MULTICHARME: A modified Chernin-type multi-pass cell designed for IR and THz long-path absorption measurements in the CHARME atmospheric simulation chamber

We have developed MULTICHARME, a modified Chernin-type multi-pass cell especially designed for IR and THz long-path absorption measurements in the CHamber for Atmospheric Reactivity and Metrology of the Environment (CHARME). By measuring the output power using a near-IR diode-laser and a THz amplified multiplication chain, we have established that the effective reflectivity of MULTICHARME is better than 94 % over approximately three decades of frequency. Absorption measurements of N2O have been performed by probing highly excited rovibrational transitions in the near-IR and ground state rotational transitions at submillimetre wavelengths. In each case the linearity of the absorbance with the pathlengths was verified. Finally, we demonstrate that THz spectroscopy is able to study the isotopic composition of greenhouse polar gases such as N2O and to absolutely quantify stable (N2O) and reactive (O3) species at trace levels. Moreover, a THz monitoring at low pressure of the ozone decay in the chamber has been performed. The deduced ozone lifetime of 3.4 ± 0.1 h is shorter compared with previous measurements performed in CHARME at atmospheric pressure. For the first time, the ability of THz rotational spectroscopy to monitor, with a very high degree of selectivity, stable and reactive polar compounds at trace level in an atmospheric simulation chamber is demonstrated. However, the sensitivity of the THz monitoring needs to be improved to reach the atmospheric trace levels. For this purpose, it is necessary to figure out the baseline variations as well as possible induced by the multiple standing waves present in MULTICHARME.

Measurement report: Variability in the composition of biogenic volatile organic compounds in a Southeastern US forest and their role in atmospheric reactivity

Despite the significant contribution of biogenic volatile organic compounds (BVOCs) to organic aerosol formation and ozone production and loss, there are few long-term, year-round, ongoing measurements of their volume mixing ratios and quantification of their impacts on atmospheric reactivity. To address this gap, we present 1 year of hourly measurements of chemically resolved BVOCs between 15 September 2019 and 15 September 2020, collected at a research tower in Central Virginia in a mixed forest representative of ecosystems in the Southeastern US. Mixing ratios of isoprene, isoprene oxidation products, monoterpenes, and sesquiterpenes are described and examined for their impact on the hydroxy radical (OH), ozone, and nitrate reactivity. Mixing ratios of isoprene range from negligible in the winter to typical summertime 24 h averages of 4–6 ppb, while monoterpenes have more stable mixing ratios in the range of tenths of a part per billion up to ∼2 ppb year-round. Sesquiterpenes are typically observed at mixing ratios of <10 ppt, but this represents a lower bound in their abundance. In the growing season, isoprene dominates OH reactivity but is less important for ozone and nitrate reactivity. Monoterpenes are the most important BVOCs for ozone and nitrate reactivity throughout the year and for OH reactivity outside of the growing season. To better understand the impact of this compound class on OH, ozone, and nitrate reactivity, the role of individual monoterpenes is examined. Despite the dominant contribution of α-pinene to total monoterpene mass, the average reaction rate of the monoterpene mixture with atmospheric oxidants is between 25 % and 30 % faster than α-pinene due to the contribution of more reactive but less abundant compounds. A majority of reactivity comes from α-pinene and limonene (the most significant low-mixing-ratio, high-reactivity isomer), highlighting the importance of both mixing ratio and structure in assessing atmospheric impacts of emissions.

Measurement Report: Variability in the composition of biogenic volatile organic compounds in a southeastern US forest and their role in atmospheric reactivity

Despite the significant contribution of biogenic volatile organic compounds (BVOCs) to organic aerosol formation and ozone production and loss, there are few long-term, year-round, ongoing measurements of their concentrations and their impacts on atmospheric reactivity. To address this gap, we present one year of hourly measurements of chemically resolved BVOCs between September 15, 2019, and September 15, 2020, collected at a research tower in Central Virginia in a mixed forest representative of ecosystems in the Southeastern U.S. Concentrations of isoprene, isoprene reaction products, monoterpenes, and sesquiterpenes are described and examined for their impact on hydroxy radical (OH), ozone, and nitrate reactivity. Concentrations of isoprene range from negligible in the winter to typical summertime 24-hour averages of 4–6 ppb, while monoterpenes have more stable concentrations in the range of tenths of a ppb up to ~ 1 ppb year-round. Sesquiterpenes are typically observed at concentrations of < 10 ppt, but this represents a lower bound in their abundance. In the growing season, isoprene dominates OH reactivity but is less important for ozone and nitrate reactivity. Monoterpenes are the most important BVOCs for ozone and nitrate reactivity throughout the year and for OH reactivity outside of the growing season. To better understand the impact of this compound class on OH, ozone, and nitrate reactivity, the role of individual monoterpenes is examined. Despite the dominant contribution of α-pinene to total monoterpene mass, the average rate constants for reaction of the monoterpene mixture with atmospheric oxidants is between 20% and 30% faster than α-pinene due to the contribution of more-reactive but less abundant compounds. A majority of reactivity comes from α-pinene and limonene (the most significant low-concentration, high-reactivity isomer), highlighting the importance of both concentration and structure in assessing atmospheric impacts of emissions.

