NO2 Photolysis Research Articles

Tropospheric aerosol as a reactive intermediate

In tropospheric chemistry, secondary organic aerosol (SOA) is deemed an end product. Here, on the basis of new evidence, we make the case that SOA is a key reactive intermediate. We present laboratory results on the catalysis by carboxylate anions of the disproportionation of NO2 'on water': 2NO2 + H2O = HONO + NO(9-) + H+ (R1), and supporting quantum chemical calculations, which we apply to reinterpret recent reports on (i) HONO daytime source strengths vis-à-vis SOA anion loadings and (ii) the weak seasonal and latitudinal dependences of NO(x) decay kinetics over several megacities. HONO daytime generation via R1 should track sunlight because it is generally catalyzed by the anions produced during the photochemical oxidation of pervasive gaseous pollutants. Furthermore, by proceeding on the everpresent substrate of aquated airborne particulates, R1 can eventually overtake the photolysis of NO2: NO2 + hv = NO + O(3P) (R2), at large zenith angles. Thus, since R1 leads directly to *OH-radical generation via HONO photolysis: HONO + hv = NO + *OH, whereas the path initiated by R2 is more circuitous and actually controlled by the slower photolysis of O3: O3 + hv (+H2O) = O2 + 2*OH, the competition between R1 and R2 provides a mechanistic switch that buffers *OH concentrations and NO2 decay (via R1 and/or NO2 + *OH = HNO3) from actinic flux variations.

Response of Atmospheric Biomarkers to NOx-Induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M Dwarf Stars

Understanding whether M dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N(2)), which leads to production of nitrogen oxides (NO(x)) in the planetary atmosphere, hence affecting biomarkers such as ozone (O(3)). We apply a stationary model, that is, without a time dependence; hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. We estimate the NO(x) production using an air shower approach and evaluate the implications using a climate-chemical model of the planetary atmosphere. O(3) formation proceeds via the reaction O+O(2)+M→O(3)+M. At high NO(x) abundances, the O atoms arise mainly from NO(2) photolysis, whereas on Earth this occurs via the photolysis of molecular oxygen (O(2)). For the flaring case, O(3) is mainly destroyed via direct titration, NO+O(3)→NO(2)+O(2), and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O(3), Rayleigh scattering by the main atmospheric gases (O(2), N(2), and CO(2)) became more important for shielding the planetary surface from UV radiation. A major result of this work is that the biomarker O(3) survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide (N(2)O) could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.

Analysis of stratospheric NO<sub>2</sub> trends above Jungfraujoch using ground-based UV-visible, FTIR, and satellite nadir observations

Abstract. The trend in stratospheric NO2 column at the NDACC (Network for the Detection of Atmospheric Composition Change) station of Jungfraujoch (46.5° N, 8.0° E) is assessed using ground-based FTIR and zenith-scattered visible sunlight SAOZ measurements over the period 1990 to 2009 as well as a composite satellite nadir data set constructed from ERS-2/GOME, ENVISAT/SCIAMACHY, and METOP-A/GOME-2 observations over the 1996–2009 period. To calculate the trends, a linear least squares regression model including explanatory variables for a linear trend, the mean annual cycle, the quasi-biennial oscillation (QBO), solar activity, and stratospheric aerosol loading is used. For the 1990–2009 period, statistically indistinguishable trends of −3.7 ± 1.1% decade−1 and −3.6 ± 0.9% decade−1 are derived for the SAOZ and FTIR NO2 column time series, respectively. SAOZ, FTIR, and satellite nadir data sets show a similar decrease over the 1996–2009 period, with trends of −2.4 ± 1.1% decade−1, −4.3 ± 1.4% decade−1, and −3.6 ± 2.2% decade−1, respectively. The fact that these declines are opposite in sign to the globally observed +2.5% decade−1 trend in N2O, suggests that factors other than N2O are driving the evolution of stratospheric NO2 at northern mid-latitudes. Possible causes of the decrease in stratospheric NO2 columns have been investigated. The most likely cause is a change in the NO2/NO partitioning in favor of NO, due to a possible stratospheric cooling and a decrease in stratospheric chlorine content, the latter being further confirmed by the negative trend in the ClONO2 column derived from FTIR observations at Jungfraujoch. Decreasing ClO concentrations slows the NO + ClO → NO2 + Cl reaction and a stratospheric cooling slows the NO + O3 → NO2 + O2 reaction, leaving more NOx in the form of NO. The slightly positive trends in ozone estimated from ground- and satellite-based data sets are also consistent with the decrease of NO2 through the NO2 + O3 → NO3 + O2 reaction. Finally, we cannot rule out the possibility that a strengthening of the Dobson-Brewer circulation, which reduces the time available for N2O photolysis in the stratosphere, could also contribute to the observed decline in stratospheric NO2 above Jungfraujoch.

