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Measurements of aerosol, N<sub>2</sub>O and OCS made in the Northern Hemisphere below 21 km altitude following the eruption of Pinatubo are presented and analyzed. After September 1999, the oxidation of OCS and sedimentation of particles in the extra-tropical overworld north of 45 N are found to maintain the aerosol in a steady state. This analysis empirically links precursor gas to aerosol abundance throughout this region. These processes are tracked with age-of-air which offers advantages over tracking as a function of latitude and altitude. In the extra-tropical, lowermost stratosphere, normalized volume distributions appear constant in time after the fall of 1999. Exchange with the troposphere is important in understanding aerosol evolution there. Size distributions of volcanically perturbed aerosol are included to distinguish between volcanic and non-volcanic conditions. This analysis suggests that model failures to correctly predict OCS and aerosol properties below 20 km in the Northern Hemisphere extra tropics result from inadequate descriptions of atmospheric circulation.
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Inverse modelling of carbon sources and sinks requires an accurate quality estimate of the modelling framework to obtain a realistic estimate of the inferred fluxes and their uncertainties. So-called "representation errors" result from our inability to correctly represent point observations with simulated average values of model grid cells. They may add substantial uncertainty to the interpretation of atmospheric CO<sub>2</sub> mixing ratio data. We simulated detailed variations in the CO<sub>2</sub> mixing ratios with a high resolution (2 km) mesoscale model (RAMS) to estimate the representation errors introduced at larger model grid sizes of 10–100 km. We found that meteorology is the main driver of representation errors in our study causing spatial and temporal variations in the error estimate. Within the nocturnal boundary layer, the representation errors are relatively large and mainly caused by unresolved topography at lower model resolutions. During the day, convective structures, mesoscale circulations, and surface CO<sub>2</sub> flux variability were found to be the main sources of representation errors. Interpreting observations near a mesoscale circulation as representative for air with the correct footprint relative to the front can reduce the representation error substantially. The remaining representation error is 0.5–1.5 ppm at 20–100 km resolution.
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A statistical analysis for the comparability of water (H<sub>2</sub>O) and ozone (O<sub>3</sub>) data sets sampled during the SPURT aircraft campaigns and the MOZAIC passenger aircraft flights is presented. The Kolmogoroff-Smirnoff test reveals that the distribution functions from SPURT and MOZAIC trace gases differ from each other with a confidence of 95%. A variance analysis shows a different variability character in both trace gas data sets. While the SPURT H<sub>2</sub>O data only contain atmospheric processes variable on a diurnal or synoptical timescale, MOZAIC H<sub>2</sub>O data also reveal processes, which vary on inter-seasonal and seasonal timescales. The SPURT H<sub>2</sub>O data set does not represent the full MOZAIC H<sub>2</sub>O variance in the UT/LS for climatological investigations, whereas the variance of O<sub>3</sub> is much better represented. SPURT H<sub>2</sub>O data are better suited in the stratosphere, where the MOZAIC RH sensor looses its sensitivity.
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A challenge for the quantitative analysis of tropospheric nitrogen dioxide (NO<sub>2</sub>) column data from satellite observations is posed partly by the lack of satellite-independent observations for validation. We performed such observations of the tropospheric NO<sub>2</sub> column using the ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique in the North China Plain (NCP) from 29 May to 29 June, 2006. Comparisons between tropospheric NO<sub>2</sub> columns measured by MAX-DOAS and the Ozone Monitoring Instrument (OMI) onboard the Aura satellite indicate that OMI data (the standard product, version 3) over NCP may have a positive bias of 1.6×10<sup>15</sup> molecules cm<sup>−2</sup> (20%), yet within the uncertainty of the OMI data. Combining these results with literature validation results for the US, Europe, and Pacific Ocean suggests that a bias of +20%/−30% is a reasonable estimate, accounting for different regions.
