Airborne observations over the North Atlantic Ocean reveal the importance of gas-phase urea in the atmosphere.

Reduced nitrogen (N) is central to global biogeochemistry, yet there are large uncertainties surrounding its sources and rate of cycling. Here, we present observations of gas-phase urea (CO(NH2)2) in the atmosphere from airborne high-resolution mass spectrometer measurements over the North Atlantic Ocean. We show that urea is ubiquitous in the lower troposphere in the summer, autumn, and winter but was not detected in the spring. The observations suggest that the ocean is the primary emission source, but further studies are required to understand the responsible mechanisms. Urea is also observed aloft due to long-range transport of biomass-burning plumes. These observations alongside global model simulations point to urea being an important, and currently unaccounted for, component of reduced-N to the remote marine atmosphere. Airborne transfer of urea between nutrient-rich and -poor parts of the ocean can occur readily and could impact ecosystems and oceanic uptake of carbon dioxide, with potentially important climate implications.

Secondary organic aerosol reduced by mixture of atmospheric vapours.

Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene 'scavenges' hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production).

Using a coupled LES aerosol-radiation model to investigate the importance of aerosol-boundary layer feedback in a Beijing haze episode.

Haze episodes, characterised by extremely high aerosol concentrations and a reduction in visibility to less than 10 km, are a frequent occurrence in wintertime Beijing, despite policy interventions leading to an overall improvement in average annual air quality. The main drivers in the onset of haze episodes in wintertime Beijing are changing synoptic conditions, however, aerosol-radiation interactions and their feedback on boundary layer meteorology are thought to play an essential role in the intensity and longevity of haze episodes. In this study we use a coupled LES aerosol-radiation model (UCLALES-SALSA), which we have recently configured for the urban environment of Beijing. The model's high resolution and control over meteorological and aerosol conditions as well as atmospheric processes means we can directly elucidate and quantify the importance of specific aspects of the aerosol-radiation-meteorology feedback in the cumulative stage of Beijing haze. The main results presented here show (a) synoptic scale meteorology has a larger impact on boundary layer suppression than high aerosol concentrations and (b) unlike previous results obtained using regional models or observationally driven analyses, there is no threshold value at which the aerosol-radiation-meteorology feedback has a significant effect on PBL height. Rather, our work shows that for the aerosol composition in this case study, the role of the feedback effect in reducing PBL height increases under shallow boundary layer conditions and with increasing pollution loading in an almost linear fashion. This lack of a threshold found for our case study has important policy implications since interventions based on such a value will not result in large reductions associated with turning off the feedback process. Furthermore, this work directly shows that although the right synoptic changes are a prerequisite for pollution episodes in Beijing, local and regional emissions drive increases in aerosol load that are sufficient to initiate the aerosol feedback loop. This further drives suppression of the boundary layer top and promotes stagnation of air and increased stability, which can be self-sustaining. This results in higher surface aerosol concentrations for extended periods of time, with severe consequences for human health [Lv et al., Atmos. Environ., 2016, 124, 98-108; Wang et al., Atmos. Chem. Phys., 2019, 19(10), 6949-6967].

Vertical profile of particle hygroscopicity and CCN effectiveness during winter in Beijing: insight into the hygroscopicity transition threshold of black carbon.

The hygroscopicity and ability of aerosol particles to act as cloud condensation nuclei (CCN) is important in determining their lifetime and role in aerosol-cloud interactions, thereby influencing cloud formation and climate. Previous studies have used the aerosol hygroscopic properties measured at the ground to evaluate the influence on cloud formation in the atmosphere, which may introduce uncertainty associated with aerosol hygroscopicity variability with altitude. In this study, the CCN behaviour and hygroscopic properties of daily filter collections of PM2.5 from three different heights (8, 120, 260 m) on a tower in Beijing were determined in the laboratory using water, water/methanol and methanol as the atomization solvents. Whilst there was substantial temporal variability in particle concentration and composition, there was little obvious difference in aerosol CCN and hygroscopic behaviour at different heights, although the planetary boundary layer height (PBLH) reduced to below the tower height during the nighttime, suggesting that use of surface hygroscopicity measurements is sufficient for the estimation of aerosol particle activation in clouds. Additionally, the critical coating thickness (in terms of mass ratio of coating/refractory BC, MRc) defining the BC transition between being hydrophobic to hydrophilic, was determined by combining hygroscopic tandem differential mobility analyser (H-TDMA), centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2) measurements. The MRc of 250 nm BC-containing particles increased from a background value of between 0.8 and 1.6 to around 4.6 at the onset of the growth event of nanoparticles, decreasing monotonically back to the background level as the event progressed. This indicates that large particles do not act as an effective pre-existing condensation sink of the hygroscopic vapours during the nanoparticle growth events, leading to the 250 nm BC particles requiring more coating materials to transition between being hydrophobic and hydrophilic. These findings show that large particles may be less important in suppressing the new particle formation and subsequent growth in the atmosphere.