Global ab Initio Potential Energy Surfaces for Low-Lying Doublet States of NO3

We report analytical global potential energy surfaces (PESs) for three low-lying doublet states (D0, D1, and D2) of NO3. The fits are made on roughly 74000 MS-CAS(17e,13o)PT2/aug-cc-pVTZ calculations of electronic energies, where these PESs are invariant of permutations of oxygen atoms. The surfaces describe two roaming pathways for NO3 → NO2-----O → NO + O2 involving different electronic states discovered in the photolysis of NO3 [ Xiao, H. Y. et al. J. Phys. Chem. Lett.2011, 2, 934 ]. These pathways become accessible at excess energy of ∼210 kJ/mol above the ground-state global minimum of NO3. The ab initio data below 360 kJ/mol are reproduced very well by the fitted PESs with the fitting rms errors of less than 5.5 kJ/mol for all the three states. Moreover, key local minima and energy profiles along the roaming pathways on the fitted PESs are compared with those on the ab initio PESs. In addition, potential contour maps in the roaming region are also compared. These careful evaluations of the fitted PESs suggest that the present fitted PESs are well suited for future dynamics calculations of this system.

Variations in surface ozone and NOx at Kannur: a tropical, coastal site in India

Continuous measurements of surface ozone (O3), NOx (NO + NO2) and meteorological parameters have been made in Kannur (11.9 °N, 75.4 °E, 5 m asl), India from November 2009 to October 2010. It was observed that O3 and NOx showed distinct diurnal and seasonal variabilities at this site. The annual average diurnal profile of O3 showed a peak of (30.3 ± 10.4) ppbv in the late afternoon and a minimum of (3.2 ± 0.7) ppbv in the early morning. The maximum value of O3 mixing ratio was observed in winter (44 ± 3.1) ppbv and minimum during monsoon (18.46 ± 3.5) ppbv. The rate of production of O3 was found to be higher in December (10.1 ppbv/h) and lower in July (1.8 ppbv/h) during the time interval 0800–1000 h. A correlation coefficient of 0.52 for the relationship between O3 and [NO2]/[NO] reveals the role of NO2 photolysis that generates O3 at this site. The correlation between O3 and meteorological parameters indicate the influence of seasonal changes on O3 production. Investigations were further extended to explore the week day weekend variations in O3 mixing ratio at an urban site reveals the enhancement of O3. The variations of O3 mixing ratio with seasonal air mass flows were elucidated with the aid of backward air trajectories. This study also indicates how vapor phase organic species present in the ambient air at this location may influence the complex chemistry involving (VOCs) that enhances the production of O3 at this location.

The dynamic chamber method: trace gas exchange fluxes (NO, NO<sub>2</sub>, O<sub>3</sub>) between plants and the atmosphere in the laboratory and in the field

