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Volatile organic compounds (VOCs) have a large impact on the oxidising capacity of the troposphere and are major precursors of tropospheric ozone and secondary atmospheric aerosols. Accurate measurements and data comparability of VOCs among monitoring networks are essential to assessing the trends of these secondary air pollutants. Metrological traceability of the measurements to the International System of Units (SI traceability) contributes to both measurement consistency and data comparability. Accurate, stable and SI-traceable reference gas mixtures (RGMs) and working standards are needed to achieve SI traceability through an unbroken chain of calibrations of the analytical instruments used to monitor VOCs. However, for many oxygenated VOCs (OVOCs), such RGMs and working standards are not available at an atmospheric amount of substance fraction levels (< 10 nmolmol-1). Here, we present the protocols developed to transfer SI traceability to the field by producing two types of SI-traceable working standards for selected OVOCs. These working standards, based on RGMs diluted dynamically with dry nitrogen and on certified spiked whole-air samples, were then assessed using a thermal desorber–gas chromatograph–flame ionisation detector (TD–GC–FID) and proton transfer reaction–time of flight–mass spectrometer (PTR–ToF–MS) as analytical methods. For that purpose, we calibrated five analytical instruments using in-house calibration standards and treated the new SI-traceable working standards as samples. Due to analytical limitations, the assessment was only possible for acetaldehyde, acetone, methanol and methyl ethyl ketone (MEK). Relative differences between assigned and measured values were used to assess the working standards based on the dilution of RGMs. The relative differences were within the measurement uncertainty for acetone, MEK, methanol and acetaldehyde at an amount of substance fractions around 10 nmolmol-1. For the working standards based on certified spiked whole-air samples in pressurised cylinders, results showed a good agreement among the laboratories (i.e. differences within the measurement expanded uncertainty (U) ranging between 0.5 and 3.3 nmolmol-1) and with the certified amount of substance fraction for acetaldehyde (15.7 nmol mol−1 ± 3.6 (U) nmol mol−1), acetone (17 nmol mol−1 ± 1.5 (U) nmol mol−1) and MEK (12.3 nmol mol−1 ± 2.3 (U) nmol mol−1). Despite the promising results for the working standards based on the dilution of RGMs and on certified spiked whole-air samples filled into pressurised cylinders, the assessment must be considered with care due to the large measurement uncertainty, particularly for methanol. Active collaboration among the metrological, meteorological and atmospheric chemistry monitoring communities is needed to tackle the challenges of OVOC monitoring, such as the lack of stable and SI-traceable calibration standards (i.e. RGMs and working standards). Besides this collaboration, other research applications, such as modelling and remote sensing, may benefit from the transfer of SI traceability to monitoring stations. [ABSTRACT FROM AUTHOR]

Tropospheric ozone results from in situ chemical formation and stratosphere–troposphere exchange (STE), with the latter being more important in the middle and upper troposphere than in the lower troposphere. Ozone photochemical formation is nonlinear and results from the oxidation of methane and non-methane hydrocarbons (NMHCs) in the presence of nitrogen oxide (NO x= NO + NO2). Previous studies showed that O3 short- and long-term trends are nonlinearly controlled by near-surface anthropogenic emissions of carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides, which may also be impacted by the long-range transport (LRT) of O3 and its precursors. In addition, several studies have demonstrated the important role of STE in enhancing ozone levels, especially in the midlatitudes. In this article, we investigate tropospheric ozone spatial variability and trends from 2005 to 2019 and relate those to ozone precursors on global and regional scales. We also investigate the spatiotemporal characteristics of the ozone formation regime in relation to ozone chemical sources and sinks. Our analysis is based on remote sensing products of the tropospheric column of ozone (TrC-O3) and its precursors, nitrogen dioxide (TrC-NO2), formaldehyde (TrC-HCHO), and total column CO (TC-CO), as well as ozonesonde data and model simulations. Our results indicate a complex relationship between tropospheric ozone column levels, surface ozone levels, and ozone precursors. While the increasing trends of near-surface ozone concentrations can largely be explained by variations in VOC and NOx concentration under different regimes, TrC-O3 may also be affected by other variables such as tropopause height and STE as well as LRT. Decreasing or increasing trends in TrC-NO2 have varying effects on TrC-O3, which is related to the different local chemistry in each region. We also shed light on the contribution of NOx lightning and soil NO and nitrous acid (HONO) emissions to trends of tropospheric ozone on regional and global scales. [ABSTRACT FROM AUTHOR]

