Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO
) 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. NO
-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 NO
-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO
radical, the difficulty of characterizing the spatial distributions of BVOC and NO
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 NO
-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 NO
-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.
The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation ...panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N2 and in N2–H2O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO2 or HONO impurities. Our rate coefficients (k 1) in N2 bath gas are parametrized in terms of low-pressure (k 0) and high-pressure (k ∞) rate coefficients and a fall-off parameter (F C) with k 1,0 N 2 = 7.24 × 10–31 (T/300 K)−2.17 cm6 molecule–2 s–1, k 1,∞ = 3.3 × 10–12 (T/300 K)−0.3 cm3 molecule–1 s–1, and F C = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H2O bath gas is a factor 5–6 larger than in N2, at room temperature (k 1,0 H 2 O = 4.55 × 10–30 (T/300 K)−4.85 cm6 molecule–2 s–1) indicating that H2O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N2–H2O mixtures, a parametrization of k 1 including both N2 and H2O as third-body quenchers was derived. Neglecting the effect of H2O results, e.g., in an underestimation of k 1 by >10% in the tropical boundary layer.
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IJS, KILJ, NUK, PNG, UL, UM
The Arabian Peninsula is characterized by high and
increasing levels of photochemical air pollution. Strong solar irradiation,
high temperatures and large anthropogenic emissions of reactive trace ...gases
result in intense photochemical activity, especially during the summer
months. However, air chemistry measurements in the region are scarce. In
order to assess regional pollution sources and oxidation rates, the first
ship-based direct measurements of total OH reactivity were performed in
summer 2017 from a vessel traveling around the peninsula during the AQABA
(Air Quality and Climate Change in the Arabian Basin) campaign. Total OH
reactivity is the total loss frequency of OH radicals due to all reactive
compounds present in air and defines the local lifetime of OH, the most
important oxidant in the troposphere. During the AQABA campaign, the total
OH reactivity ranged from below the detection limit (5.4 s−1) over the
northwestern Indian Ocean (Arabian Sea) to a maximum of 32.8±9.6 s−1 over the Arabian Gulf (also known as Persian Gulf) when air
originated from large petroleum extraction/processing facilities in Iraq and
Kuwait. In the polluted marine regions, OH reactivity was broadly comparable
to highly populated urban centers in intensity and composition. The
permanent influence of heavy maritime traffic over the seaways of the Red
Sea, Gulf of Aden and Gulf of Oman resulted in median OH sinks of
7.9–8.5 s−1. Due to the rapid oxidation of direct volatile organic
compound (VOC) emissions, oxygenated volatile organic compounds (OVOCs) were
observed to be the main contributor to OH reactivity around the Arabian
Peninsula (9 %–35 % by region). Over the Arabian Gulf, alkanes and
alkenes from the petroleum extraction and processing industry were an
important OH sink with ∼9 % of total OH reactivity each,
whereas NOx and aromatic hydrocarbons (∼10 % each)
played a larger role in the Suez Canal, which is influenced more by ship
traffic and urban emissions. We investigated the number and identity of
chemical species necessary to explain the total OH sink. Taking into account
∼100 individually measured chemical species, the observed
total OH reactivity can typically be accounted for within the measurement
uncertainty (50 %), with 10 dominant trace gases accounting for
20 %–39 % of regional total OH reactivity. The chemical regimes causing
the intense ozone pollution around the Arabian Peninsula were investigated
using total OH reactivity measurements. Ozone vs. OH reactivity
relationships were found to be a useful tool for differentiating between
ozone titration in fresh emissions and photochemically aged air masses. Our
results show that the ratio of NOx- and VOC-attributed OH reactivity
was favorable for ozone formation almost all around the Arabian Peninsula,
which is due to NOx and VOCs from ship exhausts and, often, oil/gas
production. Therewith, total OH reactivity measurements help to elucidate
the chemical processes underlying the extreme tropospheric ozone
concentrations observed in summer over the Arabian Basin.
