The analysis of the individual composition of hydrocarbon (VOC) mixtures enables us to transform observed VOC-concentrations into their respective total VOC-reactivity
versus
OH radicals (
R
VOC
= Σ(
...k
OH+VOC
i
× VOC
i
)). This is particularly useful because local ozone production essentially depends on this single parameter rather than on the details of the underlying hydrocarbon mixture (Klemp
et al.
,
Schriften des Forschungszentrums Jülich
,
Energy & Environment
, 2012,
21
). The VOC composition also enables us to pin down the major emission source of hydrocarbons in urban areas to be petrol cars with temporarily reduced catalyst efficiency (the so-called cold-start situation) whereas the source of nitrogen oxides (NO
x
= NO + NO
2
) is expected to be nowadays dominated by diesel cars. The observations in the vicinity of main roads in German cities show a decrease in the ratio of OH reactivities of VOC and NO
2
(
R
VOC
/
R
NO
2
) by a factor of 7.5 over the time period 1994-2014. This is larger than the expected decrease of a factor of 2.9 taking estimated trends of VOC and NO
x
traffic emissions in Germany (Umweltbundesamt Deutschland,
National Trend Tables for the German Atmospheric Emission Reporting
, 2015), during this time period. The observed reduction in the
R
VOC
/
R
NO
2
ratio leads to a drastic decrease in local ozone production driven by the reduction in hydrocarbons. The analysis reveals that the overall reduction of ozone production benefits from the low decrease of NO
x
emissions from road traffic which is a consequence of the eventual absence of catalytic converters for nitrogen oxide removal in diesel cars up to now.
Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like ...methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.
Greenhouse gas emissions and air pollution have changed the composition of the atmosphere, and thereby initiated global warming and reduced air quality. Our editorial board members note the need for ...a deeper understanding of atmospheric fluxes and processes to tackle climate and human health issues.Greenhouse gas emissions and air pollution have changed the composition of the atmosphere, and thereby initiated global warming and reduced air quality. Our editorial board members note the need for a deeper understanding of atmospheric fluxes and processes to tackle climate and human health issues.
It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by ...terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about -0.2 to -0.9 W m(-2). The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Soot generators are able to produce carbonaceous nanoparticles purposefully and can therefore play a vital role in the calibration of particle instruments with an actual combustion aerosol. ...Condensation particle counters (CPCs) have become the instrument of choice for legislative measurements of the particle number (PN) concentration. The Euro 5B standard introduced by the European Union in 2011 was the first regulation that introduced a PN limit for the exhaust emission from light duty diesel vehicles. Since then, several other PN regulations for soot-emitting vehicles and combustion engines have been introduced or are currently in draft stages, all with similar requirements for the periodic calibration of the particle counter’s measurement accuracy. It is for this reason that combustion particles produced under laboratory conditions have become an attractive choice of calibration aerosol. Nonetheless, it is often difficult to generate a large amount of particles in the small nanometer range from a laboratory combustion source. In this study, we evaluated the performance of a recently introduced soot generator and its suitability for calibrating the counting efficiency and lower cut-off size of CPCs in the nanometer size range. We first characterized the soot generator’s warm up time to determine when it reaches a stable output when using propane as its fuel. We then investigated the influence of a dozen propane-to-air flow settings on the resulting particle size distribution of the combustion aerosol. Finally, we determined the resulting nanoparticle concentrations for 13 size classes below 20 nm in order to achieve a high size resolution at or near the lower detection limit of common CPCs. We performed our measurements under low-pressure conditions as our group operates CPCs onboard commercial passenger aircraft that are used as an atmospheric measurement platform. Another consideration is that CPCs are also operated elsewhere at much less than standard sea-level pressure. Examples include high-altitude research stations as well as engine test rigs used for vehicle exhaust emission testing or certification operated at elevated locations, e.g., in the USA, Mexico, and China. From these experiments, we concluded that when operating this novel soot generator with 7.5 SLPM airflow and 62.5 SCPM propane flow, it is possible to generate a realistic combustion aerosol for tests and calibrations that is still adequate even in the small nanometer size range. The concentrations measured under these operating conditions were just sufficient for detection with an aerosol electrometer, which is the concentration reference typically used in CPC calibrations.
