Pattern of volatile aldehydes and aromatic hydrocarbons in the largest urban rainforest in the Americas

Origins of formaldehyde in a mountainous background atmosphere of southern China

Formaldehyde (HCHO) is one of the key indicators of severe photochemical pollution and strong atmospheric oxidation capacity in southern China. However, current information on the origins of regional HCHO and the impacts of polluted air masses remains scarce and unclear. In this study, an intensive observation of HCHO was conducted at a mountainous background site in southern China during typical photochemical pollution episodes. The concentrations of HCHO reached up to 6.14 ppbv and averaged at 2.68 ± 1.11 ppbv. Source appointment using a photochemical age-based parameterization method revealed significant contributions of secondary formation (50 %) and biomass burning (42 %). Meanwhile, under the influence of the East Asian Winter Monsoon, polluted air masses from central and western China can significantly increase the regional HCHO levels. The simulation results adopting the Rapid Adaptive Optimization Model for Atmospheric Chemistry model further demonstrated that the intrusion of active anthropogenic pollutants (e.g., small-molecule alkenes) can accelerate the net production rate of HCHO, particularly through BVOC-oxidation pathways. This study suggests a potential enhanced mechanism of HCHO production resulting from anthropogenic-biogenic interactions. It highlights that polluted air masses carrying abundant HCHO from upwind areas may facilitate severe photochemical pollution in the Greater Bay Area.

Tijuca forest contribution to the improvement of air quality and wellbeing of citizens in the city of Rio de Janeiro, Brazil

The Tijuca Forest, one of the largest urban forests in the world, is a protected area of the Brazilian Atlantic Forest, one of the world's biodiversity hotspots. The forest and the Metropolitan Region of Rio de Janeiro coexist and interact, but their mutual influence regarding air quality is still not well known and a more detailed study is needed. Here, air samples were collected inside the forest, in Tijuca National Park (TNP) and Grajaú State Park (GSP) and in two representative urban areas (Tijuca and Del Castilho Districts). Sampling was performed using stainless steel canisters, and ozone precursor hydrocarbons (HCs) were analyzed with the aid of heart-cutting multidimensional gas chromatography. The sampling points within the forest are currently visited by hundreds of people. Total HC concentrations within the green area were clearly lower than in the urbanized districts, in spite of the anthropogenic impact of visitors and the proximity of the urban area. Median values were 21.5, 35.5, 57.9 and 148.6 μg m^-3 at TNP, GSP, Tijuca and Del Castilho, respectively. Total HC concentrations were Del Castilho > Tijuca > GSP > TNP. The kinetic reactivity and ozone-forming potential of individual HCs were evaluated, as well as the intrinsic reactivity of air masses. The air masses in the urbanized area showed a higher average reactivity in all scales. In fact, in spite of the forest's contribution to isoprene emissions, its net contribution to ozone formation was lower than that of urbanized air masses, owing to a reduction in HC concentration, particularly for alkenes and monoaromatic compounds. It is not clear if the forest plays a role in the adsorption of pollutants or if it acts as a physical natural barrier to air masses carrying pollutants. Nonetheless, improving air quality within Tijuca Forest is essential to the welfare of citizens.

Accelerated toluene degradation over forests around megacities in southern China

Toluene is a typical anthropogenic pollutant that has profound impacts on air quality, climate change, and human health, but its sources and sinks over forests surrounding megacities remain unclear. The Nanling Mountains (NM) is a large subtropical forest and is adjacent to the Pearl River Delta (PRD) region, a well-known hotspot for toluene emissions in southern China. However, unexpectedly low toluene concentrations (0.16 ± 0.20 ppbv) were observed at a mountaintop site in NM during a typical photochemical period. A backward trajectory analysis categorized air masses received at the site into three groups, namely, air masses from the PRD, those from central China, and from clean areas. The results revealed more abundant toluene and its key oxidation products, for example, benzaldehyde in air masses mixed with urban plumes from the PRD. Furthermore, a more than three times faster degradation rate of toluene was found in this category of air masses, indicating more photochemical consumption in NM under PRD outflow disturbance. Compared to the categorized clean and central China plumes, the simulated OH peak level in the PRD plumes (15.8 ± 2.2 × 106 molecule cm-3) increased by approximately 30% and 55%, respectively, and was significantly higher than the reported values at other background sites worldwide. The degradation of toluene in the PRD plumes was most likely accelerated by increased atmospheric oxidative capacity, which was supported by isoprene ozonolysis reactions. Our results indicate that receptor forests around megacities are not only highly polluted by urban plumes, but also play key roles in environmental safety by accelerating the degradation rate of anthropogenic pollutants.

