We present a newly constructed, two-channel thermal dissociation cavity ring-down spectrometer (TD-CRDS) for the measurement ofNOx (NO+NO2), NOy (NOx+HNO3+RO2NO2+2N2O5 etc.), NOz (NOy-NOx) and ...particulate nitrate (pNit). NOy-containing trace gases are detected as NO2 by the CRDS at 405 nm following sampling through inlets at ambient temperature (NOx) or at 850 ∘C (NOy). In both cases,O3 was added to the air sample directly upstream of the cavities to convert NO (either ambient or formed in the 850 ∘C oven) toNO2. An activated carbon denuder was used to remove gas-phase components of NOy when sampling pNit. Detection limits, defined as the 2σ precision for 1 min averaging, are 40 pptv for both NOx and NOy. The total measurement uncertainties (at 50 % relative humidity, RH) in theNOx and NOy channels are 11%+10 pptv and 16%+14 pptv for NOz respectively. Thermograms of various trace gases of theNOz family confirm stoichiometric conversion to NO2 (and/or NO) at the oven temperature and rule out significant interferences from NH3 detection (<2 %) or radical recombination reactions under ambient conditions. While fulfilling the requirement of high particle transmission (>80 % between 30 and 400 nm) and essentially complete removal of reactive nitrogen under dry conditions (>99 %), the denuder suffered from NOx breakthrough and memory effects (i.e. release of stored NOy) under humid conditions, which may potentially bias measurements of particle nitrate.Summertime NOx measurements obtained from a ship sailing through the Red Sea, Indian Ocean and Arabian Gulf (NOx levels from <20 pptv to 25 ppbv) were in excellent agreement with those taken by a chemiluminescence detector of NO and NO2. A data set obtained locally under vastly different conditions (urban location in winter) revealed large diel variations in the NOz to NOy ratio which could be attributed to the impact of local emissions by road traffic.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO3 in pulsed laser photolysis kinetic experiments. Dispersed (220–330 nm) and time-dependent emission at ...(310 ± 5) nm following the 193 nm excitation of HNO3 in N2, air and He was recorded and analysed to characterise the OH(A2Σ) and NO(A2Σ+) electronic excited states involved. The limit of detection for HNO3 using TPEFS was ∼5 × 109 molecule cm−3 (at 60 torr N2 and 180 μs integration time). Detection of HNO3 using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO2, especially in the presence of O2 which quenches NO(A2Σ+) more efficiently than OH(A2Σ). While H2O2 (and possibly HO2) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO3) was very low. The viability of real-time TPEFS detection of HNO3 using emission at (310 ± 5) nm was demonstrated by monitoring HNO3 formation in the reaction of OH + NO2 and deriving the rate coefficient, k2. The value of k2 obtained at 293 K and pressures of 50–200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values.
Characterization of daytime sources of nitrous acid (HONO) is crucial to understand atmospheric oxidation and radical cycling in the planetary boundary layer. HONO and numerous other atmospheric ...trace constituents were measured on the Mediterranean island of Cyprus during the CYPHEX (CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume mixing ratios of HONO were 35 pptv (±25 pptv) with a HONO ∕ NOx ratio of 0.33, which was considerably higher than reported for most other rural and urban regions. Diel profiles of HONO showed peak values in the late morning (60 ± 28 pptv around 09:00 local time) and persistently high mixing ratios during daytime (45 ± 18 pptv), indicating that the photolytic loss of HONO is compensated by a strong daytime source. Budget analyses revealed unidentified sources producing up to 3.4 × 106 molecules cm−3 s−1 of HONO and up to 2.0 × 107 molecules cm−3 s−1 NO. Under humid conditions (relative humidity > 70 %), the source strengths of HONO and NO exhibited a close linear correlation (R2 = 0.72), suggesting a common source that may be attributable to emissions from microbial communities on soil surfaces.
