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Large uncertainties exist in global climate model predictions of radiative forcing due to insufficient understanding and simplified numerical representation of cloud formation and cloud–aerosol interactions. The Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) campaign was conducted near the DOE's Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site in north-central Oklahoma to provide a better understanding of land–atmosphere interactions, aerosol and cloud properties, and the influence of aerosol and land–atmosphere interactions on cloud formation. The HI-SCALE campaign consisted of two intensive observational periods (IOPs) (April–May and August–September, 2016), during which coincident measurements were conducted both on the G-1 aircraft platform and at the SGP ground site. In this study we focus on the observations at the SGP ground site. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and an Ionicon proton-transfer-reaction mass spectrometer (PTR-MS) were deployed, characterizing chemistry of non-refractory aerosol and trace gases, respectively. Contributions from various aerosol sources, including biogenic and biomass burning emissions, were retrieved using factor analysis of the AMS data. In general, the organic aerosols at the SGP site was highly oxidized, with oxygenated organic aerosol (OOA) identified as the dominant factor for both the spring and summer IOP though more aged in spring. Cases of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX SOA) and biomass burning events were further investigated to understand additional sources of organic aerosol. Unlike other regions largely impacted by IEPOX chemistry, the IEPOX SOA at SGP was more highly oxygenated, likely due to the relatively weak local emissions of isoprene. Biogenic emissions appear to largely control the formation of organic aerosol (OA) during the HI-SCALE campaign. Potential HOM (highly oxygenated molecule) chemistry likely contributes to the highly oxygenated feature of aerosols at the SGP site, with impacts on new particle formation and global climate.

Shallow convective clouds are common, occurring over many areas of the world, and are an important component in the atmospheric radiation budget. In addition to synoptic and mesoscale meteorological conditions, land–atmosphere interactions and aerosol–radiation–cloud interactions can influence the formation of shallow clouds and their properties. These processes exhibit large spatial and temporal variability and occur at the subgrid scale for all current climate, operational forecast, and cloud-system-resolving models; therefore, they must be represented by parameterizations. Uncertainties in shallow cloud parameterization predictions arise from many sources, including insufficient coincident data needed to adequately represent the coupling of cloud macrophysical and microphysical properties with inhomogeneity in the surface-layer, boundary layer, and aerosol properties. Predictions of the transition of shallow to deep convection and the onset of precipitation are also affected by errors in simulated shallow clouds. Coincident data are a key factor needed to achieve a more complete understanding of the life cycle of shallow convective clouds and to develop improved model parameterizations. To address these issues, the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) campaign was conducted near the Atmospheric Radiation Measurement (ARM) Southern Great Plains site in north-central Oklahoma during the spring and summer of 2016. We describe the scientific objectives of HI-SCALE as well as the experimental approach, overall weather conditions during the campaign, and preliminary findings from the measurements. Finally, we discuss scientific gaps in our understanding of shallow clouds that can be addressed by analysis and modeling studies that use HI-SCALE data.

Carbonaceous aerosols affect the global radiative balance by absorbing and scattering radiation, which leads to warming or cooling of the atmosphere, respectively. Black carbon is the main light-absorbing component. A portion of the organic aerosol known as brown carbon also absorbs light. The climate sensitivity to absorbing aerosols rapidly increases with altitude, but brown carbon measurements are limited in the upper troposphere. Here we present aircraft observations of vertical aerosol distributions over the continental United States in May and June 2012 to show that light-absorbing brown carbon is prevalent in the troposphere, and absorbs more short-wavelength radiation than black carbon at altitudes between 5 and 12 km. We find that brown carbon is transported to these altitudes by deep convection, and that in-cloud heterogeneous processing may produce brown carbon. Radiative transfer calculations suggest that brown carbon accounts for about 24% of combined black and brown carbon warming effect at the tropopause. Roughly two-thirds of the estimated brown carbon forcing occurs above 5 km, although most brown carbon is found below 5 km. The highest radiative absorption occurred during an event that ingested a wildfire plume. We conclude that high-altitude brown carbon from biomass burning is an unappreciated component of climate forcing. Brown carbon absorbs light, but its climate impacts in the upper troposphere are not well known. A series of aircraft observations in the US reveals that convection lofts brown carbon to high altitudes, causing greater warming than at lower altitudes.