Heterogeneous reactions of isoprene and ozone on α-Al2O3: The suppression effect of relative humidity

The heterogeneous reactions of α-Al₂O₃ particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO^-) ions on the surfaces of α-Al₂O₃ particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-Al₂O₃ particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.

Levels of Volatile Carbonyl Compounds in the Atlantic Rainforest, in the City of Rio de Janeiro

When Europeans arrived in America, the Brazilian Atlantic rainforest covered approximately 1,290,000 km². Now, only 8% of the biome's original vegetation remains. One of the largest areas is Tijuca Forest National Park. In this work, the concentrations of 13 carbonyl compounds in an isolated area inside Tijuca Forest, in an urban park with primary and secondary vegetation (Gericinó Natural Park) and in two typical urban areas (Tijuca District and the city of Nilópolis) were determined. The main compounds were formaldehyde and acetaldehyde. The formaldehyde mean concentrations were 0.98 ± 1.00, 1.27 ± 1.67, 3.09 ± 1.60 and 2.33 ± 2.17 μg m^-3 for Tijuca Forest, Gericinó Natural Park, Tijuca District and the city of Nilópolis, respectively. The mean acetaldehyde concentrations were, for the same locations, 0.93 ± 1.05, 2.94 ± 2.54, 2.78 ± 0.91 and 5.48 ± 1.90 μg m^-3. The results indicate that the compounds measured within the forest are transported from the city and that the trees play an important role in removing air pollutants. In contrast, the Gericinó protected area is heavily affected by urban emissions, and its capacity to dilute or absorb pollutants is low because of the sparse vegetation.

Organic Analysis of Environmental Samples Using Liquid Chromatography with Diode Array and Fluorescence Detectors: An Overview

This overview is focused to provide an useful guide of the families of organic pollutants that can be determined by liquid chromatography operating in reverse phase and ultraviolet/fluorescence detection. Eight families have been classified as the main groups to be considered: carbonyls, carboxyls, aromatics, phenols, phthalates, isocyanates, pesticides and emerging. The references have been selected based on analytical methods used in the environmental field, including both the well-established procedures and those more recently developed.

Observations of particulate matter, NO2, SO2, O3, H2S and selected VOCs at a semi-urban environment in the Amazon region

This research aims to assess air quality in a transitional location between city and forest in the Amazon region. Located downwind of the Manaus metropolitan region, this study is part of the large-scale experiment GoAmazon2014/5. Based on their pollutant potential, inhalable particulate matter (PM_2.5), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), hydrogen sulfide (H₂S), benzene, toluene, ethylbenzene and meta-, orto-, para-xylene (BTEX) were selected for analysis. Sampling took place during the wet season (March-April 2014) and dry season (August-October 2014). The number of forest fires in the surroundings was higher during the dry wet season. Results show significant increase during the dry season in mass concentration (wet: <0.01-10 μg m^-3; dry: 9.8-69 μg m^-3), NH₄⁺ soluble content (wet: 13-125 μg m^-3; dry: 86-323 μg m^-3) and K⁺ soluble content (wet: 11-168 μg m^-3; dry 60-356 μg m^-3) of the PM_2.5, and O₃ levels (wet: 1.4-14 μg m^-3; dry: 1.0-40 μg m^-3), indicating influence of biomass burning emissions. BTEX concentrations were low in both periods, but also increased during the dry season. A weak correlation in the time series of the organic and inorganic gaseous pollutants indicates a combination of different sources in both seasons and NO2 results suggest a spatial heterogeneity in gaseous pollutants levels beyond initial expectations.