In this study, trends of 21st-century ground-level ozone and ozone precursors were examined across South America, a less-studied region where trend estimates have rarely been comprehensively addressed. Therefore, we provided an updated regional analysis based on validated surface observations. We tested the hypothesis that the recent increasing ozone trends, mostly in urban environments, resulted from intense wildfires driven by extreme meteorological events impacting cities where preexisting volatile organic compound (VOC)-limited regimes dominate. We applied the quantile regression method based on monthly anomalies to estimate trends, quantify their uncertainties and detect trend change points. Additionally, the maximum daily 8 h average (MDA8) and peak-season metrics were used to assess short- and long-term exposure levels, respectively, for the present day (2017–2021). Our results showed lower levels in tropical cities (Bogotá and Quito), varying between 39 and 43 nmol mol -1 for short-term exposure and between 26 and 27 nmol mol -1 for long-term exposure. In contrast, ozone mixing ratios were higher in extratropical cities (Santiago and São Paulo), with a short-term exposure level of 61 nmol mol -1 and long-term exposure levels varying between 40 and 41 nmol mol -1. Santiago (since 2017) and São Paulo (since 2008) exhibited positive trends of 0.6 and 0.3 nmol mol -1 yr -1 , respectively, with very high certainty. We attributed these upward trends, or no evidence of variation, such as in Bogotá and Quito, to a well-established VOC-limited regime. However, we attributed the greater increase in the extreme percentile trends (≥ 90th) to heat waves and, in the case of southwestern South America, to wildfires associated with extreme meteorological events. [ABSTRACT FROM AUTHOR]

Numerous studies have highlighted the health benefits associated with reducing aerosol particles and other pollutants. Recent studies have shown a positive correlation between exposure to aerosol particles and COVID-19 cases. In this study, we investigate the relationship between aerosol particle concentration and COVID-19 variants' infection and mortality rates. Remote-sensing data on aerosol optical depth (AOD), a surrogate for atmospheric aerosol levels, were collected and analyzed in three regions within the states of California, Illinois, and North and South Carolina. These regions were selected to reflect the variability in aerosol concentration and anomalies during the COVID-19 period (2020–2022) compared to a reference period (2010–2019). We found consistent positive linear correlations across most regions between COVID-19 mortality rates and AOD levels below 0.2. These correlations were found to be independent of the change in aerosol levels relative to the reference period. In North and South Carolina, the Delta variant was associated with not only a high mortality rate but also a steeper increase in mortality rate per AOD of 39.6 ± 1.5 (R2 = 0.87) compared to Alpha at 10.5 ± 0.3 (R2 = 0.85). However, due to the lower mortality rate of the Delta variant compared to the Alpha variant in Illinois, the Delta variant has a lower correlation slope of 28.3 ± 1 (R2 = 0.94) compared to the Alpha variant at 52.1 ± 1.8 (R2 = 0.78). No correlation between mortality rate and AOD was observed for the Omicron variants in any of the investigated regions except in California where a weak positive correlation was evident. Our findings establish a compelling link between aerosol concentrations and SARS-CoV-2 infection and mortality. Our results underscore the urgent need for further research to elucidate the underlying mechanisms and broader implications, leading to more sustainable solutions to curb the airborne transmission of COVID-19 viruses and other viral infections in general. [ABSTRACT FROM AUTHOR]

