Cloud Processing of Gases and Aerosols in Air Quality Modeling

Nature and sources of ionic species in rainwater during monsoon periods in and around sixteenth-seventeenth century CE monuments in Yamuna River basin, India

The nature and sources of ionic species were studied in the monsoon rainwater collected from two monuments of the sixteenth-seventeenth century CE in the Yamuna River basin from 2016 to 2018. The results showed the acidic pH of the rainwater with high dissolved SO₄^-2 and NO₃^-, and soil-derived components (Ca⁺², Mg⁺², and K⁺). The anionic (SO₄^-2, NO₃^-, Cl^-, F^-, and HCO₃^-) and cationic (Ca⁺², Mg⁺², K⁺, NH₄⁺, and Na⁺) concentrations showed regional differences in yearly contribution mainly from the fossil fuel combustion, soil dust, and farm residue burning. The rainwater analysis showed low dissolved ions at SCTK (Sheikh Chilli's Tomb, Kurukshetra) compared to KBMP (Kabuli Bagh Mosque, Panipat). The mean concentration of SO₄^-2 was 1.5 times higher than the NO₃^- apportioning the sulfate as a dominant acidifying constituent in rainwater. Pearson's correlation and principal component analysis (PCA) showed terrestrial and marine origins of dissolved ions in the rainwater. The Na-normalized molar ratios and the analysis of sea salt and non-sea salt fractions indicate the dominance of non-marine contributions in the precipitation. Based on neutralization factors, cations showed neutralization of rainwater acidity as follows: NF_Ca⁺² > NF_Mg⁺² > NF_NH4⁺ > NF_K⁺. The potential index showed the dominance of the neutralization potential (NP) on acidic potential (AP) at both locations.

Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy

Fresh soot particles are generally hydrophobic, however, particle hydrophilicity can be increased through atmospheric aging processes. At present little is known on how particle chemical composition and hydrophilicity change upon atmospheric aging and associated uncertainties governing the ice cloud formation potential of soot. Here we sampled two propane flame soots referred to as brown and black soot, characterized as organic carbon rich and poor, respectively. We investigated how the ice nucleation activity of these particles changed through aging in water and aqueous acidic solutions, using a continuous flow diffusion chamber operated at cirrus cloud temperatures (T ≤ 233 K). Single aggregates of both unaged and aged soot were chemically characterized by scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM/NEXAFS) measurements. Particle wettability was determined through water sorption measurements. Unaged black and brown soot particles exhibited significantly different ice nucleation activities. Our experiments revealed significantly enhanced ice nucleation activity of the aged soot particles compared to the fresh samples, lowering the required relative humidities at which ice formation can take place at T = 218 K by up to 15% with respect to water (ΔRHi ≈ 25%). We observed an enhanced water uptake capacity for the aged compared to the unaged samples, which was more pronounced for the black soot. From these measurements we concluded that there is a change in ice nucleation mechanism when aging brown soot. Comparison of the NEXAFS spectra of unaged soot samples revealed a unique spectral feature around 287.5 eV in the case of black soot that was absent for the brown soot, indicative of carbon with hydroxyl functionalities. Comparison of the NEXAFS spectra of unaged and aged soot particles indicates changes in organic functional groups, and the aged spectra were found to be largely similar across soot types, with the exception of the water aged brown soot. Overall, we conclude that atmospheric aging is important to representatively assess the ice cloud formation activity of soot particles.

To what extent can the below-cloud washout effect influence the PM2.5? A combined observational and modeling study

The below-cloud washout (BCW) effect on PM_2.5 concentration during periods of rain is still a subject of debate. Existing BCW schemes for PM_2.5 have large deficiencies that influence its simulation in 3D chemical transport models (CTMs). In this study, a 7-year dataset with high temporal resolution (in minutes) sampled from a pristine rural site is used to calculate the BCW coefficient during the rain events. The data used for the BCW coefficient calculation cover a wide range of rain intensity from 2 mm h^-1 to 60 mm h^-1. The BCW coefficient linearly correlates with the rain intensity, with a correlation coefficient of 0.82. The coefficient has a magnitude of 10^-5 to 10^-4 s^-1 when the rain intensity ranges from 1 to 40 mm h^-1. After implementing the updated BCW scheme into the Comprehensive Air Quality Model with Extensions (CAMx) model, the performance of PM_2.5 simulation improves for the two months of heavy rain. Apart from the CAMx model, our scheme can be easily implemented into other 3D CTMs to improve PM_2.5 simulation during rainy days. The BCW effect can clean around 10-40% of the PM_2.5 over our study region, which can help to reduce the PM_2.5 exposure level for residents, and the health burdens caused by this pollutant can thus be reduced. Rainmaking is a potential way to decrease PM_2.5 concentration, but it cannot be the key method to reduce the PM_2.5 level to the standard during episodic cases (e.g., >200 μg/m³).