Abstract. We describe a dynamic chamber system to determine reactive trace gas exchange fluxes between plants and the atmosphere under laboratory and, with small modifications, also under field conditions. The system allows measurements of the flux density of the reactive NO-NO2-O3 triad and additionally of the non-reactive trace gases CO2 and H2O. The chambers are made of transparent and chemically inert wall material and do not disturb plant physiology. For NO2 detection we used a highly NO2 specific blue light converter coupled to chemiluminescence detection of the photolysis product, NO. Exchange flux densities derived from dynamic chamber measurements are based on very small concentration differences of NO2 (NO, O3) between inlet and outlet of the chamber. High accuracy and precision measurements are therefore required, and high instrument sensitivity (limit of detection) and the statistical significance of concentration differences are important for the determination of corresponding exchange flux densities, compensation point concentrations, and deposition velocities. The determination of NO2 concentrations at sub-ppb levels (<1 ppb) requires a highly sensitive NO/NO2 analyzer with a lower detection limit (3σ-definition) of 0.3 ppb or better. Deposition velocities and compensation point concentrations were determined by bi-variate weighted linear least-squares fitting regression analysis of the trace gas concentrations, measured at the inlet and outlet of the chamber. Performances of the dynamic chamber system and data analysis are demonstrated by studies of Picea abies L. (Norway Spruce) under field and laboratory conditions. Our laboratory data show that the quality selection criterion based on the use of only significant NO2 concentration differences has a considerable impact on the resulting compensation point concentrations yielding values closer to zero. The results of field experiments demonstrate the need to consider photo-chemical reactions of NO, NO2, and O3 inside the chamber for the correct determination of the exchange flux densities, deposition velocities, as well as compensation point concentrations. Under our field conditions NO2 deposition velocities would have been overestimated up to 80%, if NO2 photolysis has not been considered. We also quantified the photolysis component for some previous NO2 flux measurements. Neglecting photo-chemical reactions may have changed reported NO2 compensation point concentration by 10%. However, the effect on NO2 deposition velocity was much more intense, ranged between 50 and several hundreds percent. Our findings may have consequences for the results from previous studies and ongoing discussion of NO2 compensation point concentrations.

Daytime HONO vertical gradients during SHARP 2009 in Houston, TX

Abstract. Nitrous Acid (HONO) plays an important role in tropospheric chemistry as a precursor of the hydroxyl radical (OH), the most important oxidizing agent in the atmosphere. Nevertheless, the formation mechanisms of HONO are still not completely understood. Recent field observations found unexpectedly high daytime HONO concentrations in both urban and rural areas, which point to unrecognized, most likely photolytically enhanced HONO sources. Several gas-phase, aerosol, and ground surface chemistry mechanisms have been proposed to explain elevated daytime HONO, but atmospheric evidence to favor one over the others is still weak. New information on whether HONO formation occurs in the gas-phase, on aerosol, or at the ground may be derived from observations of the vertical distribution of HONO and its precursor nitrogen dioxide, NO2, as well as from its dependence on solar irradiance or actinic flux. Here we present field observations of HONO, NO2 and other trace gases in three altitude intervals (30–70 m, 70–130 m and 130–300 m) using UCLA's long path DOAS instrument, as well as in situ measurements of OH, NO, photolysis frequencies and solar irradiance, made in Houston, TX, during the Study of Houston Atmospheric Radical Precursor (SHARP) experiment from 20 April to 30 May 2009. The observed HONO mixing ratios were often ten times larger than the expected photostationary state with OH and NO. Larger HONO mixing ratios observed near the ground than aloft imply, but do not clearly prove, that the daytime source of HONO was located at or near the ground. Using a pseudo steady-state (PSS) approach, we calculated the missing daytime HONO formation rates, Punknown, on four sunny days. The NO2-normalized Punknown, Pnorm, showed a clear symmetrical diurnal variation with a maximum around noontime, which was well correlated with actinic flux (NO2 photolysis frequency) and solar irradiance. This behavior, which was found on all clear days in Houston, is a strong indication of a photolytic HONO source. [HONO]/[NO2] ratios also showed a clear diurnal profile, with maxima of 2–3% around noon. PSS calculations show that this behavior cannot be explained by the proposed gas-phase reaction of photoexcited NO2 (NO2*) or any other gas-phase or aerosol photolytic process occurring at similar or longer wavelengths than that of HONO photolysis. HONO formation by aerosol nitrate photolysis in the UV also seems to be unlikely. Pnorm correlated better with solar irradiance (average R2 = 0.85/0.87 for visible/UV) than with actinic flux (R2 = 0.76) on the four sunny days, clearly pointing to HONO being formed at the ground rather than on the aerosol or in the gas-phase. In addition, the observed [HONO]/[NO2] diurnal variation can be explained if the formation of HONO depends on solar irradiance, but not if it depends on the actinic flux. The vertical mixing ratio profiles, together with the stronger correlation with solar irradiance, support the idea that photolytically enhanced NO2 to HONO conversion on the ground was the dominant source of HONO in Houston.