Avoiding high ozone pollution in Delhi, India.

Surface ozone is a major pollutant threatening public health, agricultural production and natural ecosystems. While measures to improve air quality in megacities such as Delhi are typically aimed at reducing levels of particulate matter (PM), ozone could become a greater threat if these measures focus on PM alone, as some air pollution mitigation steps can actually lead to an increase in surface ozone. A better understanding of the factors controlling ozone production in Delhi and the impact that PM mitigation measures have on ozone is therefore critical for improving air quality. Here, we combine in situ observations and model analysis to investigate the impact of PM reduction on the non-linear relationship between volatile organic compounds (VOC), nitrogen oxides (NOx) and ozone. In situ measurements of NOx, VOC, and ozone were conducted in Delhi during the APHH-India programme in summer (June) and winter (November) 2018. We observed hourly averaged ozone concentrations in the city of up to 100 ppbv in both seasons. We performed sensitivity simulations with a chemical box model to explore the impacts of PM on the non-linear VOC-NOx-ozone relationship in each season through its effect on aerosol optical depth (AOD). We find that ozone production is limited by VOC in both seasons, and is particularly sensitive to solar radiation in winter. Reducing NOx alone increases ozone, such that a 50% reduction in NOx emissions leads to 10-50% increase in surface ozone. In contrast, reducing VOC emissions can reduce ozone efficiently, such that a 50% reduction in VOC emissions leads to ∼60% reduction in ozone. Reducing PM alone also increases ozone, especially in winter, by reducing its dimming effects on photolysis, such that a 50% reduction in AOD can increase ozone by 25% and it also enhances VOC-limitation. Our results highlight the importance of reducing VOC emissions alongside PM to limit ozone pollution, as well as benefitting control of PM pollution through reducing secondary organic aerosol. This will greatly benefit the health of citizens and the local ecosystem in Delhi, and could have broader application for other megacities characterized by severe PM pollution and VOC-limited ozone production.

A marine biogenic source of atmospheric ice-nucleating particles.

The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties. The formation of ice in clouds is facilitated by the presence of airborne ice-nucleating particles. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice. Sea-spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice-nucleating material is probably biogenic and less than approximately 0.2 micrometres in size. We find that exudates separated from cells of the marine diatom Thalassiosira pseudonana nucleate ice, and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice-nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol, in combination with our measurements, suggest that marine organic material may be an important source of ice-nucleating particles in remote marine environments such as the Southern Ocean, North Pacific Ocean and North Atlantic Ocean.

Mixing State of Carbonaceous Aerosols of Primary Emissions from "Improved" African Cookstoves.

Designs of "improved" stoves are introduced recently to benefit the solid fuel consumption of cooking activities in developing countries, but the uncertainties concerning the combustion processes and particulate emissions remain poorly characterized. To help understand this, combustion in three examples of "improved" African cookstoves was investigated in the laboratory. A typical European heating stove was included for comparison purpose. Detailed aerosol emissions were studied in real-time with an Aerosol Mass Spectrometer and Single Particle Soot Photometer, to explore interactions between black carbon (BC) and organic carbon aerosols, which were parametrized according to modified combustion efficiency (MCE), a common metric used within the atmospheric emission community. Greater than 50% of the total organic matter (OM) was found in BC-containing particles when MCE was >0.95 for dry oak and coal fuels, whereas at lower MCE, over 80% of the total OM for most of the fuels existed in particles without detectable BC. When the OM mass fraction of total particulate matter (PM1) > 0.9, the mass ratio of OM to refractory BC in BC-containing particles was about 2-3, but only ∼0.8 when OM mass fraction <0.9. These findings are not currently included in models and such information should be considered in the future emission scenarios.

Improving the quantification of secondary organic aerosol using a microflow reactor coupled to HPLC-MS and NMR to manufacture ad hoc calibration standards.