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In this study, the spatio-temporal and seasonal distributions of EOS/Terra Moderate Resolution Imaging Spectroradiometer (MODIS)-derived aerosol optical depth (AOD) over East Asia were analyzed in conjunction with US EPA Models-3/CMAQ v4.3 modeling. In this study, two MODIS AOD products (τ<sub>MODIS</sub>: τ<sub>M-BAER</sub> and τ<sub>NASA</sub>) retrieved through a modified Bremen Aerosol Retrieval (M-BAER) algorithm and NASA collection 5 (C005) algorithm were compared with the AOD (τ<sub>CMAQ</sub>) that was calculated from the US EPA Models-3/CMAQ model simulations. In general, the CMAQ-predicted AOD values captured the spatial and temporal variations of the two MODIS AOD products over East Asia reasonably well. Since τ<sub>MODIS</sub> cannot provide information on the aerosol chemical composition in the atmosphere, different aerosol formation characteristics in different regions and different seasons in East Asia cannot be described or identified by τ<sub>MODIS</sub> itself. Therefore, the seasonally and regionally varying aerosol formation and distribution characteristics were investigated by the US EPA Models-3/CMAQ v4.3 model simulations. The contribution of each particulate chemical species to τ<sub>MODIS</sub> and τ<sub>CMAQ</sub> showed strong spatial, temporal and seasonal variations. For example, during the summer episode, τ<sub>MODIS</sub> and τ<sub>CMAQ</sub> were mainly raised due to high concentrations of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> over Chinese urban and industrial centers and secondary organic aerosols (SOAs) over the southern parts of China, whereas during the late fall and winter episodes, τ<sub>MODIS</sub> and τ<sub>CMAQ</sub> were higher due largely to high levels of NH<sub>4</sub>NO<sub>3</sub> formed over the urban and industrial centers, as well as in areas with high NH<sub>3</sub> emissions. τ<sub>CMAQ</sub> was in general larger than τ<sub>MODIS</sub> during the year, except for spring. The high biases (τ<sub>CMAQ</sub>>τ<sub>MODIS</sub>) may be due to the excessive formation of both (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> (summer episode) and NH<sub>4</sub>NO<sub>3</sub> (fall and winter episodes) over China, possibly from the use of overestimated values for NH<sub>3</sub> emissions in the CMAQ modeling. According to CMAQ modeling, particulate NH<sub>4</sub>NO<sub>3</sub> made a 14% (summer) to 54% (winter) contribution to σ<sub>ext</sub> and τ<sub>CMAQ</sub>. Therefore, the importance of NH<sub>4</sub>NO<sub>3</sub> in estimating τ should not be ignored, particularly in studies of the East Asian air quality. In addition, the accuracy of τ<sub>M-BAER</sub> and τ<sub>NASA</sub> was evaluated by a comparison with the AOD (τ<sub>AERONET</sub>) from the AERONET sites in East Asia. Both τ<sub>M-BAER</sub> and τ<sub>NASA</sub> showed a strong correlation with τ<sub>AERONET</sub> around the 1:1 line (<I>R</I>=0.79), indicating promising potential for the application of both the M-BAER and NASA aerosol retrieval algorithms to satellite-based air quality monitoring studies in East Asia.
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Sulfur dioxide emissions from the Popocatépetl volcano in central Mexico were measured during the MILAGRO field campaign in March 2006. A stationary scanning DOAS (Differential Optical Absorption Spectrometer) was used to monitor the SO<sub>2</sub> emissions from the volcano and the results were compared with traverses done with a COSPEC from the ground and a DOAS instrument on board an ultra-light aircraft. Daytime evolutions as well as day-to-day variation of the SO<sub>2</sub> emissions are reported. A value of 2.45±1.39 Gg/day of SO<sub>2</sub> is reported from all the daily averages obtained during the month of March 2006, with large variation in maximum and minimum daily averages of 5.97 and 0.56 Gg/day, respectively. The large short-term fluctuations in the SO<sub>2</sub> emissions obtained could be confirmed through 2-D visualizations of the SO<sub>2</sub> plume measured with a scanning imaging infrared spectrometer. This instrument, based on the passive detection of thermal radiation from the volcanic gas and analysis with FTIR spectrometry, is used for the first time for plume visualization of a specific volcanic gas. A 48-h forward trajectory analysis indicates that the volcanic plume was predominantly directed towards the Puebla/Tlaxcala region (63%), followed by the Mexico City and Cuernavaca/Cuautla regions with 19 and 18% occurrences, respectively. 25% of the modeled trajectories going towards the Puebla region reached altitudes lower than 4000 m a.s.l. but all trajectories remained over this altitude for the other two regions.