Organic aerosol (OA) constitutes a significant fraction of atmospheric fine particle mass. However, the precursors and chemical processes responsible for a majority of OA are rarely conclusively identified. We use online observations of hundreds of simultaneously measured molecular components obtained from 15 laboratory OA formation experiments with constraints on their effective saturation vapor concentrations to attribute the VOC precursors and subsequent chemical pathways giving rise to the vast majority of OA mass measured in two forested regions. We find that precursors and chemical pathways regulating OA composition and volatility are dynamic over hours to days, with their variations driven by coupled interactions between multiple oxidants. The extent of physical and photochemical aging, and its modulation by NO(x), were key to a uniquely comprehensive combined composition-volatility description of OA. Our findings thus provide some of the most complete mechanistic-level guidance to the development of OA descriptions in air quality and Earth system models.

Agriculture-oriented cities in Northeastern China have experienced frequent atmospheric pollution events. Deeper understandings of the pollution characteristics, haze causes and effects of management on local air quality are crucial for conducting integrated management approaches for the sustainable development of agriculture-oriented cities. Taking a typical agriculture-dominant city (i.e., Suihua) in Northeast China, we analyzed in detail the characteristics and causes of atmospheric pollution and evaluated the straw-burning prohibition using multisource data. The results showed a clear temporal pattern of air quality index (AQI) on an annual scale (i.e., 2015-April 2019), with two typical pollution periods occurring in late autumn and early spring. The large areas of concentrated straw burning at local and regional scales accounted for the first period (i.e., October and November), while dust emissions and farming disturbances comprised the second period. The interannual variation in pollution periods among these years was large, showing similar trends from 2015 to 2017 and the postponed late-autumn pollution period in 2018. Our evaluation has shown that the prohibition effect of straw burning significantly improved air quality in 2018, with a reduction of 59% ± 88% in the PM2.5 concentrations in October and November compared to 2015-2017. However, From October to April of the following year, the improvement effect was not significant due to postponement of straw burning to February or March. Our analysis also highlighted the roles of meteorological conditions, Therefore, combined with the promotion of straw utilization, scientifically prescribed burning considering the burning amount and location, meteorological conditions and regional transportation should be implemented.

We present measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation in an environmental simulation chamber at a variety of oxidant conditions and using dry neutral seed particles to suppress acid-catalyzed multiphase chemistry. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) utilizing iodide-adduct ionization coupled to the Filter Inlet for Gases and Aerosols (FIGAERO) allowed for simultaneous online sampling of the gas and particle composition. Under high-HO2 and low-NO conditions, highly oxygenated (O : C ≥ 1) C5 compounds were major components (∼ 50 %) of SOA. The SOA composition and effective volatility evolved both as a function of time and as a function of input NO concentrations. Organic nitrates increased in both the gas and particle phases as input NO increased, but the dominant non-nitrate particle-phase components monotonically decreased. We use comparisons of measured and predicted gas-particle partitioning of individual components to assess the validity of literature-based group-contribution methods for estimating saturation vapor concentrations. While there is evidence for equilibrium partitioning being achieved on the chamber residence timescale (5.2 h) for some individual components, significant errors in group-contribution methods are revealed. In addition, > 30 % of the SOA mass, detected as low-molecular-weight semivolatile compounds, cannot be reconciled with equilibrium partitioning. These compounds desorb from the FIGAERO at unexpectedly high temperatures given their molecular composition, which is indicative of thermal decomposition of effectively lower-volatility components such as larger molecular weight oligomers.

The light-absorbing organic aerosol (OA) commonly referred to as “brown carbon” (BrC) has attracted considerable attention in recent years because of its potential to affect atmospheric radiation balance, especially in the ultraviolet region and thus impact photochemical processes. A growing amount of data has indicated that BrC is prevalent in the atmosphere, which has motivated numerous laboratory and field studies; however, our understanding of the relationship between the chemical composition and optical properties of BrC remains limited. We conducted chamber experiments to investigate the effect of various volatile organic carbon (VOC) precursors, NOx concentrations, photolysis time, and relative humidity (RH) on the light absorption of selected secondary organic aerosols (SOA). Light absorption of chamber-generated SOA samples, especially aromatic SOA, was found to increase with NOx concentration, at moderate RH, and for the shortest photolysis aging times. The highest mass absorption coefficient (MAC) value is observed from toluene SOA products formed under high-NOx conditions at moderate RH, in which nitro-aromatics were previously identified as the major light-absorbing compounds. BrC light absorption is observed to decrease with photolysis time, correlated with a decline of the organic nitrate fraction of SOA. SOA formed from mixtures of aromatics and isoprene absorb less visible (Vis) and ultraviolet (UV) light than SOA formed from aromatic precursors alone on a mass basis. However, the mixed SOA absorption was underestimated when optical properties were predicted using a two-product SOA formation model, as done in many current climate models. Further investigation, including analysis on detailed mechanisms, are required to explain the discrepancy.

