Multifunctional organic nitrates, including carbonyl nitrates, are important
species formed in
Organic nitrates play an important role as sinks or temporary reservoirs of
the reaction of the
peroxy radical, produced by the oxidation of VOCs, with NO. The major
pathway is generally Reaction (R1a), which leads to The reaction of unsaturated VOCs with the NO
Among the organic nitrates, a variety of multifunctional species such as hydroxy-nitrates, carbonyl-nitrates, and dinitrates are formed. The formed species have been shown to significantly contribute to the nitrogen budget in both rural and urban areas (Perring et al., 2013). Beaver et al. (2012) observed that carbonyl nitrates, formed as second-generation nitrates from isoprene, are an important fraction of the total organic nitrates observed over Sierra Nevada in summer. These observations are supported by several studies that investigated the photooxidation of isoprene in simulation chambers (Paulot et al., 2009; Müller et al., 2014). These multifunctional organic nitrates are also semi-volatile/nonvolatile and highly soluble species; thus, they are capable of partitioning into the atmospheric condensed phases (droplets and aerosols). Numerous field observations of the chemical composition of atmospheric particles have shown that organic nitrates represent a significant fraction (up to 75 % in mass) of the total organic aerosol (OA), demonstrating that these species are important components of total OA (Ng et al., 2017).
Several modeling studies have also confirmed that multifunctional organic
nitrates, in particular isoprene nitrates, play a key role in the transport
of reactive nitrogen and, consequently, in the formation of ozone and other
secondary pollutants at regional and global scales (Horowitz et al.,
2007; Mao et al., 2013; Squire et al., 2015). In particular, Mao et al. (2013) performed simulations based on data from the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) aircraft
campaign across the eastern US in 2004. They showed that organic
nitrates, which are mainly composed of secondary organic nitrates, including
a large fraction of carbonyl nitrates, provide an important pathway for
exporting
Recent experimental studies have revealed that hydrolysis in the aerosol phase
may be a very efficient sink of organic nitrates in the atmosphere (Bean and
Hildebrand Ruiz, 2016; Rindelaub et al., 2015). These studies also suggest
that the rate of these reactions strongly depends on the organic nitrate
chemical structure and that additional work is needed to better understand
these processes. In the gas phase, photolysis and reaction with the OH radical
are expected to dominate the fate of organic nitrates (Roberts, 1990;
Turberg et al., 1990). In a previous study, we measured the photolysis
frequencies and the rate constants for the OH oxidation of three carbonyl
nitrates (
These results are significant as they demonstrate that photolysis
frequencies of these multifunctional species cannot be calculated as the sum
of the monofunctional species' (ketone
Given the large contribution of carbonyl nitrates to the organic nitrate
pool and the importance of their photochemistry for the
As a common OH precursor in simulation chamber experiments, isopropyl nitrite
was synthesized by dropwise addition of a dilute solution of
On the contrary, 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone were
synthesized for the first time. Great care was taken to develop
a robust process: 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone syntheses
are based on Kames' method (Kames et al., 1993). This method consists of a
liquid-/gas-phase reaction where the corresponding hydroxy-ketone reacts with
The photolysis frequencies of the two carbonyl nitrates were determined by
carrying out experiments in the CESAM simulation chamber, which is only
briefly described here as detailed information can be found in Wang et al. (2011). The chamber consists of a 4.2 m
During a typical experiment, carbonyl nitrates were introduced into the
chamber which was preliminarily filled at atmospheric pressure with
During the experiment, the carbonyl nitrate loss processes can be described
as
The uncertainties were calculated by adding the respective statistical
errors (
The kinetic experiments for the OH oxidation of the carbonyl nitrates were
performed in the CSA chamber at room temperature and atmospheric pressure,
in a mixture of
The relative rate technique was used to determine the rate constant for the
OH oxidation of the carbonyl nitrates with methanol as reference compound.
We used the IUPAC recommended value
Prior to the experiments, it was verified that photolysis and wall losses of
the studied compounds were negligible under our experimental conditions.
This can be explained by the facts that (i) the irradiation system of the CSA
chamber emits photons at significantly higher wavelengths than the
CESAM chamber, and (ii) the walls of the chamber are made of Pyrex which is
more chemically inert than stainless steel. Therefore, it was assumed that
reaction with OH is the only fate of both the studied compound (carbonyl
nitrate) and the reference compound (methanol) and that neither of these
compounds is reformed at any stage during the experiment. Based on these
hypotheses, it can be shown that
Dry synthetic air was generated using
Figure 1 presents the kinetic plots obtained for the two compounds, where
Kinetic plots for
Photolysis frequencies and PAN yields for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone measured in the CESAM chamber.
