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

Abstract

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...

Differences in fine particle chemical composition on clear and cloudy days

“…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). Organic speciation at IMPROVE locations changes in time and space (Christiansen et al, 2020), and the suitability of applying a constant value for organic hygroscopicity is difficult to quantitatively assess.…”

“…2). We use RH from the NARR and estimate κ d , the particle hygroscopicity, from IMPROVEmeasured chemical composition mass concentrations and individual species κ values (κ SO 4 = 0.5, κ NO 3 = 0.7, κ org = 0.3) (Chang et al, 2010;Nguyen et al, 2014;Petters and Kreidenweis, 2007). Here, gf (D) is the hygroscopic diameter growth.…”

Reply on RC1

“…The liquid water content of PM2.5 is high during regionally (i.e., eastern US) and seasonally (i.e., summer) humid conditions (Carlton et al, 2020). High humidity can shift the partitioning of hydrophilic organic gases toward the coexisting inorganic salted solution than to the organic phase, enhancing SOA growth via aqueous-reactions (Carlton and Turpin, 2013).…”

Improvements to a laser-induced fluorescence instrument for measuring SO<sub>2</sub> – impact on accuracy and precision

“…Through reaction with the hydroxyl radical (OH), SO 2 produces sulfuric acid, which affects the pH of aqueous particles and leads to acid deposition. Sulfuric acid also condenses onto organic and black carbon particles, producing sulfate, which increases the aerosol hygroscopicity and influences the accumulation of aerosol liquid water (Fiedler et al, 2011;Carlton et al, 2020). Sulfuric acid is believed to be the most important source gas globally for homogeneous nucleation and growth of new aerosol particles, which may occur primarily in the tropical upper troposphere (Brock et al, 1995;Dunne et al, 2016;Williamson et al, 2019).…”

“…Sulfuric acid also condenses onto organic and black carbon particles, producing sulfate, which increases the aerosol hygroscopicity and influences the accumulation of aerosol liquid water (Fiedler et al, 2011;Carlton et al, 2020). Sulfuric acid is believed to be the most important source gas globally for homogeneous nucleation and growth of new aerosol particles, which may occur primarily in the tropical upper troposphere (Brock et al, 1995;Dunne et al, 2016;Williamson et al, 2019). SO 2 and sulfate particles can be transported long distances, driving the production of haze pollution in areas downwind of SO 2 emissions (Andreae et al, 1988).…”