Brown carbon (i.e., light-absorbing organic carbon, or BrC) exerts important effects on the environment and on climate in particular. Based on spectrophotometric absorption measurements on extracts of bulk aerosol samples, this study investigated the characteristics of BrC during winter in Beijing, China. Organic compounds extractable by methanol contributed approximately 85% to the organic carbon (OC) mass. Light absorption by the methanol extracts exhibited a strong wavelength dependence, with an average absorption Angstrom exponent of 7.10 (fitted between 310 and 450 nm). Normalizing the absorption coefficient ( b abs ) measured at 365 nm to the extractable OC mass yielded an average mass absorption efficiency (MAE) of 1.45 m 2 /g for the methanol extracts. This study suggests that light absorption by BrC could be comparable with black carbon in the spectral range of near-ultraviolet light. Our results also indicate that BrC absorption and thus BrC radiative forcing could be largely underestimated when using water-soluble organic carbon (WSOC) as a surrogate for BrC. Compared to previous work relying only on WSOC, this study provides a more comprehensive understanding of BrC aerosol based on methanol extraction.

Brown carbon (BrC) is increasingly included in climate models as an emerging category of particulate organic compounds that can absorb solar radiation efficiently at specific wavelengths. Water-soluble organic carbon (WSOC) has been commonly used as a surrogate for BrC; however, it only represents a limited fraction of total organic carbon (OC) mass, which could be as low as about 20% in urban atmosphere. Using methanol as the extraction solvent, up to approximately 90% of the OC in Beijing aerosol was isolated and measured for absorption spectra over the ultraviolet-to-visible wavelength range. Compared to methanol-soluble OC (MSOC), WSOC underestimated BrC absorption by about 50% at 365nm. The mass absorption efficiencies measured for BrC in Beijing aerosol were converted to the imaginary refractive indices of BrC and subsequently used to compute BrC coating-induced enhancement of light absorption (Eabs) by black carbon. Eabs attributed to lensing was reduced in the case of BrC coating relative to that caused by purely-scattering coating. However, this reduction was overwhelmed by the effect of BrC shell absorption, indicating that the overall effect of BrC coating was an increase in Eabs. Methanol extraction significantly reduced charring of OC during thermal-optical analysis, leading to a large increase in the measured elemental carbon (EC) mass and an apparent improvement in the consistency of EC measurements by different thermal-optical methods.

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene-and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (similar to 2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.

pH is a fundamental aerosol property that affects ambient particle concentration and composition, linking pH to all aerosol environmental impacts. Here, PM1 and PM2. 5 pH are calculated based on data from measurements during the California Research at the Nexus of Air Quality and Climate Change (CalNex) study from 15 May to 15 June 2010 in Pasadena, CA. Particle pH and water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to measured gas–particle partitioning of inorganic nitrate, ammonium, and chloride. The study mean ± standard deviation PM1 pH was 1.9 ± 0.5 for the SO42−–NO3−–NH4+–HNO3–NH3 system. For PM2. 5, internal mixing of sea salt components (SO42−–NO3−–NH4+–Na+–Cl−–K+–HNO3–NH3–HCl system) raised the bulk pH to 2.7 ± 0.3 and improved predicted nitric acid partitioning with PM2. 5 components. The results show little effect of sea salt on PM1 pH, but significant effects on PM2. 5 pH. A mean PM1 pH of 1.9 at Pasadena was approximately one unit higher than what we have reported in the southeastern US, despite similar temperature, relative humidity, and sulfate ranges, and is due to higher total nitrate concentrations (nitric acid plus nitrate) relative to sulfate, a situation where particle water is affected by semi-volatile nitrate concentrations. Under these conditions nitric acid partitioning can further promote nitrate formation by increasing aerosol water, which raises pH by dilution, further increasing nitric acid partitioning and resulting in a significant increase in fine particle nitrate and pH. This study provides insights into the complex interactions between particle pH and nitrate in a summertime coastal environment and a contrast to recently reported pH in the eastern US in summer and winter and the eastern Mediterranean. All studies have consistently found highly acidic PM1 with pH generally below 3.