Atmospheric non-methane hydrocarbons (NMHCs) have been extensively studied around the globe due to their importance to atmospheric chemistry and their utility in emission source and chemical sink ...identification. This study reports on shipborne NMHC measurements made around the Arabian Peninsula during the AQABA (Air Quality and climate change in the Arabian BAsin) ship campaign. The ship traversed the Mediterranean Sea, the Suez Canal, the Red Sea, the northern Indian Ocean, and the Arabian Gulf, before returning by the same route. The Middle East is one of the largest producers of oil and gas (O&G), yet it is among the least studied. Atmospheric mixing ratios of C2–C8 hydrocarbons ranged from a few ppt in unpolluted regions (Arabian Sea) to several ppb over the Suez Canal and Arabian Gulf (also known as the Persian Gulf), where a maximum of 166.5 ppb of alkanes was detected. The ratio between i-pentane and n-pentane was found to be 0.93±0.03 ppb ppb−1 over the Arabian Gulf, which is indicative of widespread O&G activities, while it was 1.71±0.06 ppb ppb−1 in the Suez Canal, which is a characteristic signature of ship emissions. We provide evidence that international shipping contributes to ambient C3–C8 hydrocarbon concentrations but not to ethane, which was not detected in marine traffic exhausts. NMHC relationships with propane differentiated between alkane-rich associated gas and methane-rich non-associated gas through a characteristic enrichment of ethane over propane atmospheric mixing ratios. Utilizing the variability–lifetime relationship, we show that atmospheric chemistry governs the variability of the alkanes only weakly in the source-dominated areas of the Arabian Gulf (bAG=0.16) and along the northern part of the Red Sea (bRSN=0.22), but stronger dependencies are found in unpolluted regions such as the Gulf of Aden (bGA=0.58) and the Mediterranean Sea (bMS=0.48). NMHC oxidative pair analysis indicated that OH chemistry dominates the oxidation of hydrocarbons in the region, but along the Red Sea and the Arabian Gulf the NMHC ratios occasionally provided evidence of chlorine radical chemistry. These results demonstrate the utility of NMHCs as source/sink identification tracers and provide an overview of NMHCs around the Arabian Peninsula.
Rate coefficients (
k
4
) for the reaction of hydroxyl radicals (OH) with methyl nitrate (CH
3
ONO
2
) were measured over the temperature range 232-343 K using pulsed laser photolysis to generate OH ...and pulsed laser-induced fluorescence to detect it in real-time and under pseudo-first-order conditions. In order to optimize the accuracy of the rate coefficients obtained, the concentration of CH
3
ONO
2
(the reactant in excess) was measured on-line by absorption spectroscopy at 213.86 nm for which the absorption cross-section was also measured (
σ
213.86
= 1.65 ± 0.09 × 10
−18
cm
2
molecule
−1
). The temperature-dependent rate coefficient is described by
k
4
(
T
) = 7.5 × 10
−13
exp(−1034 ± 40)/
T
cm
3
molecule
−1
s
−1
with a room temperature rate coefficient of
k
4
(296 ± 2 K) = (2.32 ± 0.12) × 10
−14
cm
3
molecule
−1
s
−1
where the uncertainty includes the statistical error of 2
σ
and an estimation of the potential systematic bias of 5%. This new dataset helps to consolidate the database for this rate coefficient and to reduce uncertainty in the atmospheric lifetime of CH
3
ONO
2
. As part of this study, an approximate rate coefficient for the reaction of H-atoms with CH
3
ONO
2
(
k
9
) was also derived at room temperature:
k
9
(298 K) = (1.68 ± 0.45) × 10
−13
cm
3
molecule
−1
s
−1
.
Temperature dependent rate coefficients (
k
4
) for the reaction of OH with CH
3
ONO
2
.
Volatile organic compounds (VOCs) were measured around the Arabian Peninsula using a research vessel during the AQABA campaign (Air Quality and Climate
Change in the Arabian Basin) from June to ...August 2017. In this study we
examine carbonyl compounds, measured by a proton transfer reaction mass
spectrometer (PTR-ToF-MS), and present both a regional concentration
distribution and a budget assessment for these key atmospheric species.
Among the aliphatic carbonyls, acetone had the highest mixing ratios in most
of the regions traversed, varying from 0.43 ppb over the Arabian Sea to 4.5 ppb over the Arabian Gulf, followed by formaldehyde (measured by a Hantzsch monitor, 0.82 ppb over the Arabian Sea and 3.8 ppb over the Arabian Gulf)
and acetaldehyde (0.13 ppb over the Arabian Sea and 1.7 ppb over the Arabian
Gulf). Unsaturated carbonyls (C4–C9) varied from 10 to 700 ppt during the
campaign and followed similar regional mixing ratio dependence to aliphatic carbonyls, which were identified as oxidation products of cycloalkanes over
polluted areas. We compared the measurements of acetaldehyde, acetone, and methyl ethyl ketone to global chemistry-transport model (ECHAM5/MESSy Atmospheric Chemistry – EMAC) results. A
significant discrepancy was found for acetaldehyde, with the model
underestimating the measured acetaldehyde mixing ratio by up to an order of
magnitude. Implementing a photolytically driven marine source of
acetaldehyde significantly improved the agreement between measurements and
model, particularly over the remote regions (e.g. Arabian Sea). However, the
newly introduced acetaldehyde source was still insufficient to describe the
observations over the most polluted regions (Arabian Gulf and Suez), where
model underestimation of primary emissions and biomass burning events are
possible reasons.