Isoprene Emissions and Ozone Formation in Urban Conditions: A Case Study in the City of Rio de Janeiro

The potential role of isoprene oxidative processes, as well as the possible impact of air pollution on isoprene emissions, are more important in tropical cities, surrounded by rainforests. In this study, the contribution of isoprene to ozone formation was determined considering different scenarios, mainly volatile organic compounds/NO _x (VOC/NO _x ) ratios, and typical atmospheric conditions for the city of Rio de Janeiro, where more than 36% of the urbanized area is covered by vegetation. Ozone isopleths and incremental reactivity coefficients (IR) were evaluated to understand the direct contribution of isoprene to ground-level ozone formation and the negative impact of anthropogenic NO _x emissions on the natural atmospheric balance. Although isoprene accounted for only 2.7% of the total VOC mass, excluding the isoprene concentration from the model reduced the maximum ozone value by 14.1%. The calculated IR coefficient (grams of O₃ formed per gram of added isoprene) was 2.2 for a VOC/NO _x ratio of 8.86.

Kinetic and mechanistic reactivity. Isoprene impact on ozone levels in an urban area near Tijuca Forest, Rio de Janeiro

Volatile organic compounds (VOCs) play a central role in atmospheric chemistry. In this work, the kinetic and mechanistic reactivities of VOCs are analyzed, and the contribution of the organic compounds emitted by anthropogenic and natural sources is estimated. VOCs react with hydroxyl radicals and other photochemical oxidants, such as ozone and nitrate radicals, which cause the conversion of NO to NO₂ in various potential reaction paths, including photolysis, to form oxygen atoms, which generate ozone. The kinetic reactivity was evaluated based on the reaction coefficients for hydroxyl radicals with VOCs. The mechanistic reactivity was estimated using a detailed mechanism and the incremental reactivity scale that Carter proposed. Different scenarios were proposed and discussed, and a minimum set of compounds, which may describe the tropospheric reactivity in the studied area, was determined. The role of isoprene was analyzed in terms of its contribution to ozone formation.

Volatile Organic Compounds in the Atmosphere of the Botanical Garden of the City of Rio de Janeiro: A Preliminary Study

Volatile organic compounds (VOCs) play a central role in atmospheric chemistry. In this work, VOCs in the Botanical Garden of Rio de Janeiro were determined using the TO-15 Method. The park occupies 1,370,000 m² in the southern area of the city and is next to the Tijuca Forest, which is considered the largest secondary urban forest in the world. The total VOC concentrations ranged from 43.52 to 168.75 µg m^-3, depending on the sampling site and dates. In terms of concentration isoprene represented 4 %-14 % of the total VOC masses. The results suggested that the differences in biomass, distance from the street and activities within the park affected the concentrations of VOCs. The ratios of isoprene/aromatic compounds were higher than those determined in other areas of the city, confirming that the atmosphere of this green area has the contribution of other sources. Kinetic and mechanistic reactivities were also evaluated.

In-cloud multiphase behaviour of acetone in the troposphere: gas uptake, Henry's law equilibrium and aqueous phase photooxidation

Acetone is ubiquitous in the troposphere. Several papers have focused in the past on its gas phase reactivity and its impact on tropospheric chemistry. However, acetone is also present in atmospheric water droplets where its behaviour is still relatively unknown. In this work, we present its gas/aqueous phase transfer and its aqueous phase photooxidation. The uptake coefficient of acetone on water droplets was measured between 268 and 281K (γ=0.7 x 10(-2)-1.4 x 10(-2)), using the droplet train technique coupled to a mass spectrometer. The mass accommodation coefficient α (derived from γ) was found in the range (1.0-3.0±0.25) x 10(-2). Henry's law constant of acetone was directly measured between 283 and 298K using a dynamic equilibrium system (H((298K))=(29±5)Matm(-1)), with the Van't Hoff expression lnH(T)=(5100±1100)/T-(13.4±3.9). A recommended value of H was suggested according to comparison with literature. The OH-oxidation of acetone in the aqueous phase was carried out at 298K, under two different pH conditions: at pH=2, and under unbuffered conditions. In both cases, the formation of methylglyoxal, formaldehyde, hydroxyacetone, acetic acid/acetate and formic acid/formate was observed. The formation of small amounts of four hydroperoxides was also detected, and one of them was identified as peroxyacetic acid. A drastic effect of pH was observed on the yields of formaldehyde, one hydroperoxide, and, (to a lesser extent) acetic acid/acetate. Based on the experimental observations, a chemical mechanism of OH-oxidation of acetone in the aqueous phase was proposed and discussed. Atmospheric implications of these findings were finally discussed.