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Fluorescence characteristics of aerosol particles in a polluted atmosphere were studied using a wideband integrated bioaerosol spectrometer (WIBS-4A) in Nanjing, Yangtze River Delta area of China. We observed strong diurnal and day-to-day variations of fluorescent aerosol particles (FAPs). The average number concentrations of FAPs (1–15 µm) detected in the three WIBS measurement channels (FL1: 0.6 cm<sup>−3</sup>, FL2: 3.4 cm<sup>−3</sup>, FL3: 2.1 cm<sup>−3</sup>) were much higher than those observed in forests and rural areas, suggesting that FAPs other than bioaerosols were detected. We found that the number fractions of FAPs were positively correlated with the black carbon mass fraction, especially for the FL1 channel, indicating a large contribution of combustion-related aerosols. To distinguish bioaerosols from combustion-related FAPs, we investigated two classification schemes for use with WIBS data. Our analysis suggests a strong size dependence for the fractional contributions of different types of FAPs. In the FL3 channel, combustion-related particles seem to dominate the 1–2 µm size range while bioaerosols dominate the 2–5 µm range. The number fractions of combustion-related particles and non-combustion-related particles to total aerosol particles were ∼ 11 and ∼ 5 %, respectively.
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This work presents an analysis of the physical properties of sub-micrometer aerosol particles measured at the high Arctic site Villum Research Station, Station Nord (VRS), northeast Greenland, between July 2010 and February 2013. The study focuses on particle number concentrations, particle number size distributions and the occurrence of new particle formation (NPF) events and their seasonality in the high Arctic, where observations and characterization of such aerosol particle properties and corresponding events are rare and understanding of related processes is lacking.<br><br>A clear accumulation mode was observed during the darker months from October until mid-May, which became considerably more pronounced during the prominent Arctic haze months from March to mid-May. In contrast, nucleation- and Aitken-mode particles were predominantly observed during the summer months. Analysis of wind direction and wind speed indicated possible contributions of marine sources from the easterly side of the station to the observed summertime particle number concentrations, while southwesterly to westerly winds dominated during the darker months. NPF events lasting from hours to days were mostly observed from June until August, with fewer events observed during the months with less sunlight, i.e., March, April, September and October. The results tend to indicate that ozone (O<sub>3</sub>) might be weakly anti-correlated with particle number concentrations of the nucleation-mode range (10–30 nm) in almost half of the NPF events, while no positive correlation was observed. Calculations of air mass back trajectories using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for the NPF event days suggested that the onset or interruption of events could possibly be explained by changes in air mass origin. A map of event occurrence probability was computed, indicating that southerly air masses from over the Greenland Sea were more likely linked to those events.
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An important current problem in micrometeorology is the characterization of turbulence in the roughness sublayer (RSL), where most of the measurements above tall forests are made. There, scalar turbulent fluctuations display significant departures from the predictions of Monin–Obukhov similarity theory (MOST). In this work, we analyze turbulence data of virtual temperature, carbon dioxide, and water vapor in the RSL above an Amazonian forest (with a canopy height of 40 m), measured at 39.4 and 81.6 m above the ground under unstable conditions. We found that dimensionless statistics related to the rate of dissipation of turbulence kinetic energy (TKE) and the scalar variance display significant departures from MOST as expected, whereas the vertical velocity variance follows MOST much more closely. Much better agreement between the dimensionless statistics with the Obukhov similarity variable, however, was found for the subset of measurements made at a low zenith angle <i>Z</i>, in the range 0° < |<i>Z</i>| < 20°. We conjecture that this improvement is due to the relationship between sunlight incidence and the “activation–deactivation” of scalar sinks and sources vertically distributed in the forest. Finally, we evaluated the relaxation coefficient of relaxed eddy accumulation: it is also affected by zenith angle, with considerable improvement in the range 0° < |<i>Z</i>| < 20°, and its values fall within the range reported in the literature for the unstable surface layer. In general, our results indicate the possibility of better stability-derived flux estimates for low zenith angle ranges.
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Sulfate is an important component of global atmospheric aerosol, and has partially compensated for greenhouse gas-induced warming during the industrial period. The magnitude of direct and indirect radiative forcing of aerosols since preindustrial times is a large uncertainty in climate models, which has been attributed largely to uncertainties in the preindustrial environment. Here, we report observations of the oxygen isotopic composition (Δ<sup>17</sup>O) of sulfate aerosol collected in the remote marine boundary layer (MBL) in spring and summer in order to evaluate sulfate production mechanisms in pristine-like environments. Model-aided analysis of the observations suggests that 33–50 % of sulfate in the MBL is formed via oxidation by hypohalous acids (HOX = HOBr + HOCl), a production mechanism typically excluded in large-scale models due to uncertainties in the reaction rates, which are due mainly to uncertainties in reactive halogen concentrations. Based on the estimated fraction of sulfate formed via HOX oxidation, we further estimate that daily-averaged HOX mixing ratios on the order of 0.01–0.1 parts per trillion (ppt = pmol/mol) in the remote MBL during spring and summer are sufficient to explain the observations.
