Changing Nature of Organic Carbon over the United States

Christiansen, Amy; Carlton, Annmarie G.; Porter, William C.

doi:10.1021/acs.est.0c02225

Cited by 16 publications

(21 citation statements)

References 93 publications

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“…Modeling and field experiments suggest that organic chemistry in ALW is a dominant SOA formation pathway in the eastern U.S. 1−3,15−18 and imparts a determining impact in western arid regions of the U.S. as well. 19 An aqSOA mechanism makes it possible for sulfate and nitrate containing particles to grow in organic mass under conditions of elevated relative humidity (RH). Inorganic salts can react directly to form S-and N-containing species 20,21 or alter the partitioning and volatility of organic compounds at atmospherically relevant conditions.…”

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confidence: 99%

Multiphase Atmospheric Chemistry in Liquid Water: Impacts and Controllability of Organic Aerosol

et al. 2020

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Conspectus Liquid water is a dominant and critical tropospheric constituent. Over polluted land masses low level cumulus clouds interact with boundary layer aerosol. The planetary boundary layer (PBL) is the lowest atmospheric layer and is directly influenced by Earth’s surface. Water–aerosol interactions are critical to processes that govern the fate and transport of trace species in the Earth system and their impacts on air quality, radiative forcing, and regional hydrological cycling. In the PBL, air parcels rise adiabatically from the surface, and anthropogenically influenced hygroscopic aerosols take up water and serve as cloud condensation nuclei (CCN) to form clouds. Water-soluble gases partition to liquid water in wet aerosols and cloud droplets and undergo aqueous-phase photochemistry. Most cloud droplets evaporate, and low volatility material formed during aqueous phase chemistry remains in the condensed phase and adds to aerosol mass. The resulting cloud-processed aerosol has different physicochemical properties compared to the original CCN. Organic species that undergo multiphase chemistry in atmospheric liquid water transform gases to highly concentrated, nonideal ionic aqueous solutions and form secondary organic aerosol (SOA). In recent years, SOA formation modulated by atmospheric waters has received considerable interest. Key uncertainties are related to the chemical nature of hygroscopic aerosols that become CCN and their interaction with organic species. Gas-to-droplet or gas-to-aqueous aerosol partitioning of organic compounds is affected by the intrinsic chemical properties of the organic species in addition to the pre-existing condensed phase. Environmentally relevant conditions for atmospheric aerosol are nonideal. Salt identity and concentration, in addition to aerosol phase state, can dramatically affect organic gas miscibility for many compounds, in particular when ionic strength and salt molality are outside the bounds of limiting laws. For example, Henry’s law and Debye–Hückel theory are valid only for dilute aqueous systems uncharacteristic of real atmospheric conditions. Chemical theory is incomplete, and at ambient conditions, this chemistry plays a determining role in total aerosol mass and particle size, controlling factors for air quality and climate-relevant aerosol properties. Accurate predictive skill to understand the impacts of societal choices and policies on air quality and climate requires that models contain correct chemical mechanisms and appropriate feedbacks. Globally, SOA is a dominant contributor to the atmospheric organic aerosol burden, and most mass can be traced back to precursor gas-phase volatile organic compounds (VOCs) emitted from the biosphere. However, organic aerosol concentrations in the Amazon Rainforest, the largest emitter of biogenic VOCs, are generally lower than in U.S. national parks. The Interagency Monitoring of Protected Visual Environments (IMPROVE) air quality network, with sites located predominantly in national parks, provides the longest...

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Multiphase Atmospheric Chemistry in Liquid Water: Impacts and Controllability of Organic Aerosol

et al. 2020

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“…IMPROVE measurements are 24 h samples. Further, we analyze surface measurements only and do not address aloft extinction, which can be substantial at IM-PROVE sites (Christiansen et al, 2019). The mismatch in timing and poorly captured vertical and diurnal variations in both clouds and particle composition add uncertainty to this analysis and represent a limitation.…”

Section: Methodsmentioning

confidence: 99%

“…Organic hygroscopicity values are uncertain (Metzger et al, 2018;Nguyen et al, 2015), and the magnitude of water uptake varies by location (Jathar et al, 2016) but is typically less than the contribution from inorganic constituents at IM-PROVE sites (Christiansen et al, 2019) and surface locations globally (Nguyen et al, 2016b). We provide an estimate of organic ALW using a relevant hygroscopicity value for rural aerosol of 0.3 (Chang et al, 2010;Nguyen et al, 2014).…”

Section: Methodsmentioning

confidence: 99%

“…Here, the water activity (a w ) is assumed to be equivalent to RH, V o and V w,o are the volumes of organic matter and water from organic species, respectively, and κ org is the organic hygroscopicity parameter. V o is determined by dividing organic mass (OM) by 1.4 g cm −3 (Christiansen et al, 2019). OM is calculated from IMPROVE-measured OC with siteand time-specific OM : OC ratios, which are estimated via a mass balance method, as described in Malm et al (2020) and Christiansen et al (2020).…”

Section: Methodsmentioning

confidence: 99%

“…Less attention has been given to clear-sky and cloudy differences in PM 2.5 chemical composition, especially with regards to particle hygroscopicity and water uptake. Aerosol mass concentrations and chemical speciation including aerosol liquid water (ALW) influence AOT (Christiansen et al, 2019;Malm et al, 1994;Nguyen et al, 2016a;Pitchford et al, 2007), cloud microphysics, and mesoscale convective systems (Kawecki and Steiner, 2018), including storm morphology and precipitation patterns (Kawecki et al, 2016). ALW provides a plausible contribution to reconcile surface PM 2.5 and remotely sensed AOT (Babila et al, 2020;Nguyen et al, 2016a).…”

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Differences in fine particle chemical composition on clear and cloudy days

2020

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Abstract. Clouds are prevalent and alter fine particulate matter (PM2.5) mass and chemical composition. Cloud-affected satellite retrievals are subject to higher uncertainty and are often removed from data products, hindering quantitative estimates of tropospheric chemical composition during cloudy times. We examine surface PM2.5 chemical constituent concentrations in the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network in the United States during cloudy and clear-sky times defined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags from 2010 to 2014 with a focus on differences in particle species that affect hygroscopicity and aerosol liquid water (ALW). Cloudy and clear-sky periods exhibit significant differences in PM2.5 mass and chemical composition that vary regionally and seasonally. In the eastern US, relative humidity alone cannot explain differences in ALW, suggesting that emissions and in situ chemistry related to anthropogenic sources exert determining impacts. An implicit clear-sky bias may hinder efforts to quantitatively understand and improve representation of aerosol–cloud interactions, which remain dominant uncertainties in models.

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