Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H₂O₂ formation through HO₂ recombination. Surprisingly, H₂O₂ mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H₂O₂, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H₂O₂ mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H₂O₂ formation rates of about 0.2 ppbv h^-1 during the initial irradiation periods are consistent with the H₂O₂ rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H₂O₂ formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM_2.5 ≥ 75 μg m^-3 agree with the observed H₂O₂ concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO₂ reaction with OH and HSO₃^- oxidation by H₂O₂. Overall, under high pollution, the H₂O₂-caused sulfate formation rate is above 250 ng m^-3 h^-1, contributing to the sulfate formation by more than 70%.

Guaiacol Nitration in a Simulated Atmospheric Aerosol with an Emphasis on Atmospheric Nitrophenol Formation Mechanisms

Atmospheric nitrophenols are pollutants of concern due to their toxicity and light-absorption characteristics and their low reactivity resulting in relatively long residence times in the environment. We investigate multiphase nitrophenol formation from guaiacol in a simulated atmospheric aerosol and support observations with the corresponding chemical mechanisms. The maximal secondary organic aerosol (SOA) yield (42%) is obtained under illumination at 80% relative humidity. Among the identified nitrophenols, 4-nitrocatechol (3.6% yield) is the prevailing species in the particulate phase. The results point to the role of water in catechol and further 4-nitrocatechol formation from guaiacol. In addition, a new pathway of dark nitrophenol formation is suggested, which prevailed in dry air and roughly yielded 1% nitroguaiacols. Furthermore, the proposed mechanism possibly leads to oligomer formation via a phenoxy radical formation by oxidation with HONO.

Organosulfates in Ambient Aerosol: State of Knowledge and Future Research Directions on Formation, Abundance, Fate, and Importance

Organosulfates (OSs), also referred to as organic sulfate esters, are well-known and ubiquitous constituents of atmospheric aerosol particles. Commonly, they are assumed to form upon mixing of air masses of biogenic and anthropogenic origin, that is, through multiphase reactions between organic compounds and acidic sulfate particles. However, in contrast to this simplified picture, recent studies suggest that OSs may also originate from purely anthropogenic precursors or even directly from biomass and fossil fuel burning. Moreover, besides classical OS formation pathways, several alternative routes have been discovered, suggesting that OS formation possibly occurs through a wider variety of formation mechanisms in the atmosphere than initially expected. During the past decade, OSs have reached a constantly growing attention within the atmospheric science community with evermore studies reporting on large numbers of OS species in ambient aerosol. Nonetheless, estimates on OS concentrations and implications on atmospheric physicochemical processes are still connected to large uncertainties, calling for combined field, laboratory, and modeling studies. In this Critical Review, we summarize the current state of knowledge in atmospheric OS research, discuss unresolved questions, and outline future research needs, also in view of reductions of anthropogenic sulfur dioxide (SO₂) emissions. Particularly, we focus on (1) field measurements of OSs and measurement techniques, (2) formation pathways of OSs and their atmospheric relevance, (3) transformation, reactivity, and fate of OSs in atmospheric particles, and (4) modeling efforts of OS formation and their global abundance.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

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.

Highly Oxidized Multifunctional Organic Compounds Observed in Tropospheric Particles: A Field and Laboratory Study

Very recent studies have reported the existence of highly oxidized multifunctional organic compounds (HOMs) with O/C ratios greater than 0.7. Because of their low vapor pressure, these compounds are often referred as extremely low-volatile organic compounds (ELVOCs), and thus, they are able to contribute significantly to organic mass in tropospheric particles. While HOMs have been successfully detected in the gas phase, their fate after uptake into particles remains unclear to date. Hence, the present study was designed to detect HOMs and related oxidation products in the particle phase and, thus, to shed light on their fate after phase transfer. To this end, aerosol chamber investigations of α-pinene ozonolysis were conducted under near environmental precursor concentrations (2.4 ppb) in a continuous flow reactor. The chemical characterization shows three classes of particle constituents: (1) intact HOMs that contain a carbonyl group, (2) particle-phase decomposition products, and (3) highly oxidized organosulfates (suggested to be addressed as HOOS). Besides chamber studies, HOM formation was also investigated during a measurement campaign conducted in summer 2013 at the TROPOS research station Melpitz. During this field campaign, gas-phase HOM formation was found to be correlated with an increase in the oxidation state of the organic aerosol.

2-hydroxyterpenylic acid: an oxygenated marker compound for α-pinene secondary organic aerosol in ambient fine aerosol

