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In the urban area of Guangzhou, observations on aerosol light extinction effect were conducted at a monitoring site of the South China Institute of Environmental Sciences (SCIES) during April 2009, July 2009, October 2009 and January 2010. The main goal of these observations is to recognise the impact of relative humidity (RH) and particles number distribution on aerosol light extinction. PM<sub>2.5</sub> was sampled by Model PQ200 air sampler; ions and OC/EC in PM<sub>2.5</sub> were identified by the Dionex ion chromatography and the DRI model 2001 carbon analyser, respectively; particles number size distribution was measured by TSI 3321 APS, while total light scattering coefficient was measured by TSI 3563 Nephelometer. Chemical composition of PM<sub>2.5</sub> was reconstructed by the model ISORROPIA II. As a result, possible major components in PM<sub>2.5</sub> were (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, Na<sub>2</sub>SO<sub>4</sub>, K<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub>, HNO<sub>3</sub>, water, POM and EC. Regarding ambient RH, mass concentration of PM<sub>2.5</sub> ranged from 26.1 to 279.1 μg m<sup>−3</sup> and had an average of 94.8, 44.6, 95.4 and 130.8 μg m<sup>−3</sup> in April, July, October and January, respectively. With regard to the total mass of PM<sub>2.5</sub>, inorganic species, water, POM, EC and the Residual accounted for 34–47%, 19–31%, 14–20%, 6–8% and 8–17%, respectively. Under the assumption of "internal mixture", optical properties of PM<sub>0.5–20</sub> were estimated following the Mie Model. Optical refractive index, hygroscopic growth factor and the dry aerosol density required by the Mie Model were determined with an understanding of chemical composition of PM<sub>2.5</sub>. With these three parameters and the validated particles number size distribution of PM<sub>0.5–20</sub>, the temporal variation trend of optical property of PM<sub>0.5–20</sub> was estimated with good accuracy. The highest average of <i>b</i><sub>ep,pm0.5–20</sub> was 300 Mm<sup>−1</sup> in April while the lowest one was 78.6 Mm<sup>−1</sup> in July. Regarding size distribution of <i>b</i><sub>ep,pm0.5–20</sub>, peak value was almost located in the diameter range between 0.5 and 1.0 μm. Furthermore, hygroscopic growth of optical properties of PM<sub>0.5–20</sub> largely depended on RH. As RH increased, <i>b</i><sub>ep,pm0.5–20</sub> grew and favoured a more rapid growth when aerosol had a high content of inorganic water-soluble salts. Averagely, <i>f</i><sub>bep,pm0.5–20</sub> enlarged 1.76 times when RH increased from 20% to 90%. With regard to the temporal variation of ambient RH, <i>f</i><sub>bep,pm0.5–20</sub> was 1.29, 1.23, 1.14 and 1.26 on average in April, July, October and January, respectively.
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A modified cumulus convection parametrisation scheme is presented. This scheme computes the mass of air transported upward in a cumulus cell using conservation of moisture and a detailed distribution of convective precipitation provided by a reanalysis dataset. The representation of vertical transport within the scheme includes entrainment and detrainment processes in convective updrafts and downdrafts. Output from the proposed parametrisation scheme is employed in the National Institute for Environmental Studies (NIES) global chemical transport model driven by JRA-25/JCDAS reanalysis. The simulated convective precipitation rate and mass fluxes are compared with observations and reanalysis data. A simulation of the short-lived tracer <sup>222</sup>Rn is used to further evaluate the performance of the cumulus convection scheme. Simulated distributions of <sup>222</sup>Rn are evaluated against observations at the surface and in the free troposphere, and compared with output from models that participated in the TransCom-CH<sub>4</sub> Transport Model Intercomparison. From this comparison, we demonstrate that the proposed convective scheme in general is consistent with observed and modeled results.