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Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive aerosol single scattering albedo, ϖ<sub>0, λ</sub>, as a function of wavelength, λ. We find that in the near-UV spectral range (250 to 400 nm) ϖ<sub>0, λ</sub> is much lower compared to ϖ<sub>0, λ</sub> at 500 nm indicating enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or gas cannot account for this enhanced absorption leaving the organic carbon component of the aerosol (OA) as the most likely absorber. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred ϖ<sub>0, λ</sub> to estimate the Mass Absorption Cross section (MAC) for the organic aerosol. We find that the MAC is about 10.5 m<sup>2</sup>/g at 300 nm and falls close to zero at about 500 nm; values that are roughly consistent with other estimates of organic aerosol MAC. These MAC values can be considered as "radiatively correct", because when used in radiative transfer calculations, the calculated irradiances/actinic fluxes match those measured at the wavelengths considered here. For an illustrative case study described here, we estimate that the light absorption by the "brown" (organic) carbonaceous aerosol can add about 40% to the light absorption of black carbon in Mexico City. This contribution will vary depending on the relative abundance of organic aerosol relative to black carbon. Furthermore, our analysis indicates that organic aerosol would slow down photochemistry by selectively scavenging the light reaching the ground at those wavelengths that drive photochemical reactions. Finally, satellite retrievals of trace gases that are used to infer emissions currently assume that the MAC of organic carbon is zero. For trace gases that are retrieved using wavelengths shorter then 420 nm (i.e. SO<sub>2</sub>, HCHO, halogenoxides, NO<sub>2</sub>), the assumption of non-zero MAC values will induce an upward correction to the inferred emissions. This assumption will be particularly relevant in polluted urban atmospheres and areas of biomass burning where organic aerosols are particularly abundant.
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Proton transfer reaction mass spectrometry (PTR-MS) is a technique for online measurements of atmospheric concentrations, or volume mixing ratios, of volatile organic compounds (VOCs). This paper gives a detailed description of our measurement, calibration, and volume mixing ratio calculation methods, which have been designed for long-term stand-alone field measurements by PTR-MS. The PTR-MS instrument has to be calibrated regularly with a gas standard to ensure the accuracy needed in atmospheric VOC measurements. We introduce a novel method for determining an instrument specific relative transmission curve using information obtained from a calibration. This curve enables consistent mixing ratio calculation for VOCs not present in a calibration gas standard. Our method proved to be practical, systematic, and sensitive enough to capture changes in the transmission over time. We also propose a new approach to considering the abundance of H<sub>3</sub>O<sup>+</sup>H<sub>2</sub>O ions in mixing ratio calculation. The approach takes into account the difference in the transmission efficiencies for H<sub>3</sub>O<sup>+</sup> and H<sub>3</sub>O<sup>+</sup>H<sub>2</sub>O ions. To illustrate the functionality of our measurement, calibration, and calculation methods, we present a one-month period of ambient mixing ratio data measured in a boreal forest ecosystem at the SMEAR II station in southern Finland. During the measurement period 27 March–26 April 2007, the hourly averages of the mixing ratios were 0.051–0.57 ppbv for formaldehyde, 0.19–3.1 ppbv for methanol, 0.038–0.39 ppbv for benzene, and 0.020–1.3 ppbv for monoterpenes. The detection limits for the hourly averages were 0.020, 0.060, 0.0036, and 0.0092 ppbv, respectively.
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The evaporation rate of D<sub>2</sub>O has been determined by Raman thermometry of a droplet train (12–15 μm diameter) injected into vacuum (~10<sup>-5</sup> torr). The cooling rate measured as a function of time in vacuum was fit to a model that accounts for temperature gradients between the surface and the core of the droplets, yielding an evaporation coefficient (γ<sub>e</sub>) of 0.57±0.06. This is nearly identical to that found for H<sub>2</sub>O (0.62±0.09) using the same experimental method and model, and indicates the existence of a kinetic barrier to evaporation. The application of a recently developed transition-state theory (TST) model suggests that the kinetic barrier is due to librational and hindered translational motions at the liquid surface, and that the lack of an isotope effect is due to competing energetic and entropic factors. The implications of these results for cloud and aerosol particles in the atmosphere are discussed.
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In many investigations of tropospheric chemistry information about the two dimensional distribution of trace gases on a small scale (e.g. tens to hundreds of metres) is highly desirable. An airborne instrument based on imaging Differential Optical Absorption Spectroscopy has been built to map the two dimensional distribution of a series of relevant trace gases including NO<sub>2</sub>, HCHO, C<sub>2</sub>H<sub>2</sub>O<sub>2</sub>, H<sub>2</sub>O, O<sub>4</sub>, SO<sub>2</sub>, and BrO on a scale of 100 m. <br><br> Here we report on the first tests of the novel aircraft instrument over the industrialised South African Highveld, where large variations in NO<sub>2</sub> column densities in the immediate vicinity of several sources e.g. power plants or steel works, were measured. The observed patterns in the trace gas distribution are interpreted with respect to flux estimates, and it is seen that the fine resolution of the measurements allows separate sources in close proximity to one another to be distinguished.