Secondary organic aerosol (SOA) is a key uncertainty in quantifying the impact of humans on Earth's climate. SOA is a complex mixture of oxidized organic species, and a fundamental hurdle in determining its composition is the lack of authentic standards for comparison and quantification. Organic synthesis can be used to produce pure standards, but is limited to compounds for which there is a degree of confidence in the proposed structure and can be expensive and time-consuming. In this study, a flow reactor was developed to form SOA in sufficient quantities to be collected and pure compounds subsequently isolated from the mixture using semipreparative high performance liquid chromatography. The purity and yield of each isolated compound were obtained using proton nuclear magnetic resonance ((1)H NMR), whereas molecular formulas were confirmed by high resolution Fourier transform ion cyclotron mass spectrometry (FTICR-MS). The effectiveness of the methodology has been evaluated here by using α-pinene as the precursor because it is the monoterpene with the most well characterized SOA chemistry. Eleven individual α-pinene SOA compounds were produced from α-pinene oxidation experiments and used for quantitative analysis of SOA formed during chamber experiments carried out close to ambient conditions. These compounds represented 25% of the total SOA mass, a significant improvement in mass balance compared to previous studies. This relatively simple approach may be extended to produce other SOA components not available commercially to improve quantification of aerosol sources.

An assessment of vapour pressure estimation methods.

Laboratory measurements of vapour pressures for atmospherically relevant compounds were collated and used to assess the accuracy of vapour pressure estimates generated by seven estimation methods and impacts on predicted secondary organic aerosol. Of the vapour pressure estimation methods that were applicable to all the test set compounds, the Lee-Kesler [Reid et al., The Properties of Gases and Liquids, 1987] method showed the lowest mean absolute error and the Nannoolal et al. [Nannoonal et al., Fluid Phase Equilib., 2008, 269, 117-133] method showed the lowest mean bias error (when both used normal boiling points estimated using the Nannoolal et al. [Nannoolal et al., Fluid Phase Equilib., 2004, 226, 45-63] method). The effect of varying vapour pressure estimation methods on secondary organic aerosol (SOA) mass loading and composition was investigated using an absorptive partitioning equilibrium model. The Myrdal and Yalkowsky [Myrdal and Yalkowsky, Ind. Eng. Chem. Res., 1997, 36, 2494-2499] vapour pressure estimation method using the Nannoolal et al. [Nannoolal et al., Fluid Phase Equilib., 2004, 226, 45-63] normal boiling point gave the most accurate estimation of SOA loading despite not being the most accurate for vapour pressures alone.

HIPTox-Hazard Identification Platform to Assess the Health Impacts from Indoor and Outdoor Air Pollutant Exposures, through Mechanistic Toxicology: A Single-Centre Double-Blind Human Exposure Trial Protocol.

Over the past decade, our understanding of the impact of air pollution on short- and long-term population health has advanced considerably, focusing on adverse effects on cardiovascular and respiratory systems. There is, however, increasing evidence that air pollution exposures affect cognitive function, particularly in susceptible groups. Our study seeks to assess and hazard rank the cognitive effects of prevalent indoor and outdoor pollutants through a single-centre investigation on the cognitive functioning of healthy human volunteers aged 50 and above with a familial predisposition to dementia. Participants will all undertake five sequential controlled exposures. The sources of the air pollution exposures are wood smoke, diesel exhaust, cleaning products, and cooking emissions, with clean air serving as the control. Pre- and post-exposure spirometry, nasal lavage, blood sampling, and cognitive assessments will be performed. Repeated testing pre and post exposure to controlled levels of pollutants will allow for the identification of acute changes in functioning as well as the detection of peripheral markers of neuroinflammation and neuronal toxicity. This comprehensive approach enables the identification of the most hazardous components in indoor and outdoor air pollutants and further understanding of the pathways contributing to neurodegenerative diseases. The results of this project have the potential to facilitate greater refinement in policy, emphasizing health-relevant pollutants and providing details to aid mitigation against pollutant-associated health risks.

Critical assessment of liquid density estimation methods for multifunctional organic compounds and their use in atmospheric science.

In order to model the properties and chemical composition of secondary organic aerosol (SOA), estimated physical property data for many thousands of organic compounds are required. Seven methods for estimating liquid density are assessed against experimental data for a test set of 56 multifunctional organic compounds. The group contribution method of Schroeder coupled with the Rackett equation using critical properties by Nannoolal was found to give the best liquid density values for this test set. During this work some problems with the representation of certain groups (aromatic amines and phenols) within the critical property estimation methods were identified, highlighting the importance (and difficulties) of deriving the parameters of group contribution methods from good quality experimental data. A selection of the estimation methods are applied to the 2742 compounds of an atmospheric chemistry mechanism, which showed that they provided consistent liquid density values for compounds with such atmospherically important (but poorly studied) functional groups as hydroperoxide, peroxide, peroxyacid, and PAN. Estimated liquid density values are also presented for a selection of compounds predicted to be important in atmospheric SOA. Hygroscopic growth factor (a property expected to depend on liquid density) has been calculated for a wide range of particle compositions. A low sensitivity of the growth factor to liquid density was found, and a single density value of 1350 kg·m(-3) could be used for all multicomponent SOA in the calculation of growth factors for comparison with experimentally measured values in the laboratory or the field without incurring significant error.