Determination of Hydroxyl Radical in Seto Inland Sea and its Potential to Degrade Irgarol

Photo-production rates and overall scavenging rate constants of the highly reactive hydroxyl radicals (•OH) were measured by irradiation of surface seawater samples collected from the Seto Inland Sea, Japan. Photo-production rates ofOH ranged from 7.2 × 10 -12 M s -1 to 424 × 10 12 M s -1 and scavenging rate constants were 1.3 - 4.1×10 6 s -1 . The steady state concentrations ofOH in seawater, which were calculated from the photo-production rates and scavenging rate constants were in the range 2.6 -189 × 10 -18 M. Estimation from the scavenging rate constant showed that the lifetime ofOH in seawater ranged from 0.29 ×10 -6 s to 0.55 ×10 -6 s. Nitrates and hydrogen peroxide contributions to OH radical formation rates were negligible in the samples studied. However, direct photolysis of NO2 - accounted for up to 73% of the observed •OH formation rates. The rate constant for the reactionOH with irgarol was found to be (9.7 ± 0.52) × 10 9 M - 1 s -1. Based on the steady state concentration ofOH, the calculated half-lives for Irgarol due to the reaction with hydroxyl radicals in the Seto Inland Sea were in the range 4.3-318 days.

Study of inhibition mechanism of [formula omitted] on photoreduction of Hg(II) in artificial water

Photoreduction is of immense importance to mercury transfer from water to atmosphere. In this investigation, the reduction of HgCl2 and Hg(NO3)2 under dark, natural light and ultraviolet radiation (UV) was estimated using series of laboratory experiments. The result showed that the role of light irradiation in HgCl2 reduction was entirely different from that in Hg(NO3)2. The rate of HgCl2 reduction was in the order of UV>natural light>dark, compared with dark>natural light>UV of Hg(NO3)2. The experiments of mercury reduction treated by KNO3 and CH3OH, which was used as a NO3- provider and a OH scavenger respectively, indicated that OH which was produced via direct photolysis of NO3- in water should be responsible for the unexpected inhibition of UV to Hg(NO3)2 reduction. The reduction of Hg(NO3)2 under different radiation intensity each of UVA, UVB and UVC was investigated. The result showed that UVB induced the highest mercury reduction while mercury reduction increased with light intensity. In addition, the kinetic study of mercury reduction under natural light was conducted through both concentration gradient method and trial method. The pseudo first rate constant was estimated to be 7×10−4min−1.

A CFD modeling study of the impacts of NOx and VOC emissions on reactive pollutant dispersion in and above a street canyon

Abstract A computational fluid dynamics (CFD) model that includes the carbon bond mechanism IV (CBM-IV) is developed and used to investigate reactive pollutant dispersion in and above a street canyon with an aspect ratio of 1. Fourteen emission scenarios of NOx and volatile organic compounds (VOCs) are considered. Dispersion types are classified into NO-type, NO2-type, and O3-type dispersion that exhibit concentration maxima at the street bottom, near the center of the street canyon, and above the street canyon, respectively. For the base emission scenario, the number of reactive species is 9 in the NO-type dispersion, 10 in the NO2-type dispersion, and 15 in the O3-type dispersion. As the NOx emission level decreases or the VOC emission level increases, some species in the O3-type dispersion are shifted to the NO2-type dispersion. The VOC-to-NOx emission ratio is found to be an important factor in determining the transition of dispersion type. In this transition process, OH plays a key role through a radical chain including HO2, RO, and RO2. Because of their high OH reactivities, XYL (xylene) and OLE (olefin carbon bond) among VOCs are largely responsible for the transition of dispersion type. The O3 sensitivity is examined by reducing NOx or VOC emission level by a half. Because the NO titration of O3 is more pronounced than the NO2 photolysis and the radical chain process in the street canyon, the O3 concentration therein is negatively correlated with the NOx emission level and weakly correlated with the VOC emission level. As a result, the street canyon is a negatively NOx-sensitive regime.