Characteristic Vertical Profiles of Cloud Water Composition in Marine Stratocumulus Clouds and Relationships With Precipitation

This study uses airborne cloud water composition measurements to characterize the vertical structure of air-equivalent mass concentrations of water-soluble species in marine stratocumulus clouds off the California coast. A total of 385 cloud water samples were collected in the months of July and August between 2011 and 2016 and analyzed for water-soluble ionic and elemental composition. Three characteristic profiles emerge: (i) a reduction of concentration with in-cloud altitude for particulate species directly emitted from sources below cloud without in-cloud sources (e.g., Cl^- and Na⁺), (ii) an increase of concentration with in-cloud altitude (e.g., NO₂ ^- and formate), and (iii) species exhibiting a peak in concentration in the middle of cloud (e.g., non-sea-salt SO₄ ^2-, NO₃ ^-, and organic acids). Vertical profiles of rainout parameters such as loss frequency, lifetime, and change in concentration with respect to time show that the scavenging efficiency throughout the cloud depth depends strongly on the thickness of the cloud. Thin clouds exhibit a greater scavenging loss frequency at cloud top, while thick clouds have a greater scavenging loss frequency at cloud base. The implications of these results for treatment of wet scavenging in models are discussed.

Evaluation and Intercomparison of Five North American Dry Deposition Algorithms at a Mixed Forest Site

To quantify differences between dry deposition algorithms commonly used in North America, five models were selected to calculate dry deposition velocity (V _d) for O₃ and SO₂ over a temperate mixed forest in southern Ontario, Canada, where a 5-year flux database had previously been developed. The models performed better in summer than in winter with correlation coefficients for hourly V _d between models and measurements being approximately 0.6 and 0.3, respectively. Differences in mean V _d values between models were on the order of a factor of 2 in both summer and winter. All models produced lower V _d values than the measurements of O₃ in summer and SO₂ in summer and winter, although the measured V _d may be biased. There was not a consistent tendency in the models to overpredict or underpredict for O₃ in winter. Several models produced magnitudes of the diel variation of V _d (O₃) comparable to the measurements, while all models produced slightly smaller diel variations than the measurements of V _d (SO₂) in summer. A few models produced larger diel variations than the measurements of V _d for O₃ and SO₂ in winter. Model differences were mainly due to different surface resistance parameterizations for stomatal and nonstomatal uptake pathways, while differences in aerodynamic and quasi-laminar resistances played only a minor role. It is recommended to use ensemble modeling results for ecosystem impact assessment studies, which provides mean values of all the used models and thus can avoid too much overestimations or underestimations.

A framework for expanding aqueous chemistry in the Community Multiscale Air Quality (CMAQ) model version 5.1

This paper describes the development and implementation of an extendable aqueous-phase chemistry option (AQCHEM -KMT(I)) for the Community Multiscale Air Quality (CMAQ) modeling system, version 5.1. Here, the Kinetic PreProcessor (KPP), version 2.2.3, is used to generate a Rosenbrock solver (Rodas3) to integrate the stiff system of ordinary differential equations (ODEs) that describe the mass transfer, chemical kinetics, and scavenging processes of CMAQ clouds. CMAQ's standard cloud chemistry module (AQCHEM) is structurally limited to the treatment of a simple chemical mechanism. This work advances our ability to test and implement more sophisticated aqueous chemical mechanisms in CMAQ and further investigate the impacts of microphysical parameters on cloud chemistry. Box model cloud chemistry simulations were performed to choose efficient solver and tolerance settings, evaluate the implementation of the KPP solver, and assess the direct impacts of alternative solver and kinetic mass transfer on predicted concentrations for a range of scenarios. Month-long CMAQ simulations for winter and summer periods over the US reveal the changes in model predictions due to these cloud module updates within the full chemical transport model. While monthly average CMAQ predictions are not drastically altered between AQCHEM and AQCHEM-KMT, hourly concentration differences can be significant. With added in-cloud secondary organic aerosol (SOA) formation from biogenic epoxides (AQCHEM-KMTI), normalized mean error and bias statistics are slightly improved for 2-methyltetrols and 2-methylglyceric acid at the Research Triangle Park measurement site in North Carolina during the Southern Oxidant and Aerosol Study (SOAS) period. The added in-cloud chemistry leads to a monthly average increase of 11-18 % in "cloud" SOA at the surface in the eastern United States for June 2013.

Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen

Vegetation emits a class of oxygenated hydrocarbons--the green leaf volatiles (GLVs)--under stress or damage. Under foggy conditions GLVs might be a source of secondary organic aerosol (SOA) via aqueous reactions with hydroxyl radical (OH), singlet oxygen ((1)O2*), and excited triplet states ((3)C*). To examine this, we determined the aqueous kinetics and SOA mass yields for reactions of (3)C* and (1)O2* with five GLVs: methyl jasmonate (MeJa), methyl salicylate (MeSa), cis-3-hexenyl acetate (HxAc), cis-3-hexen-1-ol (HxO), and 2-methyl-3-butene-2-ol (MBO). Second-order rate constants with (3)C* and (1)O2* range from (0.13-22) × 10(8) M(-1) s(-1) and (8.2-60) × 10(5) M(-1) s(-1) at 298 K, respectively. Rate constants with (3)C* are independent of temperature, while values with (1)O2* show significant temperature dependence (Ea = 20-96 kJ mol(-1)). Aqueous SOA mass yields for oxidation by (3)C* are (84 ± 7)%, (80 ± 9)%, and (38 ± 18)%, for MeJa, MeSa, and HxAc, respectively; we did not measure yields for other conditions because of slow kinetics. The aqueous production of SOA from GLVs is dominated by (3)C* and OH reactions, which form low volatility products at a rate that is approximately half that from the parallel gas-phase reactions of GLVs.