An oxygenated MW 188 compound is commonly observed in substantial abundance in atmospheric aerosol samples and was proposed in previous studies as an α-pinene-related marker compound that is associated with aging processes. Owing to difficulties in producing this compound in sufficient amounts in laboratory studies and the occurrence of isobaric isomers, a complete assignment for individual MW 188 compounds could not be achieved in these studies. Results from a comprehensive mass spectrometric analysis are presented here to corroborate the proposed structure of the most abundant MW 188 compound as a 2-hydroxyterpenylic acid diastereoisomer with 2R,3R configuration. The application of collision-induced dissociation with liquid chromatography/electrospray ionization-ion trap mass spectrometry in both negative and positive ion modes, as well as chemical derivatization to methyl ester derivatives and analysis by the latter technique and gas chromatography/electron ionization mass spectrometry, enabled a comprehensive characterization of MW 188 isomers, including a detailed study of the fragmentation behavior using both mass spectrometric techniques. Furthermore, a MW 188 positional isomer, 4-hydroxyterpenylic acid, was tentatively identified, which also is of atmospheric relevance as it could be detected in ambient fine aerosol. Quantum chemical calculations were performed to support the diastereoisomeric assignment of the 2-hydroxyterpenylic acid isomers. Results from a time-resolved α-pinene photooxidation experiment show that the 2-hydroxyterpenylic acid 2R,3R diastereoisomer has a time profile distinctly different from that of 3-methyl-1,2,3-butanetricarboxylic acid, a marker for oxygenated (aged) secondary organic aerosol. This study presents a comprehensive chemical data set for a more complete structural characterization of hydroxyterpenylic acids in ambient fine aerosol, which sets the foundation to better understand the atmospheric fate of α-pinene in future studies.

Parental provisioning behaviour plays a key role in linking personality with reproductive success

Repeatable behavioural traits ('personality') have been shown to covary with fitness, but it remains poorly understood how such behaviour-fitness relationships come about. We applied a multivariate approach to reveal the mechanistic pathways by which variation in exploratory and aggressive behaviour is translated into variation in reproductive success in a natural population of blue tits, Cyanistes caeruleus. Using path analysis, we demonstrate a key role for provisioning behaviour in mediating the link between personality and reproductive success (number of fledged offspring). Aggressive males fed their nestlings at lower rates than less aggressive individuals. At the same time, their low parental investment was associated with increased female effort, thereby positively affecting fledgling production. Whereas male exploratory behaviour was unrelated to provisioning behaviour and reproductive success, fast-exploring females fed their offspring at higher rates and initiated breeding earlier, thus increasing reproductive success. Our findings provide strong support for specific mechanistic pathways linking components of behavioural syndromes to reproductive success. Importantly, relationships between behavioural phenotypes and reproductive success were obscured when considering simple bivariate relationships, underlining the importance of adopting multivariate views and statistical tools as path analysis to the study of behavioural evolution.

Ozone-driven secondary organic aerosol production chain

Acidic sulfate particles are known to enhance secondary organic aerosol (SOA) mass in the oxidation of biogenic volatile organic compounds (BVOCs) through accretion reactions and organosulfate formation. Enhanced phase transfer of epoxides, which form during the BVOC oxidation, into the acidified sulfate particles is shown to explain the latter process. We report here a newly identified ozone-driven SOA production chain that increases SOA formation dramatically. In this process, the epoxides interact with acidic sulfate particles, forming a new generation of highly reactive VOCs through isomerization. These VOCs partition back into the gas phase and undergo a new round of SOA forming oxidation reactions. Depending on the nature of the isomerized VOCs, their next generation oxidation forms highly oxygenated terpenoic acids or organosulfates. Atmospheric evidence is presented for the existence of marker compounds originating from this chain. The identified process partly explains the enhanced SOA formation in the presence of acidic particles on a molecular basis and could be an important source of missing SOA precursor VOCs that are currently not included in atmospheric models.

Denuder sampling techniques for the determination of gas-phase carbonyl compounds: a comparison and characterisation of in situ and ex situ derivatisation methods

Two denuder sampling techniques have been compared for the analysis of gaseous carbonyl compounds. One type of denuder was coated with XAD-4 resin and the other type of denuder was coated with XAD-4 and 2,4-dinitrophenylhydrazine (DNPH) to derivatise gaseous carbonyl compounds to their hydrazone forms simultaneously. A detailed protocol for the denuder coating procedure is described. The collection efficiency under dry (RH <3%) and humid conditions (RH 50%) as well as filter positive artefacts were evaluated. The XAD-4/DNPH coated denuders showed significantly less break-through potential and hence collection than the XAD-4-only coated denuders. The performance of the XAD-4/DNPH denuder was better under humid conditions with no detected break-through for hydroxyacetone, methacrolein, methylglyoxal, campholenic aldehyde and nopinone. Calibration experiments were performed in a simulation chamber and carbonyl-hydrazone concentrations determined in the extracts of both the denuder types were related to the mixing ratios of gaseous carbonyl compounds in the chamber to overcome losses and errors associating with the denuder sampling, extraction and sample preparation. The application of on-tube conversion for the XAD-4/DNPH denuders resulted in higher R(2) values than the XAD-4 denuder, ranging up to 0.991 for nopinone. The XAD-4-only coated denuders showed acceptable calibration curves only for lower vapour pressure carbonyl compounds though larger relative standard deviations (RSD) were observed. Carbonyl compounds that were formed during the oxidation of nopinone were collected using the XAD-4/DNPH denuders. The results showed that the denuder sampling device was able to provide reproducible nopinone mixing ratios that remained in the chamber after about 1h of the oxidation. One isomer of oxo-nopinones was tentatively identified from off-line HPLC/(-)ESI-TOFMS analysis. Based on the TOFMS response of the nopinone-DNPH derivative, the oxo-nopinone molar yield of 0.7±0.1% (n=3) was determined from the reaction of nopinone with OH radicals. Depending on target analytes, accuracy and sensitivity requirements, the present method can be employed for the determination of gaseous carbonyl compounds that are formed during the oxidation of monoterpenes.