The HNO<sub>3</sub> forming branch of the HO<sub>2</sub> + NO reaction: pre-industrial-to-present trends in atmospheric species and radiative forcings

Abstract. Recent laboratory measurements have shown the existence of a HNO3 forming branch of the HO2 + NO reaction. This reaction is the main source of tropospheric O3, through the subsequent photolysis of NO2, as well as being a major source of OH. The branching of the reaction to HNO3 reduces the formation of these species significantly, affecting O3 abundances, radiative forcing and the oxidation capacity of the troposphere. The Oslo CTM2, a three-dimensional chemistry transport model, is used to calculate atmospheric composition and trends with and without the new reaction branch. Results for the present day atmosphere, when both temperature and pressure effects on the branching ratio are accounted for, show an 11 % reduction in the calculated tropospheric burden of O3, with the main contribution from the tropics. An increase of the global, annual mean methane lifetime by 10.9 %, resulting from a 14.1 % reduction in the global, annual mean OH concentration is also found. Comparisons with measurements show that including the new branch improves the modelled O3 in the Oslo CTM2, but that it is not possible to conclude whether the NOy distribution improves. We model an approximately 11 % reduction in the tropical tropospheric O3 increase since pre-industrial times, and a 4 % reduction of the increase in total tropospheric burden. Also, an 8 % decrease in the trend of OH concentrations is calculated, when the new branch is accounted for. The radiative forcing due to changes in O3 over the industrial era was calculated as 0.33 W m−2, reducing to 0.26 W m−2 with the new reaction branch. These results are significant, and it is important that this reaction branching is confirmed by other laboratory groups.

Temporal and Spatial Variations of NOx and Ozone Concentrations in Seoul during the Solar Eclipse of 22 July 2009

Abstract The temporal and spatial variations of NO, NO2, and O3 concentrations in Seoul, South Korea, during the solar eclipse of 22 July 2009 are investigated by analyzing data measured at 25 environmental monitoring stations. The NO2 concentration increases and the NO and O3 concentrations decrease because the efficiency of NO2 photolysis decreases during the solar eclipse. About an hour after the maximum obscuration, the reduction in the average O3 concentration over Seoul is estimated to be 45%. The maximum reduction in the O3 concentration downwind of the NOx source area is higher and occurs later than that in the downtown region. Deviations from the NO–NO2–O3 photostationary state in the downwind region are larger than those in the downtown region. This result implies that, in addition to the photochemical effect, the effect of transport by winds increases the reduction potential of the O3 concentration in the downwind region during the solar eclipse.

Modeling of photolysis rates over Europe: impact on chemical gaseous species and aerosols

Abstract. This paper evaluates the impact of photolysis rate calculation on simulated European air composition and air quality. In particular, the impact of the cloud parametrisation and the impact of aerosols on photolysis rates are analysed. Photolysis rates are simulated using the Fast-JX photolysis scheme and gas and aerosol concentrations over Europe are simulated with the regional chemistry-transport model Polair3D of the Polyphemus platform. The photolysis scheme is first used to update the clear-sky tabulation of photolysis rates used in the previous Polair3D version. Important differences in photolysis rates are simulated, mainly due to updated cross-sections and quantum yields in the Fast-JX scheme. In the previous Polair3D version, clouds were taken into account by multiplying the clear-sky photolysis rates by a correction factor. In the new version, clouds are taken into account more accurately by simulating them directly in the photolysis scheme. Differences in photolysis rates inside clouds can be large but outside clouds, and especially at the ground, differences are small. To take into account the impact of aerosols on photolysis rates, Polair3D and Fast-JX are coupled. Photolysis rates are updated every hour. Large impact on photolysis rates is observed at the ground, decreasing with altitude. The aerosol specie that impact the most photolysis rates is dust especially in south Europe. Strong impact is also observed over anthropogenic emission regions (Paris, The Po and the Ruhr Valley) where mainly nitrate and sulphate reduce the incoming radiation. Differences in photolysis rates lead to changes in gas concentrations, with the largest impact simulated on OH and NO concentrations. At the ground, monthly mean concentrations of both species are reduced over Europe by around 10 to 14% and their tropospheric burden by around 10%. The decrease in OH leads to an increase of the life-time of several species such as VOC. NO2 concentrations are not strongly impacted and O3 concentrations are mostly reduced at the ground (−3%). O3 peaks are systematically decreased because of the NO2 photolysis rate coefficient decrease. Not only gas are impacted but also secondary aerosols, due to changes in gas precursors concentrations. However changes in aerosol species concentrations often compensate each other resulting in a low impact on PM10 and PM2.5 concentrations (lower than 2%). The changes in gas concentrations at the ground induced by the modification of photolysis rates (by aerosols and clouds) are compared to changes induced by 29 different model parametrisations in Roustan et al. (2010). Among the 31 model parametrisations, "including aerosols on photolysis rates calculation" has the strongest impact on OH concentrations and on O3 bias in July. In terms of air quality, ground concentrations (NO2, O3, PM10) are compared with measurements. Changes arising from cloud parametrisation are small. Simulation performances are often slightly better when including aerosol in photolysis rates calculation. The systematic O3 peak reduction leads to large differences in the exceedances of the European O3 standard as calculated by the model, in better agreement with measurements. The number of exceedances of the information and the alert threshold is divided by 2 when the aerosol impact on photochemistry is simulated. This shows the importance of taking into account aerosols impact on photolysis rates in air quality studies.