In Table 2, these photolysis frequencies are compared to those we
obtained in a previous study (Suarez-Bertoa et al., 2012) for
3-nitrooxy-2-propanone, 3-nitrooxy-2-butanone, and
3-methyl-3-nitrooxy-2-butanone using the same experimental conditions and
methodology. Experimental photolysis frequencies have also been compared to
those calculated using cross sections published in the literature and by
assuming a quantum yield equal to unity. The intensity of the actinic flux
in the CESAM chamber was determined by combining measurement of the spectrum of
the lamps with a spectroradiometer and determination of
Comparison of the experimental photolysis frequencies of carbonyl nitrates with those calculated for CESAM irradiation conditions and by assuming a quantum yield equal to unity.
Products formed by the photolysis of the carbonyl nitrates were investigated
using FTIR spectrometry. For both compounds, only peroxy acetyl nitrate (PAN)
was detected. To calculate its formation yield, the concentration of PAN was
plotted as a function of –
For 4-nitrooxy-2-butanone, PAN is formed with a yield equal to unity. Its
formation can be explained by the dissociation of the C(O)–C bond, as shown
in Scheme 1. This pathway also leads to the formation of the alkyl radical
Photolysis pathways of 4-nitrooxy-2-butanone. Detected products are indicated using a gray background, and their formation yield is given (in percent).
As discussed above, as the enhancement in the cross sections is larger at
the higher wavelengths, where absorption by the nitrate chromophore is very
small, it was proposed by Müller et al. (2014) that the absorption by
the carbonyl chromophore is enhanced due to the neighboring nitrate group.
The authors also suggested that the photodissociation proceeds by a
dissociation of the weak O–
For 5-nitrooxy-2-pentanone, the formation yield of PAN has been observed to be
much lower:
Photolysis pathways of 5-nitrooxy-2-pentanone. Detected products are indicated using a gray background, and their formation yield is given (in percent).
Rate constants of the OH oxidation were measured for
4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone. Prior to the experiments,
it was checked that the carbonyl nitrates did not photolyze nor
decompose/adsorb on the walls of the chamber. Figure 2 represents the
kinetic plots obtained for the two carbonyl nitrates. For each compound,
several independent kinetic experiments were performed and data were
combined to provide the
Kinetic plots for the oxidation of
4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone by OH radicals. For
5-nitrooxy-2-pentanone, data have been shifted by 0.2 on the
Rate constants for the OH oxidation of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone.
Rate constants provided in this study, as well as those previously reported
for a series of
Comparison of the experimental rate constants for the OH oxidation of carbonyl nitrates with those estimated by SARs.
Rate constants are expressed in cubed centimeters per molecule per second (cm
Atmospheric lifetimes of carbonyl nitrates towards photolysis and reaction with OH radicals.
From these experiments, several oxidation products have been detected: HCHO, PAN, and methylglyoxal for 4-nitrooxy-2-butanone, and HCHO, PAN, and 3-nitrooxy-propanal for 5-nitrooxy-2-pentanone. However, their quantification was highly uncertain because the infrared spectra were complex due to the presence of methanol, isopropyl nitrite, impurities (in particular acetic acid), and their oxidation products. Dedicated mechanistic experiments should currently/in the near future be performed using HONO as the OH source in order to simplify the chemical mixture.
Atmospheric lifetimes of the investigated compounds are presented in Table 5. The photolysis frequencies were estimated using
In order to evaluate the impact of these carbonyl nitrates on the nitrogen
budget and the transport of
This paper presents the first study on the atmospheric reactivity of
4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone. Thanks to experiments in
simulation chambers, photolysis frequencies and rate constants of the
OH oxidation were measured for the first time. From these results, it is
concluded that, similarly to
Rate constants for the OH oxidation and photolysis frequencies of carbonyl nitrates are provided in Tables 1 and 3. They are also available through the Library of Advanced Data Products (LADP) of the EUROCHAMP data center (
Simulation chamber experiments which were used to retrieve these parameters are available through the Database of Atmospheric Simulation Chamber Studies (DASCS) of the EUROCHAMP data center (
The supplement related to this article is available online at:
BPV coordinated the research project. BPV, RSB, and JFD designed the experiments in the simulation chambers. RSB performed the experiments with the technical support of MC and EP. RSB and MDa performed the organic syntheses. BPV, RSB, and MDu performed the data treatment and interpretation. BPV and RSB wrote the paper, and BPV was responsible for the final version of the paper. All coauthors revised the content of the original manuscript and approved the final version of the paper.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Simulation chambers as tools in atmospheric research (AMT/ACP/GMD inter-journal SI)”. It is not associated with a conference.
The authors thank Mila Ródenas (mila@ceam.es)
(CEAM, Paterna-Valencia, Spain) for the development and free
distribution of the ANIR software via the EUROCHAMP-2020 Data Centre
website (
This work was supported by the French National Agency for Research (project no. ONCEM-ANR-12-BS06-0017-01) and by the European Commission's Seventh Framework Programme (EUROCHAMP-2; grant no. 228335), H2020 Research Infrastructures (EUROCHAMP-2020; grant no. 730997).
This paper was edited by Andreas Hofzumahaus and reviewed by two anonymous referees.