Characteristics and Chemical Reactivity of Biogenic Volatile Organic Compounds from Dominant Forest Species in the Jing-Jin-Ji Area, China

Background: Biogenic volatile organic compounds (BVOCs) play an essential role in tropospheric atmospheric chemical reactions. There are few studies on BVOCs emission of dominant tree species in the Jing-Jin-Ji area. Based on the field survey, forest resources data and the measured standard emission factors, this paper used Guenther model in 1993 (G93) to estimate the emissions of BVOCs from dominant forest species (Platycladus orientalis , Quercus variabilis, Betula platyphylla, Populus tomentosa, Pinus tabuliformis, Robinia pseudoacacia, Ulmus pumila, Salix babylonica, Larix gmelinii) in the Jing-Jin-Ji area in 2017, analyzed their spatiotemporal emission characteristics and evaluated their amospheric chemical reactivity. Results: Results showed that the total annual BVOCs emissions were estimated to be 70.8 Gg C·a-1, consisting of 40.5% (28.7 Gg C·a-1) isoprene, 36.0% (25.5 Gg C·a-1) monoterpenes, and 23.4% (16.6 Gg C·a-1) other VOCs. The emissions of Platycladus orientalis, Quercus variabilis, Populus tomentosa and Pinus tabulaeformis contributed 56.1%, 41.2%, 36.0% and 31.1%, respectively. In summer and winter, BVOCs emissions from the Jing-Jin-Ji area accounted for 61.9% and 1.8% of the annual total. Emissions from Chengde contributed to 28.8%, followed by Beijing, accounting for 24.9%, which was mainly distributed in the Taihang Mountains and the Yanshan Mountains. Robinia pseudoacacia, Populus tomentosa, Quercus variabilis, and Pinus tabulaeformis contributed higher BVOCs reaction activity. Conclusions: Emissions peaked in summer (June, July, and August) and bottomed out in winter (December, January, and February). Chengde contributed the most, followed by Beijing. Platycladus orientalis, Quercus variabilis, Populus tomentosa, Pinus tabulaeformis and Robinia pseudoacacia are the primary BVOCs emission and atmospheric reactivity contributors, the planting rates of these species with significant emissions or atmospheric reactivity of BVOCs should be considered for reduction.

Atmospheric mercury in the Southern Hemisphere – Part 2: Source apportionment analysis at Cape Point station, South Africa

Mercury (Hg) contamination is ubiquitous. In order to assess its emissions, transport, atmospheric reactivity, and deposition pathways, worldwide Hg monitoring has been implemented over the past 10–20 years, albeit with only a few stations in the Southern Hemisphere. Consequently, little is known about the relative contribution of marine and terrestrial Hg sources, which is important in the context of growing interest in effectiveness evaluation of Hg mitigation policies. This paper constitutes Part 2 of the study describing a decade of atmospheric Hg concentrations at Cape Point, South Africa, i.e. the first long-term (> 10 years) observations in the Southern Hemisphere. Building on the trend analysis reported in Part 1, here we combine atmospheric Hg data with a trajectory model to investigate sources and sinks of Hg at Cape Point. We find that the continent is the major sink, and the ocean, especially its warm regions (i.e. the Agulhas Current), is the major source for Hg. Further, we find that mercury concentrations and trends from long-range transport are independent of the source region (e.g. South America, Antarctica) and thus indistinguishable. Therefore, by filtering out air masses from source and sink regions we are able to create a dataset representing a southern hemispheric background Hg concentrations. Based on this dataset, we were able to show that the interannual variability in Hg concentrations at Cape Point is not driven by changes in atmospheric circulation but rather due to changes in global emissions (gold mining and biomass burning).

Atmospheric reactivity and oxidation capacity during summer at a suburban site between Beijing and Tianjin

Hydroxyl (OH) radicals, nitrate (NO3) radicals and ozone (O3) play central roles in the troposphere because they control the lifetimes of many trace gases that result from anthropogenic and biogenic origins. To estimate the air chemistry, the atmospheric reactivity and oxidation capacity were comprehensively analyzed based on a parameterization method at a suburban site in Xianghe in the North China Plain from 6 July 2018 to 6 August 2018. The total OH, NO3 and O3 reactivities at the site varied from 9.2 to 69.6, 0.7 to 27.5 and 3.3×10-4 to 1.8×10-2 s−1 with campaign-averaged values of 27.5±9.7, 2.2±2.6 and 1.2±1.7×10-3 s−1 (± standard deviation), respectively. NOx (NO+NO2) was by far the main contributor to the reactivities of the three oxidants, with average values of 43 %–99 %. Alkenes dominated the OH, NO3 and O3 reactivities towards total nonmethane volatile organic compounds (NMVOCs), accounting for 42.9 %, 77.8 % and 94.0 %, respectively. The total OH, NO3 and O3 reactivities displayed similar diurnal variations with the lowest values during the afternoon but the highest values during rush hours, and the diurnal profile of NOx appears to be the major driver for the diurnal profiles of the reactivities of the three oxidants. A box model (a model to Simulate the concentrations of Organic vapors, Sulfuric Acid and Aerosols; SOSAA) derived from a column chemical transport model was used to simulate OH and NO3 concentrations during the observation period. The calculated atmospheric oxidation capacity (AOC) reached 4.5×108 moleculescm-3s-1, with a campaign-averaged value of 7.8×107 moleculescm-3s-1 dominated by OH (7.7×107 moleculescm-3s-1, 98.2 %), O3 (1.2×106 moleculescm-3s-1, 1.5 %) and NO3 (1.8×105 moleculescm-3s-1, 0.3 %). Overall, the integration of OH, NO3 and O3 reactivities analysis could provide useful insights for NMVOC pollution control in the North China Plain. We suggest that further studies, especially direct observations of OH and NO3 radical concentrations and their reactivities, are required to better understand trace gas reactivity and AOC.