A Lagrangian particle model with chemical reactions: application in real atmosphere

In this work, a Lagrangian particle model able to account for simple chemical reactions between NO and O3 has been improved to consider the photolysis of NO2. A system of chemical equations is numerically solved on a Eulerian grid, while the particle trajectories are moved in a Lagrangian frame. The NOx emissions of a power plant in real atmosphere, situated in a complex topography environment, have been considered as a test case. The simulated episodes refer to the diurnal time, when the ultraviolet radiation activates the NO2. Comparisons between NO/NO2’s concentrations ratio are presented in terms of scatter plots and statistical indexes analysis.

Influence of solar eclipse of 15 January 2010 on surface ozone

Abstract This study pertains to the variations observed in the mixing ratios of surface ozone, its prominent precursor NOx∗ and the meteorological parameters (solar radiation, temperature, relative humidity and wind speed) during the annular solar eclipse that occurred on 15 January 2010 at Kannur (11.9°N, 75.4°E, 5 m amsl), a tropical coastal site in the southern region of India. The solar eclipse started at 11:05 IST (IST is 5.5 h ahead of UTC), reached to the maximum obscuration at 13:20 IST and ended at 15:05 IST. The observations made at Kannur show influences of the solar eclipse phenomenon on the surface ozone and NOx∗ mixing ratios. A sharp decline in the surface ozone was observed during the eclipse, due to the decreased efficiency of the photochemical ozone formation. The NO2∗ levels were found to increase during the eclipse period while the NO levels remained unchanged. The eclipse induced reduction in surface ozone and enhancement in NOx∗are estimated to be 57.5% and 62.5% respectively. Reductions in the air temperature, relative humidity and wind speed were also observed during the event of solar eclipse. Simulation from a chemical box model indicates about 94% reduction in the NO2 photolysis rates during the eclipse period, which is leading to about 59% reduction in the surface ozone. Observations as well as model simulations indicate that the reduced photochemical ozone production from NO2 photolysis is possibly the main driver of ozone reduction during the eclipse at this site.

NO<sub>2</sub> photolysis frequencies in street canyons

Abstract. Photolysis frequencies for NO2 are modeled for the conditions in urban streets, which are taken into account as canyons with variable height and width. The effect of a street canyon is presented with absolute values and as a ratio RJ of the photolysis frequency within the street compared to that with free horizon. This allows further use of the existing photolysis parameterizations. Values are presented for variable solar elevation and azimuth angles, varying atmospheric conditions and different street properties. The NO2 photolysis frequency in a street depends strongly on the relative width of the street and its orientation towards the sun. Averaged over atmospheric conditions and street orientation, the NO2 photolysis frequency is reduced in comparison with the values for free horizon: to less than 20% for narrow skyscraper streets, to about 40% for typical urban streets, and only to about 80% for garden streets. A parameterization with the global solar irradiance is given for the averaged RJ values.