A chamber study was carried out to investigate the stable carbon isotopic composition (δ13C) of secondary organic aerosol (SOA) formed from ozonolysis of β‐pinene. β‐Pinene (600 ppb) with a known ...δ13C value (−30.1‰) and 500 ppb ozone were injected into the chamber in the absence of light and the resulting SOA was collected on preheated quartz fiber filters. Furthermore, δ13C values of the gas‐phase β‐pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC‐IRMS). β‐Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of β‐pinene with ozone was measured to be 1.0026 2.6 ± 1.5‰). The δ13C value of total secondary organic aerosol was very similar to that of its precursor (average = −29.6 ± 0.2‰) independent of experiment time. Nopinone, one of the major oxidation products of β‐pinene, was found in both the gas and aerosol phases. The gas‐phase nopinone was heavier than the initial β‐pinene by 1.3‰ but lighter than the corresponding aerosol‐phase nopinone. On average, the gas‐phase nopinone was lighter by 2.3‰ than the corresponding aerosol‐phase nopinone. The second product found in the SOA was detected as acetone, but it desorbed from the filter at a higher temperature than nopinone, which indicates that it is a pyrolysis product. The acetone showed a much lower δ13C (−36.6‰) compared to the initial β‐pinene δ13C.
Heterogeneous uptake of hydroperoxyl radicals (HO2)
onto aerosols has been proposed to be a significant sink of HOx, hence
impacting the atmospheric oxidation capacity. Accurate calculation of the
...HO2 uptake coefficient γHO2 is key to
quantifying the potential impact of this atmospheric process. Laboratory
studies show that γHO2 can vary by orders of
magnitude due to changes in aerosol properties, especially aerosol soluble
copper (Cu) concentration and aerosol liquid water content (ALWC). In this
study we present a state-of-the-art model called MARK to simulate both gas-
and aerosol-phase chemistry for the uptake of HO2 onto Cu-doped
aerosols. Moreover, a novel parameterization of HO2 uptake was
developed that considers changes in relative humidity (RH) and condensed-phase
Cu ion concentrations and which is based on a model optimization using
previously published and new laboratory data included in this
work. This new parameterization will be applicable to wet aerosols, and it
will complement current IUPAC recommendations. The new parameterization is
as follows (the explanations for symbols are in the Appendix): 1γHO2=1αHO2+3×υHO24×106×RdHcorrRT×(5.87+3.2×ln(ALWC/PM+0.067))×PM-0.2×Cu2+eff0.65+υHO2l4RTHorgDorgε. All parameters used in the paper are summarized in Table A1. Using this new
equation, field data from a field campaign were used to evaluate the impact
of the HO2 uptake onto aerosols on the ROx (= OH + HO2 + RO2) budget. Highly variable values for HO2 uptake were
obtained for the North China Plain (median value < 0.1).
The temperature dependence of the kinetic isotope effect (KIE) of β‐pinene ozonolysis was investigated experimentally at 258, 273 and 303 K in the AIDA atmospheric simulation chamber. Compound ...specific carbon isotopic analysis of gas phase samples was performed off‐line with a Thermo Desorption‐Gas Chromatography‐Isotope Ratio Mass Spectrometry (TD‐GC‐IRMS) system. From the temporal behavior of the δ13C of β‐pinene a KIE of 1.00358 ± 0.00013 was derived at 303 K, in agreement with literature data. Furthermore, KIE values of 1.00380 ± 0.00014 at 273 K and 1.00539 ± 0.00012 at 258 K were determined, showing an increasing KIE with decreasing temperature. A parameterization of the observed KIE temperature dependence was deduced and used in a sensitivity study carried out with the global chemistry transport model MOZART‐3. Two scenarios were compared, the first neglecting, the second implementing the KIE temperature dependence in the simulations. β‐Pinene stable carbon isotope ratio and concentration were computed, with emphasis on boreal zones. For early spring it is shown that when neglecting the temperature dependence of KIE, the calculated average age of β‐pinene in the atmosphere can be up to two times over‐ or underestimated. The evolution of the isotopic composition of the major β‐pinene oxidation product, nopinone, was examined using Master Chemical Mechanism (MCM) simulations. The tested hypothesis that formation of nopinone and its associated KIE are the determining factors for the observed δ13C values of nopinone is supported at high β‐pinene conversion levels.
