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RO 2 + OH reactions at atmospheric conditions lead to widespread levels of the previously omitted strong oxidizing agent ROOOH. 

Abstract. Acidity, defined as pH, is a central component of aqueouschemistry. In the atmosphere, the acidity of condensed phases (aerosolparticles, cloud water, and fog droplets) governs the phase partitioning ofsemivolatile gases such as HNO3, NH3, HCl, and organic acids andbases as well as chemical reaction rates. It has implications for theatmospheric lifetime of pollutants, deposition, and human health. Despiteits fundamental role in atmospheric processes, only recently has this fieldseen a growth in the number of studies on particle acidity. Even with thisgrowth, many fine-particle pH estimates must be based on thermodynamic modelcalculations since no operational techniques exist for direct measurements.Current information indicates acidic fine particles are ubiquitous, butobservationally constrained pH estimates are limited in spatial and temporalcoverage. Clouds and fogs are also generally acidic, but to a lesser degreethan particles, and have a range of pH that is quite sensitive toanthropogenic emissions of sulfur and nitrogen oxides, as well as ambientammonia. Historical measurements indicate that cloud and fog droplet pH haschanged in recent decades in response to controls on anthropogenicemissions, while the limited trend data for aerosol particles indicateacidity may be relatively constant due to the semivolatile nature of thekey acids and bases and buffering in particles. This paper reviews andsynthesizes the current state of knowledge on the acidity of atmosphericcondensed phases, specifically particles and cloud droplets. It includesrecommendations for estimating acidity and pH, standard nomenclature, asynthesis of current pH estimates based on observations, and new modelcalculations on the local and global scale. 

Chemical processes in clouds and fogs can substantially alter atmospheric oxidant budgets and lead to aerosol mass formation. However, many regional and global models do not include detailed aqueous‐phase chemical mechanisms due to the (a) lack of complete understanding of the chemical processes and (b) computational burden of adding constituents. Current gas‐aqueous chemistry 0‐dimensional models were evaluated in a cloud‐chemistry box model intercomparison based on a mid‐September 2016 cloud chemistry event at Whiteface Mountain, New York. Multiphase mechanisms in the five participating models ranged from those appropriate for 3‐d models to highly complex with thousands of reactions. This study focused on oxidant levels in both phases and aqueous‐phase sulfate and organic acid formation. Comparison of gas‐phase‐only chemistry gives very similar oxidant predictions at night but shows significant differences during daytime with the hydroxyl radical (OH) variability of about an order of magnitude. The variability in the model results increases substantially with aqueous chemistry due to different Henry's Law constants, aqueous‐phase reaction rate constants, and chemical mechanisms. Using a prescribed liquid water content and pH value of 4.5, modeled aqueous OH, aldehyde, and organic acid concentrations differ by over an order of magnitude in daytime. Simulations were also conducted at a pH = 5.1, predicted variable pH, and with added transition metal ion chemistry. While we compare predicted and measured inorganic anions and water‐soluble organic carbon, we cannot do so for aqueous‐phase oxidant concentrations due to the lack of measurements. We highlight a need for recommended equilibrium and aqueous‐phase rate constants.

 

Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τ_{exp, flow}) of SOM after poking the material with a needle. In the first set of experiments, we show that τ_{exp, flow} increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m⁻³. Based on simulations, the viscosities of the particles were between 6  ×  10⁵ and 5  ×  10⁷ Pa s at < 0.5 % RH and between 3  ×  10² and 9  ×  10³ Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τ_{exp, flow} decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m⁻³. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m⁻³ was determined to be between 4  ×  10⁴ and 1.5  ×  10⁶ Pa s compared to between 6  ×  10⁵ and 5  ×  10⁷ Pa s for SOM with a production mass concentration of 121 µg m⁻³. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere. 

Abstract. Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models.

This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.