Evidence of Roaming Dynamics and Multiple Channels for Molecular Elimination in NO3 Photolysis

We report correlated state distributions arising from NO3 photolysis at 588 nm using velocity map ion imaging. State-selected NO images reveal clear evidence for two dissociation pathways for the NO + O2 channel. Vibrationally excited O2 (3Σg−, ν = 5−10) is formed in coincidence with low rotational states of NO (2Π) in the dominant mechanism. We also observe a small channel consisting of low vibrational levels of O2 (3Σg−, ν = 0−4) coincident with high NO rotational states. We discuss the results in the context of the recent studies of formaldehyde dissociation and postulate the involvement of roaming-type dynamics.

Evaluation of ultraviolet light-emitting diodes for detection of atmospheric NO2 by photolysis - chemiluminescence

Commercially available ultraviolet light-emitting diodes (UV-LEDs) are evaluated in the laboratory as a light source for photolysis of atmospheric nitrogen dioxide (NO2) followed by chemiluminescence detection (P-CL) of the resulting nitric oxide (NO). Sensitivity, selectivity, and engineering simplicity of three UV-LED sources are compared. The most powerful source uses two 9-W Nichia LED modules and provides an NO2 photolysis frequency (j) of 8.4 s−1 corresponding to a nine-fold improvement over a 100-W Hg arc lamp; this source provides the most sensitivity, but requires water cooling. A pair of Hamamatsu 0.25-W LED modules provides intermediate sensitivity with photolysis efficiency comparable to a 100-W Hg arc lamp. The Hamamatsu LEDs require no additional cooling and are a simple and inexpensive package, providing sensitivity and stability appropriate for long-term, unattended measurements at remote surface sites. A Droplet Measurement Technologies Blue-Light Converter (BLC) represents the most complete off-the-shelf system available for atmospheric measurements of NO2. The BLC provides the least sensitivity of the LED-based converters, but is less subject to interference from HONO. Implementing the Nichia LEDs into the NOAA WP-3D aircraft P-CL instrument, in conjunction with improved sample gas handling, improves instrument time response by a factor of eight and permits 1-Hz retrieval of ambient NO and NO2 using a single detector simply by modulating the LEDs on and off. The performance and stability of the Nichia LEDs are further assessed during the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May and June of 2010.

Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance

Abstract. Representative values of the atmospheric NO2 photolysis frequency j(NO2) are required for the adequate calculation and interpretation of NO and NO2 concentrations and exchange fluxes near the surface. Direct measurements of j(NO2) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j(NO2) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j(NO2) and G. We have measured j(NO2) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept = 0): j(NO2)=(1+α)× (B1×G+B2×G2), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients: B1=(1.47± 0.03)×10-5 W−1 m2 s−1 and B2=(-4.84±0.31)×10-9 W−2 m4 s−1 can be used to estimate ground-level j(NO2) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j(NO2) residual of the empirical function is ±6×10-4 s−1(2σ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α<0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.

Photochemical Formation of Hydroxyl Radicals in Red Soil-Polluted Seawater on the North of Okinawa Island, Japan

Land development has caused runoff of red soil into the ocean on the north side of Okinawa Island, Japan. In an attempt to clarify the impacts of this “red soil pollution” on the oxidizing power of seawater, we studied the formation of hydroxyl radical (•OH), the most potent oxidant in the environment, in red soil-polluted waters using a 313-nm monochromatic light. •OH was photochemically formed in the red soil-polluted water samples, and the formation rates of •OH decreased as salinity increased, i.e., as red soil-polluted river water gets mixed with seawater. The photo-formation rates of •OH showed good correlations with dissolved Fe concentrations (R 2 = 0.96) and [NO2 −] + [NO3 −] concentrations (R 2 = 0.87), while a negative and weak correlation was found with dissolved organic carbon concentrations (R = −0.78). Theoretical calculation showed that direct photolysis of NO3 −, Fe(OH)2+, and hydrogen peroxide all together accounted for less than 10% of the observed •OH formation in the red soil-polluted waters. Comparison between filtered and unfiltered samples showed that red soil particles were not the main sources of •OH, and the photolysis of NO2 − could account for at most 78% of the observed •OH formation rates. We found that the Fenton’s reaction (a reaction between Fe(II) and H2O2) could possibly account for the observed formation of •OH in the red soil-polluted waters.