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We use satellite observations of aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectrometer (MODIS) together with the GEOS-Chem global chemical transport model to contrast export of aerosols from East Asia and North America during 2004–2010. The GEOS-Chem model reproduces the spatial distribution and temporal variations of Asian aerosol outflow generally well, although a low bias (−30%) is found in the model fine mode AOD, particularly during summer. We use the model to identify 244 aerosol pollution export events from E. Asia and 251 export events from N. America over our 7-year study period. When these events are composited by season, we find that the AOD in the outflow is enhanced by 50–100% relative to seasonal mean values. The composite Asian plume splits into one branch going poleward to the Arctic in 3–4 days, with the other crossing the Pacific Ocean in 6–8 days. A fraction of the aerosols is trapped in the subtropical Pacific High during spring and summer. The N. American plume travels to the northeast Atlantic, reaching Europe after 4–5 days. Part of the composite plume turns anticyclonically in the Azores High, where it slowly decays. Both the Asian and N. American export events are favored by a dipole structure in sea-level pressure anomalies, associated with mid-latitude cyclone activity over the respective source regions. This dipole structure during outflow events is a strong feature for all seasons except summer, when convection becomes more important. The observed AOD in the E. Asian outflow exhibits stronger seasonality, with a spring maximum, than the N. American outflow, with a broad spring/summer maximum. The large spring AOD in the Asian outflow is the result of enhanced sulfate and dust aerosol concentrations, but is also due to a larger export efficiency of sulfate and SO<sub>2</sub> from the Asian boundary layer relative to the N. American boundary layer. While the N. American sulfate outflow is mostly found in the lower troposphere (1–3 km altitude), the Asian sulfate outflow occurs at higher altitudes (2–6 km). In the Asian outflow 42–59% of the sulfate column is present above 2 km altitude, with only 24–35% in the N. American outflow. We link this to the factor of 2–5 lower precipitation in the warm conveyor belts (WCB) of midlatitude cyclones over E. Asia compared to N. America. This relative lack of precipitation makes Asian WCB very efficient for injecting aerosols in the middle troposphere.
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Very short-lived substances (VSLS) still represent a major factor of uncertainty in the quantification of stratospheric bromine loading. One of the major obstacles for short-lived source gases in contributing to the stratosphere is generally thought to be loss of inorganic bromine (Br<sub>y</sub>) in the tropical tropopause layer (TTL) due to dehydration. We use sensitivity calculations with a three-dimensional chemistry transport model comprising a consistent parametrization of convective transport and a comprehensive chemistry scheme to investigate the associated processes. The model considers the two most important bromine VSLS, bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>). The organic bromine source gases as well as the resulting profile of inorganic bromine in the model are consistent with available observations. In contrast to its organic precursors, Br<sub>y</sub> is assumed to have a significant sorption capacity regarding sedimenting liquid or frozen particles thus the fraction of intact source gases during their ascent through the TTL is a critical factor. We find that source gas injection is the dominant pathway into the stratosphere, about 50% of CHBr<sub>3</sub> and 94% of CH<sub>2</sub>Br<sub>2</sub> is able to overcome the cold point tropopause at approximately 17 km altitude, modulated by the interannual variability of the vertical transport efficiency. In fact, our sensitivity calculations indicate that the extent of source gas injection of CHBr<sub>3</sub> is highly sensitive to the strength of convection and large-scale ascent; in contrast, modifying the photolysis or the destruction via OH yields a significantly smaller response. In principle, the same applies as well to CH<sub>2</sub>Br<sub>2</sub>, though it is considerably less responsive due to its longer lifetime. The next important aspect we identified is that the partitioning of available Br<sub>y</sub> from short-lived sources is clearly shifted away from HBr, according to our current state of knowledge the only member of the Br<sub>y</sub> family which is efficiently adsorbed on ice particles. This effect is caused by very efficient heterogeneous reactions on ice surfaces which reduce the HBr/Br<sub>y</sub> fraction below 15% at the tropical tropopause. Under these circumstances there is no significant loss of Br<sub>y</sub> due to dehydration in the model, VSLS contribute fully to stratospheric bromine. In addition, we conduct several sensitivity calculations to test the robustness of this result. If heterogeneous chemistry is ignored, the HBr/Br<sub>y</sub> fraction exceeds 50% and about 10% of bromine from VSLS is scavenged. Dehydration plays a minor role for Br<sub>y</sub> removal under the assumption that HOBr is efficiently adsorbed on ice as well since the heterogeneous reactions alter the partitioning equilibrium of Br<sub>y</sub> in favor of HOBr. In this case, up to 12% of bromine from VSLS is removed. Even in the extreme and unrealistic case that adsorbed species on ice particles are instantaneously removed the maximum loss of bromine does not exceed 25%. Assuming 6 parts per trillion by volume (pptv) of bromine short-lived source gases in convective updrafts, a value that is supported by observational data, we find a most likely contribution of VSLS to stratospheric bromine in the range of 4.5–6 pptv.