Key Points
The KIE of b‐pinene ozonolysis increases with decreasing temperature
Nopinone loss processes can be neglected for its isotopic evolution
Temperature dependence of KIE should be implemented in global modeling
An intercomparison of different aerosol chemical characterization techniques has been performed as part of a chamber study of biogenic secondary organic aerosol (BSOA) formation and aging at the ...atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). Three different aerosol sampling techniques – the aerosol collection module (ACM), the chemical analysis of aerosol online (CHARON) and the collection thermal-desorption unit (TD) were connected to proton transfer reaction time-of-flight mass spectrometers (PTR-ToF-MSs) to provide chemical characterization of the SOA. The techniques were compared among each other and to results from an aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS). The experiments investigated SOA formation from the ozonolysis of β-pinene, limonene, a β-pinene–limonene mix and real plant emissions from Pinus sylvestris L. (Scots pine). The SOA was subsequently aged by photo-oxidation, except for limonene SOA, which was aged by NO3 oxidation. Despite significant differences in the aerosol collection and desorption methods of the PTR-based techniques, the determined chemical composition, i.e. the same major contributing signals, was found by all instruments for the different chemical systems studied. These signals could be attributed to known products expected from the oxidation of the examined monoterpenes. The sampling and desorption method of ACM and TD provided additional information on the volatility of individual compounds and showed relatively good agreement. Averaged over all experiments, the total aerosol mass recovery compared to an SMPS varied within 80 ± 10, 51 ± 5 and 27 ± 3 % for CHARON, ACM and TD, respectively. Comparison to the oxygen-to-carbon ratios (O : C) obtained by AMS showed that all PTR-based techniques observed lower O : C ratios, indicating a loss of molecular oxygen either during aerosol sampling or detection. The differences in total mass recovery and O : C between the three instruments resulted predominantly from differences in the field strength (E∕N) in the drift tube reaction ionization chambers of the PTR-ToF-MS instruments and from dissimilarities in the collection/desorption of aerosols. Laboratory case studies showed that PTR-ToF-MS E∕N conditions influenced fragmentation which resulted in water and further neutral fragment losses of the detected molecules. Since ACM and TD were operated in higher E∕N than CHARON, this resulted in higher fragmentation, thus affecting primarily the detected oxygen and carbon content and therefore also the mass recovery. Overall, these techniques have been shown to provide valuable insight on the chemical characteristics of BSOA and can address unknown thermodynamic properties such as partitioning coefficient values and volatility patterns down to a compound-specific level.
Hydroxyl (OH) and peroxy radicals (HO2 and RO2)
were measured in the Pearl River Delta, which is one of the most polluted
areas in China, in autumn 2014. The radical observations were complemented
by ...measurements of OH reactivity (inverse OH lifetime) and a comprehensive
set of trace gases including carbon monoxide (CO), nitrogen oxides (NOx=NO, NO2) and volatile organic compounds (VOCs). OH reactivity was in
the range from 15 to 80 s−1, of which about 50 % was
unexplained by the measured OH reactants. In the 3 weeks of the
campaign, maximum median radical concentrations were 4.5×106 cm−3
for OH at noon and 3×108 and
2.0×108 cm−3 for HO2 and RO2, respectively, in
the early afternoon. The completeness of the daytime radical measurements
made it possible to carry out experimental budget analyses for all radicals
(OH, HO2, and RO2) and their sum (ROx). The maximum loss rates for
OH, HO2, and RO2 reached values between 10 and 15 ppbv h−1
during the daytime. The largest fraction of this can be attributed to radical
interconversion reactions while the real loss rate of ROx remained below
3 ppbv h−1. Within experimental uncertainties, the destruction rates of HO2
and the sum of OH, HO2, and RO2 are balanced by their respective
production rates. In case of RO2, the budget could be closed by
attributing the missing OH reactivity to unmeasured VOCs. Thus, the
presumption of the existence of unmeasured VOCs is supported by RO2
measurements. Although the closure of the RO2 budget is greatly
improved by the additional unmeasured VOCs, a significant imbalance in the
afternoon remains, indicating a missing RO2 sink. In case of OH, the
destruction in the morning is compensated by the quantified OH sources from
photolysis (HONO and O3), ozonolysis of alkenes, and OH recycling
(HO2+NO). In the afternoon, however, the OH budget indicates a
missing OH source of 4 to 6 ppbv h−1. The diurnal variation of the missing OH
source shows a similar pattern to that of the missing RO2 sink so that
both largely compensate each other in the ROx budget. These observations
suggest the existence of a chemical mechanism that converts RO2 to OH
without the involvement of NO, increasing the RO2 loss rate during the daytime
from 5.3 to 7.4 ppbv h−1 on average. The photochemical net ozone
production rate calculated from the reaction of HO2 and RO2 with
NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated
ROx primary sources being 22 ppbv in this campaign. The produced ozone can
be attributed to the oxidation of measured (18 %) and unmeasured (60 %)
hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger
integrated net ozone production of 140 ppbv would be calculated from the
oxidation rate of VOCs with OH if HO2 and all RO2 radicals
react with NO. However, the unknown RO2 loss (evident in the RO2
budget) causes 30 ppbv less ozone production than would be expected from the
VOC oxidation rate.
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