Strongly enhanced tropospheric ozone (O3) mixing ratios have
been reported in the Arabian Basin, a region with intense solar radiation
and high concentrations of O3 precursors such as nitrogen oxides ...(NOx) and
volatile organic compounds (VOCs). To analyze photochemical O3 production in the
marine boundary layer (MBL) around the Arabian Peninsula, we use shipborne
observations of NO, NO2, O3, OH, HO2, HCHO, the actinic flux,
water vapor, pressure and temperature obtained during the summer 2017 Air
Quality and Climate in the Arabian Basin (AQABA) campaign, and we compare them to
simulation results from the ECHAM-MESSy Atmospheric Chemistry (EMAC) general
circulation model. Net O3 production rates (NOPRs) were greatest over both the Gulf of Oman and the northern Red Sea (16 ppbv d−1) and
over the Arabian Gulf (32 ppbv d−1). The NOPR over the
Mediterranean, the southern Red Sea and the Arabian Sea did not
significantly deviate from zero; however, the results for the Arabian Sea
indicated weak net O3 production of 5 ppbv d−1 as well as net O3
destruction over the Mediterranean and the southern Red Sea with values of −1 and −4 ppbv d−1, respectively. Constrained
by HCHO∕NO2 ratios, our photochemistry calculations show that net O3
production in the MBL around the Arabian Peninsula mostly occurs in
NOx-limited regimes with a significant share of O3 production
occurring in the transition regime between NOx limitation and VOC limitation over
the Mediterranean and more significantly over the northern Red Sea and Oman
Gulf.
This article, the eighth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the gas-phase ...thermal and photochemical reactions of organic species with four, or more, carbon atoms (≥ C4) available on the IUPAC website in 2021, including thermal reactions of closed-shell organic species with HO and NO3 radicals and their photolysis. The present work is a continuation of volume II (Atkinson et al., 2006), with new reactions and updated data sheets for reactions of HO (77 reactions) and NO3 (36 reactions) with ≥ C4 organics, including alkanes, alkenes, dienes, aromatics, oxygenated, organic nitrates and nitro compounds in addition to photochemical processes for nine species. The article consists of a summary table (Table 1), containing the recommended kinetic parameters for the evaluated reactions, and a supplement containing the data sheets, which provide information upon which recommendations are made.
This article, the seventh in the series, presents kinetic
and photochemical data sheets evaluated by the IUPAC Task Group on
Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of ...the
gas-phase and photochemical reactions related to Criegee intermediates
previously published in Atmospheric Chemistry and Physics (ACP) in 2006 and implemented on the IUPAC website up
to 2020. The article consists of an introduction, description of laboratory
measurements, a discussion of rate coefficients for reactions of O3 with
alkenes producing Criegee intermediates, rate coefficients of unimolecular
and bimolecular reactions and photochemical data for reactions of Criegee
intermediates, and an overview of the atmospheric chemistry of Criegee
intermediates. Summary tables of the recommended kinetic and mechanistic
parameters for the evaluated reactions are provided. Data sheets summarizing
information upon which the recommendations are based are given in two files,
provided as a Supplement to this article.
Aerosols influence the Earth's energy balance directly by modifying the radiation transfer and
indirectly by altering the cloud microphysics.
Anthropogenic aerosol emissions dropped considerably when ...the global COVID-19 pandemic
resulted in severe restraints on mobility, production, and public life in spring 2020.
We assess the effects of these reduced emissions
on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails.
We simulate the atmospheric composition with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model
in a baseline (business-as-usual) and a reduced emission scenario.
The model results are compared to aircraft observations
from the BLUESKY aircraft campaign performed in May–June 2020 over Europe.
The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate,
and nitrate in the upper troposphere, likely due to a biased representation of stratospheric aerosol chemistry
and missing information about volcanic eruptions.
The comparison with a baseline scenario shows that the largest relative differences for tracers
and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the
reduced aircraft emissions, while the largest absolute changes are present at the surface.
We also find an increase in all-sky shortwave radiation of 0.21 ± 0.05 W m−2 at the surface in Europe for May 2020,
solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight,
followed by reduced aerosol absorption caused by lower concentrations of inorganic and black carbon aerosols in the troposphere.
A further increase in shortwave radiation from aerosol indirect effects was found to be much
smaller than its variability.
Impacts on ice crystal concentrations, cloud droplet number concentrations, and effective crystal radii are found to be negligible.
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