The Mediterranean is a climatically sensitive region located at the crossroads of air masses from three continents: Europe, Africa, and Asia. The chemical processing of air masses over this region ...has implications not only for the air quality but also for the long-range transport of air pollution. To obtain a comprehensive understanding of oxidation processes over the Mediterranean, atmospheric concentrations of the hydroxyl radical (OH) and the hydroperoxyl radical (HO2) were measured during an intensive field campaign (CYprus PHotochemistry EXperiment, CYPHEX-2014) in the northwest of Cyprus in the summer of 2014. Very low local anthropogenic and biogenic emissions around the measurement location provided a vantage point to study the contrasts in atmospheric oxidation pathways under highly processed marine air masses and those influenced by relatively fresh emissions from mainland Europe.The CYPHEX measurements were used to evaluate OH and HO2 simulations using a photochemical box model (CAABA/MECCA) constrained with CYPHEX observations of O3, CO, NOx, hydrocarbons, peroxides, and other major HOx (OH + HO2) sources and sinks in a low-NOx environment (< 100 pptv of NO). The model simulations for OH agreed to within 10 % with in situ OH observations. Model simulations for HO2 agreed to within 17 % of the in situ observations. However, the model strongly under-predicted HO2 at high terpene concentrations, this under-prediction reaching up to 38 % at the highest terpene levels. Different schemes to improve the agreement between observed and modelled HO2, including changing the rate coefficients for the reactions of terpene-generated peroxy radicals (RO2) with NO and HO2 as well as the autoxidation of terpene-generated RO2 species, are explored in this work. The main source of OH in Cyprus was its primary production from O3 photolysis during the day and HONO photolysis during early morning. Recycling contributed about one-third of the total OH production, and the maximum recycling efficiency was about 0.7. CO, which was the largest OH sink, was also the largest HO2 source. The lowest HOx production and losses occurred when the air masses had higher residence time over the oceans.
Formaldehyde (HCHO) is the most abundant aldehyde in the troposphere. While its background mixing ratio is mostly determined by the oxidation
of methane, in many environments, especially in the ...boundary layer, HCHO can have a large variety of precursors, in particular biogenic and
anthropogenic volatile organic compounds (VOCs) and their oxidation products. Here we present shipborne observations of HCHO, hydroxyl radical (OH)
and OH reactivity (R(OH)), which were obtained during the Air Quality and Climate Change in the Arabian Basin (AQABA) campaign in summer 2017. The loss
rate of HCHO was inferred from its reaction with OH, measured photolysis rates and dry deposition. In photochemical steady state, the HCHO loss is
balanced by production via OH-initiated degradation of VOCs, photolysis of oxygenated VOCs (OVOCs) and the ozonolysis of alkenes. The slope
αeff from a scatter plot of the HCHO production rate versus the product of OH and R(OH)eff (excluding inorganic
contribution) yields the fraction of OH reactivity that contributes to HCHO production. Values of αeff varied between less than
2 % in relatively clean air over the Arabian Sea and the southern Red Sea and up to 32 % over the polluted Arabian Gulf (also known as
Persian Gulf), signifying that polluted areas harbor a larger variety of HCHO precursors. The separation of R(OH)eff into individual
compound classes revealed that elevated values of αeff coincided with increased contribution of alkanes and OVOCs, with the highest
reactivity of all VOCs over the Arabian Gulf.
Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OH + isoprene (Reaction R1), OH +α-pinene (Reaction R2) ...and OH + Δ-3-carene (Reaction R3). Gas-phase rate coefficients were characterized by non-Arrhenius kinetics for all three reactions. For (R1), k1 (241–356 K) = (1.93±0.08) × 10−11exp{(466±12)∕T} cm3 molecule−1 s−1 was determined, with a room temperature value of k1 (297 K) = (9.3±0.4) × 10−11 cm3 molecule−1 s−1, independent of bath-gas pressure (5–200 Torr) and composition (M = N2 or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption cross section, σisoprene = (1.28±0.06) × 10−17 cm2 molecule−1 at λ = 184.95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative-rate determinations of k1 result in part from σ values used to derive the isoprene concentration in high-precision absolute determinations.Similar methods were used to determine rate coefficients (in 10−11 cm3 molecule−1 s−1) for (R2)–(R3): k2 (238–357 K) = (1.83±0.04) × exp{(330±6)∕T} and k3 (235–357 K) = (2.48±0.14) × exp{(357±17)∕T}. This is the first temperature-dependent dataset for (R3) and enables the calculation of reliable atmospheric lifetimes with respect to OH removal for e.g. boreal forest springtime conditions. Room temperature values of k2 (296 K) = (5.4±0.2) × 10−11 cm3 molecule−1 s−1 and k3 (297 K) = (8.1±0.3) × 10−11 cm3 molecule−1 s−1 were independent of bath-gas pressure (7–200 Torr, N2 or air) and in good agreement with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: σ = (1.54±0.08) × 10−17 cm2 molecule−1 for α-pinene and (2.40±0.12) × 10−17 cm2 molecule−1 for Δ-3-carene.
Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human ...activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.
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We present a chemical ionization quadrupole mass
spectrometer (CI-QMS) with a radio-frequency (RF) discharge ion source through
N2∕CH3I as a source of primary ions. In addition to the expected
...detection of PAN, peracetic acid (PAA) and ClNO2 through well-established
ion–molecule reactions with I− and its water cluster, the instrument is
also sensitive to SO2, HCl and acetic acid (CH3C(O)OH) through
additional ion chemistry unique to our ion source. We present ionization
schemes for detection of SO2, HCl and acetic acid along with
illustrative datasets from three different field campaigns underlining the
potential of the CI-QMS with an RF discharge ion source as an alternative to
210Po. The additional sensitivity to SO2 and HCl makes the CI-QMS
suitable for investigating the role of sulfur and chlorine chemistry in the
polluted marine and coastal boundary layer.
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During the summertime CYPHEX campaign (CYprus PHotochemical EXperiment 2014) in the eastern Mediterranean, multiple volatile organic compounds (VOCs) were measured from a 650 m hilltop site in ...western Cyprus (34° 57′ N/32° 23′ E). Periodic shifts in the northerly Etesian winds resulted in the site being alternately impacted by photochemically processed emissions from western (Spain, France, Italy) and eastern (Turkey, Greece) Europe. Furthermore, the site was situated within the residual layer/free troposphere during some nights which were characterized by high ozone and low relative humidity levels. In this study we examine the temporal variation of VOCs at the site. The sparse Mediterranean scrub vegetation generated diel cycles in the reactive biogenic hydrocarbon isoprene, from very low values at night to a diurnal median level of 80–100 pptv. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited weak diel cycles and were approximately an order of magnitude higher in mixing ratio (ca. 2.5–3 ppbv median level by day, range: ca. 1–8 ppbv) than the locally emitted isoprene and aromatic compounds such as benzene and toluene. Acetic acid was present at mixing ratios between 0.05 and 4 ppbv with a median level of ca. 1.2 ppbv during the daytime. When data points directly affected by the residual layer/free troposphere were excluded, the acid followed a pronounced diel cycle, which was influenced by various local effects including photochemical production and loss, direct emission, dry deposition and scavenging from advecting air in fog banks. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times (between 12 h and several days) of air masses originating from eastern and western Europe. Ozone and many OVOC levels were ∼ 20 and ∼ 30–60 % higher, respectively, in air arriving from the east. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol and acetone decreased with residence time in the marine boundary layer (MBL) with loss rate constants of 0.74 and 0.53 day−1 from eastern Europe and 0.70 and 0.34 day−1 from western Europe, respectively. Simulations using the EMAC model underestimate these loss rates. The missing sink in the calculation is most probably an oceanic uptake enhanced by microbial consumption of methanol and acetone, although the temporal and spatial variability in the source strength on the continents might play a role as well. Correlations between acetone and methanol were weaker in western air masses (r2 = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r2 = 0.73).
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