In this study, we investigate the environmental perception changes in college students during the COVID-19 pandemic. During the almost worldwide lockdown between March and December 2020, many students experienced dramatic changes in their socioeconomic life that have affected their well-being, ability to learn, and environmental perceptions. Students were sent home from college campuses, many businesses were shut down, and changes were seen in the environment throughout the world. In many places, lockdown policies lead to improved air, water, and land quality, mainly due to fewer vehicle and anthropogenic emissions. An online survey conducted on students enrolled in the University of South Florida and Florida State University demonstrates that most students have become more environmentally conscious after the COVID-19 lockdown, indicating significant perception changes. The students' demographic backgrounds played a major role in affecting their perception changes. Furthermore, mass media was found to have affected the student's perceptions, which demonstrates the important role media plays in directing public opinion, especially among college students and millennia. This study also highlights the environmental issues that underrepresented students face and calls for policies to address them on both educational and socioeconomic levels. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOx) are key intermediates in the atmospheric cycling of reactive nitrogen, the spatiotemporal distribution of which modulates ozone (O3) production. Field campaigns were conducted at the Tudor Hill Marine Atmospheric Observatory, Bermuda, in the spring and summer of 2019 to explore atmospheric cycling of NOx and its modulation of photochemical O3 production in the marine boundary layer. In aged, clean marine air, an atypical NO2 diel profile with a solar noon peak of 69 ± 5 pptv was recorded, challenging the classic U‐shaped diel profile with a solar noon valley characterized by fast photolysis and oxidation consumption in the daytime. This result indicated an unknown daytime NOx source excluded from the current near‐explicit chemical model, which underestimated the solar noon NOx level by 20–56 pptv and source rate by 9.7–33.5 pptv hr−1, considering the upper and lower limits of total OH reactivity and halogen photochemistry in the marine boundary layer. The observed HONO level accounted for ∼56% of the unknown NOx source, implying an unknown NOx regeneration pathway with HONO as an intermediate. The photochemical nature of the unknown NOx source maximized perturbation of photochemical OH and O3 production. The O3 abundance and production rate were underestimated by 2–4 ppbv and 28%–80%, respectively, and the OH abundance and source rate were 7%–55% and 21%–57% lower than the estimated levels with the constraint of NOx, respectively. The unknown NOx source requires urgent revision of the current understanding of reactive nitrogen cycling and the oxidative capacity of the clean marine atmosphere. Plain Language Summary: This study presents measurements and a chemical budget analysis of NO2, O3, and OH in the clean marine boundary layer at the Tudor Hill Marine Atmospheric Observatory, Bermuda. The high solar noon abundance and atypical diel profile of NO2 challenged the classic U‐shape, indicating an unknown daytime NOx source. Assessments using a near‐explicit MCM chemical model (v3.3.1) yielded an unknown NOx source by 9.7–33.5 pptv hr−1 at solar noon and demonstrated that the photochemical nature of the unknown NOx source maximized perturbation of the O3 and OH budgets. In addition to halogen photochemistry, the unknown NOx source necessitates revision of the photochemistry in the clean marine boundary layer. Key Points: The atypical NO2 diel profile against its classic U‐shape indicates an unknown daytime source of NOx in the marine boundary layerThe unknown daytime NOx source, in addition to halogen photochemistry, greatly perturbs NOx, O3, and OH budgetsThe unknown NOx source appears to be due in part to a missing HONO source, that is, NOx is regenerated with HONO as an intermediate [ABSTRACT FROM AUTHOR]

The chemical nocturnal rate of ozone removal in a downtown location in the city of Santiago de Chile was evaluated during March (end of summer), and May and June (autumn) of 2005. In all the measured days, ground level ozone concentrations drop below 1 ppb during night- time. The simultaneous measurements of NO and VOCs concentrations allows an estimation of the chemical rates of ozone removal by these pollutants. Calculated chemical removal rates are considerably larger than those experimentally determined. The main chemical ozone removal process is its reaction with NO. This process is partially compensated by ozone transport, most probably from the residual layer and / or advection of ozone enriched air masses. [ABSTRACT FROM AUTHOR]

A new instrument (LOPAP: LOng Path liquid Absorption Photometer) for the sensitive detection of nitric acid (HNO3) in the atmosphere is described. HNO3 is sampled in a temperature controlled stripping coil mounted in an external sampling module to minimize sampling artefacts in sampling lines. After conversion into a strongly absorbing dye, HNO3 is detected in long path absorption in special Teflon® AF 2400 tubes used as liquid core wave guides. For the correction of some interferences, due to for example HONO and particle nitrate, two channels are used in series. The interferences from several potential interfering compounds including particle nitrate were quantified in the laboratory and in a large outdoor simulation chamber. With the exception of the interference caused by N2O5, which is quantitatively measured by the instrument, all tested interferences can be corrected under atmospheric conditions. Thus, in the instrument only the sum of N( V) from HNO3 and N2O5 is determined, which is expected to be a common problem of wet chemical HNO3 instruments. The instrument has a detection limit of 5–30 pptv for a time response of 6–2 min, respectively and was validated against the FTIR technique in a large outdoor simulation chamber. In addition, the applicability of the instrument was demonstrated in a field campaign. [ABSTRACT FROM AUTHOR]