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Interannual variabilities in atmospheric ammonia (NH<sub>3</sub>) during the most recent 7–11 years were investigated at 14 sites across North America using the monitored data obtained from NAPS, CAPMoN and AMoN networks. The long-term average of atmospheric NH<sub>3</sub> ranged from 0.8 to 2.6 ppb, depending on location, at four urban and two rural/agricultural sites in Canada. The annual average at these sites did not show any deceasing trend with largely decreasing anthropogenic NH<sub>3</sub> emission. An increasing trend was actually identified from 2003 to 2014 at the downtown Toronto site using either the Mann–Kendall or the ensemble empirical mode decomposition method, but “no” or “stable” trends were identified at other sites. The ∼ 20 % increase during the 11-year period at the site was likely caused by changes in NH<sub>4</sub><sup>+</sup>–NH<sub>3</sub> partitioning and/or air–surface exchange process as a result of the decreased sulfur emission and increased ambient temperature. The long-term average from 2008 to 2015 was 1.6–4.9 ppb and 0.3–0.5 ppb at four rural/agricultural and at four remote US sites, respectively. A stable trend in NH<sub>3</sub> mixing ratio was identified at one rural/agricultural site while increasing trends were identified at three rural/agricultural (0.6–2.6 ppb, 20–50 % increase from 2008 to 2015) and four remote sites (0.3–0.5 ppb, 100–200 % increase from 2008 to 2015). Increased ambient temperature was identified to be a cause for the increasing trends in NH<sub>3</sub> mixing ratio at four out of the seven US sites, but what caused the increasing trends at other US sites needs further investigation.
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Fluorinated compounds such as NF<sub>3</sub> and C<sub>2</sub>F<sub>5</sub>Cl (CFC-115) are characterised by very large global warming potentials (GWPs), which result from extremely long atmospheric lifetimes and strong infrared absorptions in the atmospheric window. In this study we have experimentally determined the infrared absorption cross sections of NF<sub>3</sub> and CFC-115, calculated the radiative forcing and efficiency using two radiative transfer models and identified the effect of clouds and stratospheric adjustment. The infrared cross sections are within 10 % of previous measurements for CFC-115 but are found to be somewhat larger than previous estimates for NF3, leading to a radiative efficiency for NF<sub>3</sub> that is 25 % larger than that quoted in the Intergovernmental Panel on Climate Change Fifth Assessment Report. A whole atmosphere chemistry–climate model was used to determine the atmospheric lifetimes of NF<sub>3</sub> and CFC-115 to be (509 ± 21) years and (492 ± 22) years, respectively. The GWPs for NF<sub>3</sub> are estimated to be 15 600, 19 700 and 19 700 over 20, 100 and 500 years, respectively. Similarly, the GWPs for CFC-115 are 6030, 7570 and 7480 over 20, 100 and 500 years, respectively.
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The influence of the solar corpuscular radiation on weather is demonstrated by characterizing the corpuscular impact by means of the geomagnetic aa-index and the terrestrial response by European temperature data. Considering different spatial and vectorial circumstances the following conclusions can be drawn: 1. the efficiency of the corpuscular impact depends on the Sun-Earth attitude (semiannual fluctuation); 2. this regularity depends on the polarity of the solar main magnetic dipole field; and 3. the whole complex of phenomena also depends on the geographic position - it exists at European middle latitudes but it does not exist in northern and southern Europe. We conclude that the periods of differently or oppositely working mechanisms should be separated in order to recognize the regularities.
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A linear Eady model with a parameterization of heat influx from the lower boundary is studied analytically in order to obtain the characteristics of baroclinic normal modes modified by this non-adiabatic source. The results display a secondary maximum of growth rate at high wave numbers and a range of absolutely unstable waves, thus suggesting that the property observed among mid-latitude explosive cyclones of being near-stationary in the phase of maximum growth may be captured by this representation of the air-sea energy exchange.
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Based on radiative transfer calculations, it is studied whether polar stratospheric clouds (PSCs) can be detected by the new Global Ozone Monitoring Experiment (GOME) on board the second European Research Satellite (ERS-2) planned to be launched in 1995. It is proposed to identify PSC-covered areas by use of an indicator, the Normalized Radiance Difference (NRD), which relates the difference of two spectral radiances at 0.515 µm and 0.67 µm to one radiance measured in the centre of the oxygen A-band at 0.76 µm. Simulations are carried out for two solar zenith angles, θ=78.5° and θ=86.2°. They indicate that, in presence of PSCs and with increasing solar zenith angles above θ=80°, the NRD decrease to values clearly below those derived under conditions of a cloud-free stratosphere. Results for θ=86.2° show that the method is successful independent of existing tropospheric clouds, of different tropospheric aerosol loadings, and of surface albedos. Results for θ=78.5° illustrate that PSC detection under conditions of smaller solar zenith angles θ80° needs additional information about tropospheric clouds.
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Ray-tracing techniques are used to computationally investigate the propagation of gravity waves through the middle atmosphere, as characterized by the vertically varying CIRA-86 wind and temperature models, plus a tidal wind model that varies temporally as well as vertically. For the wave parameters studied here, the background wind variation has a much stronger influence on the ray path and changes in wave characteristics than does the temperature variation. The temporal variation of the tidal component of the wind changes the observed frequency, sometimes substantially, while leaving the intrinsic frequency unaltered. It also renders temporary any critical levels that occur in the tidal region. Different starting times for the rays relative to the tidal phase provide different propagation environments, so that the temporary critical levels appear at different heights. The lateral component of the tidal wind is shown to advect propagating wave packets; the maximum lateral displacement of a packet varies inversely with its vertical group velocity. Time-dependent effects are more pronounced in local winter than in summer.
