coefficients that differ by an order of magnitude. The stereospecificity observed here for crotonaldehyde is general and will lead to a significant diastereomeric-specific chemistry in the atmosphere. This enhancement of specific stereoisomers by stereoselective gas-phase reactions could have widespread implications given the ubiquity of chirality in nature.

Atmospheric Acetaldehyde: Importance of Air-Sea Exchange and a Missing Source in the Remote Troposphere.

We report airborne measurements of acetaldehyde (CHCHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CHCHO is examined using the Community Atmospheric Model with chemistry (CAM-chem), with a newly-developed online air-sea exchange module. The upper limit of the global ocean net emission of CHCHO is estimated to be 34 Tg a (42 Tg a if considering bubble-mediated transfer), and the ocean impacts on tropospheric CHCHO are mostly confined to the marine boundary layer.

Mapping hydroxyl variability throughout the global remote troposphere via synthesis of airborne and satellite formaldehyde observations.

The hydroxyl radical (OH) fuels tropospheric ozone production and governs the lifetime of methane and many other gases. Existing methods to quantify global OH are limited to annual and global-to-hemispheric averages. Finer resolution is essential for isolating model deficiencies and building process-level understanding.

Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-Pinene and β-Pinene by Hydroxyl Radicals.

Atmospheric oxidation of monoterpenes (emitted primarily by evergreen trees) is known to contribute to the formation and growth of aerosol particles. While recent research has tied the formation of organic aerosol to unimolecular chemistry of the organic peroxy radicals (RO) formed in the oxidation of monoterpenes, the fundamental physical chemistry of these RO remains obscure. Here we use isomer-specific measurements and ab initio calculations to determine the unimolecular reaction rates and products of RO derived from the hydroxyl radical (OH) oxidation of α-pinene and β-pinene.

Intramolecular Hydrogen Shift Chemistry of Hydroperoxy-Substituted Peroxy Radicals.

Gas-phase autoxidation - the sequential regeneration of peroxy radicals (RO) via intramolecular hydrogen shifts (H-shifts) followed by oxygen addition - leads to the formation of organic hydroperoxides. The atmospheric fate of these peroxides remains unclear, including the potential for further H-shift chemistry. Here, we report H-shift rate coefficients for a system of RO with hydroperoxide functionality produced in the OH-initiated oxidation of 2-hydroperoxy-2-methylpentane.