Understanding the patterns and health impact of indoor air pollutant exposures in Bradford, UK: a study protocol.

INTRODUCTION: Relative to outdoor air pollution, there is little evidence examining the composition and concentrations of indoor air pollution and its associated health impacts. The INGENIOUS project aims to provide the comprehensive understanding of indoor air pollution in UK homes. METHODS AND ANALYSIS: 'Real Home Assessment' is a cross-sectional, multimethod study within INGENIOUS. This study monitors indoor air pollutants over 2 weeks using low-cost sensors placed in three rooms in 300 Born in Bradford (BiB) households. Building audits are completed by researchers, and participants are asked to complete a home survey and a health and behaviour questionnaire, in addition to recording household activities and health symptoms on at least 1 weekday and 1 weekend day. A subsample of 150 households will receive more intensive measurements of volatile organic compound and particulate matter for 3 days. Qualitative interviews conducted with 30 participants will identify key barriers and enablers of effective ventilation practices. Outdoor air pollution is measured in 14 locations across Bradford to explore relationships between indoor and outdoor air quality. Data will be analysed to explore total concentrations of indoor air pollutants, how these vary with building characteristics, and whether they are related to health symptoms. Interviews will be analysed through content and thematic analysis. ETHICS AND DISSEMINATION: Ethical approval has been obtained from the NHS Health Research Authority Yorkshire and the Humber (Bradford Leeds) Research Ethics Committee (22/YH/0288). We will disseminate findings using our websites, social media, publications and conferences. Data will be open access through the BiB, the Open Science Framework and the UK Data Service.

Global organic and inorganic aerosol hygroscopicity and its effect on radiative forcing.

The climate effects of atmospheric aerosol particles serving as cloud condensation nuclei (CCN) depend on chemical composition and hygroscopicity, which are highly variable on spatial and temporal scales. Here we present global CCN measurements, covering diverse environments from pristine to highly polluted conditions. We show that the effective aerosol hygroscopicity, κ, can be derived accurately from the fine aerosol mass fractions of organic particulate matter (ϵorg) and inorganic ions (ϵinorg) through a linear combination, κ = ϵorg ⋅ κorg + ϵinorg ⋅ κinorg. In spite of the chemical complexity of organic matter, its hygroscopicity is well captured and represented by a global average value of κorg = 0.12 ± 0.02 with κinorg = 0.63 ± 0.01 as the corresponding value for inorganic ions. By showing that the sensitivity of global climate forcing to changes in κorg and κinorg is small, we constrain a critically important aspect of global climate modelling.

A significant role for nitrate and peroxide groups on indoor secondary organic aerosol.

This paper reports indoor secondary organic aerosol, SOA, composition based on the results from an improved model for indoor air chemistry. The model uses a detailed chemical mechanism that is near-explicit to describe the gas-phase degradation of relevant indoor VOC species. In addition, gas-to-particle partitioning is included for oxygenated products formed from the degradation of limonene, the most ubiquitous terpenoid species in the indoor environment. The detail inherent in the chemical mechanism permits the indoor SOA composition to be reported in greater detail than currently possible using experimental techniques. For typical indoor conditions in the suburban UK, SOA concentrations are ~1 µg m(-3) and dominated by nitrated material (~85%), with smaller contributions from peroxide (12%), carbonyl (3%), and acidic (1%) material. During cleaning activities, SOA concentrations can reach 20 µg m(-3) with the composition dominated by peroxide material (73%), with a smaller contribution from nitrated material (21%). The relative importance of these different moieties depends crucially (in order) on the outdoor concentration of O(3), the deposition rates employed and the scaling factor value applied to the partitioning coefficient. There are currently few studies that report observation of aerosol composition indoors, and most of these have been carried out under conditions that are not directly relevant. This study highlights the need to investigate SOA composition in real indoor environments. Further, there is a need to measure deposition rates for key indoor air species on relevant indoor surfaces and to reduce the uncertainties that still exist in gas-to-particle phase parametrization for both indoor and outdoor air chemistry models.