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We describe measurements of peroxyacetyl nitrate (CH<sub>3</sub>C(O)O<sub>2</sub>NO<sub>2</sub>, PAN) and peroxyacetic acid (CH<sub>3</sub>C(O)OOH, PAA) in the Boreal forest using iodide chemical ionization mass spectrometry (ICIMS). The measurements were made during the Hyytiälä United Measurement of Photochemistry and Particles – Comprehensive Organic Particle and Environmental Chemistry (HUMPPA-COPEC-2010) measurement intensive. Mixing ratios of PAN and PAA were determined by measuring the acetate ion signal (CH<sub>3</sub>C(O)O<sup>−</sup>, <i>m/z</i> = 59) resulting from reaction of CH<sub>3</sub>C(O)O<sub>2</sub> (from the thermal dissociation of PAN) or CH<sub>3</sub>C(O)OOH with iodide ions using alternatively heated and ambient temperature inlet lines. During some periods of high temperature (~ 30 °C) and low NO<sub>x</sub> (< 1 ppbv), PAA mixing ratios were similar to, or exceeded those of PAN and thus contributed a significant fraction of the total acetate signal. PAA is thus a potential interference for ICIMS measurements of PAN, and especially eddy covariance flux measurements in environments where the PAA flux is likely to be a significant proportion of the (short timescale) acetate ion variability. Within the range of mixing ratios of NO<sub>x</sub> measured during HUMPPA-COPEC, the modelled ratio of PAA-to-PAN was found to be sensitive to temperature (through the thermal decomposition rate of PAN) and the HO<sub>2</sub> mixing ratio, thus providing some constraint to estimates of photochemical activity and oxidation rates in the Boreal environment.
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The Eyjafjallajökull volcano in Iceland emitted a cloud of ash into the atmosphere during April and May 2010. Over the UK the ash cloud was observed by the FAAM BAe-146 Atmospheric Research Aircraft which was equipped with in-situ probes measuring the concentration of volcanic ash carried by particles of varying sizes. The UK Met Office Numerical Atmospheric-dispersion Modelling Environment (NAME) has been used to simulate the evolution of the ash cloud emitted by the Eyjafjallajökull volcano during the period 4–18 May 2010. In the NAME simulations the processes controlling the evolution of the concentration and particle size distribution include sedimentation and deposition of particles, horizontal dispersion and vertical wind shear. For travel times between 24 and 72 h, a 1/<i>t</i> relationship describes the evolution of the concentration at the centre of the ash cloud and the particle size distribution remains fairly constant. Although NAME does not represent the effects of microphysical processes, it can capture the observed decrease in concentration with travel time in this period. This suggests that, for this eruption, microphysical processes play a small role in determining the evolution of the distal ash cloud. Quantitative comparison with observations shows that NAME can simulate the observed column-integrated mass if around 4% of the total emitted mass is assumed to be transported as far as the UK by small particles (< 30 μm diameter). NAME can also simulate the observed particle size distribution if a distal particle size distribution that contains a large fraction of < 10 μm diameter particles is used, consistent with the idea that phraetomagmatic volcanoes, such as Eyjafjallajökull, emit very fine particles.
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Aerosol loading over Interior Alaska displays a strong seasonality, with pristine conditions generally prevailing during winter months. Long term aerosol research from the University of Alaska Fairbanks indicates that the period around April typically marks the beginning of the transition from winter to summer conditions. In April 2008, the NASA-sponsored "Arctic Research of the Composition of the Troposphere from Aircraft and Satellites" (ARCTAS) field campaign was conducted to analyze incursions of aerosols transported over Alaska and the Canadian North. In and around Fairbanks, Alaska, data concerning aerosol characteristics were gathered by polarization (0.693 μm) lidar, DRUM Impactor sampler, and balloon-borne optical particle counter. These data provide information on the vertical distribution and type of aerosol, their size distributions, the chemical nature of aerosol observed at the surface, and timing of aerosol loading. A detailed synoptic analysis placed these observations into their transport and source-region context. Evidence suggests four major aerosol loading periods in the 25 March–30 April 2008 timeframe: a period during which typical Arctic haze conditions prevailed, several days of extremely clear conditions, rapid onset of a period dominated by Asian dust with some smoke, and a period dominated by Siberian wildfire smoke. A focused case study analysis conducted on 19 April 2008 using a balloon-borne optical particle counter suggests that, on this day, the majority of the suspended particulate matter consisted of coarse mode desiccated aerosol having undergone long-range transport. Backtrack trajectory analysis suggests aged Siberian wildfire smoke. In the last week of April, concentrations gradually decreased as synoptic conditions shifted away from favoring transport to Alaska. An important result is a strong suggestion of an Asian dust incursion in mid-April that was not well identified in other ARCTAS measurements. The lidar and OPC-sonde unambiguously discern aerosols height stratification patterns indicative of long range transport. Identification of a dust component is suggested by DRUM sampler results, which indicate crustal species, and supported by synoptic and trajectory analysis, which indicates both a source-region lifting event and appropriate air-mass pathways.
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Continuous carbon monoxide (CO) total column densities above the Universidad Nacional Autónoma de México (UNAM) campus in Mexico City have been derived from solar absorption infrared spectroscopic measurements since October 2007. Its diurnal evolution is used in the present study in conjunction with other ground-based and satellite data to develop a top-down emission estimate of the annual CO emission of the Mexico City Metropolitan Area (MCMA). The growth-rate of the total column around noon under low ventilation conditions is calculated and allows us to derive the average surface emission-flux at UNAM, while similar measurements taken at the edge of the MCMA in Tecámac provide information on background CO levels in the Mexico basin. Based on 3 yr of measurements, CO column measurements from the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument are used to reconstruct the spatial distribution of this anthropogenic pollutant over the MCMA. The agreement between the measured columns of the satellite and ground-based measurements is excellent, particularly when a comparison strategy based on time-displaced air masses is used. The annual emission of the Mexico Megacity is estimated to be (2.15 ± 0.5) Tg yr<sup>−1</sup> for the year 2008, while the official inventory for that year reported 1.6 Tg yr<sup>−1</sup>. The difference is slightly higher than the conservative uncertainty estimated in this work suggesting that the emission might be underestimated by the conventional bottom-up method. A larger discrepancy is found in the spatial distribution of the emissions, when comparing the emission flux over UNAM (derived from the ground-based measurement) with that of the inventory integrated over a representative area. The methodology presented here represents a new and useful strategy to evaluate the contribution of megacities to the global anthropogenic gas emissions. Additionally, three different strategies to compare ground and space-based measurements above an inhomogeneous and strongly contaminated area like Mexico City are presented and discussed.
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We provide a regional characterization of coarse particulate matter (PM<sub>10–2.5</sub>) spanning the western United States based on the analysis of measurements from 50 sites reported in the US EPA Air Quality System (AQS) and two state agencies. We found that the observed PM<sub>10–2.5</sub> concentrations show significant spatial variability and distinct spatial patterns, associated with the distributions of land use/land cover and soil moisture. The highest concentrations were observed in the southwestern US, where sparse vegetation, shrublands or barren lands dominate with lower soil moistures, whereas the lowest concentrations were observed in areas dominated by grasslands, forest, or croplands with higher surface soil moistures. The observed PM<sub>10–2.5</sub> concentrations also show variable seasonal, weekly, and diurnal patterns, indicating a variety of sources and their relative importance at different locations. The observed results were compared to modeled PM<sub>10–2.5</sub> concentrations from an annual simulation using the Community Multiscale Air Quality modeling system (CMAQ) that has been designed for regulatory or policy assessments of a variety of pollutants including PM<sub>10</sub>, which consists of PM<sub>10–2.5</sub> and fine particulate matter (PM<sub>2.5</sub>). The model under-predicts PM<sub>10–2.5</sub> observations at 49 of 50 sites, among which 14 sites have annual observation means that are at least five times greater than model means. Model results also fail to reproduce their spatial patterns. Important sources (e.g. pollen, bacteria, fungal spores, and geogenic dust) were not included in the emission inventory used and/or the applied emissions were greatly under-estimated. Unlike the observed patterns that are more complex, modeled PM<sub>10–2.5</sub> concentrations show the similar seasonal, weekly, and diurnal pattern; the temporal allocations in the modeling system need improvement. CMAQ does not include organic materials in PM<sub>10–2.5</sub>; however, speciation measurements show that organics constitute a significant component. The results improve our understanding of sources and behavior of PM<sub>10–2.5</sub> and suggest avenues for future improvements to models that simulate PM<sub>10–2.5</sub> emissions, transport and fate.
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This paper describes the optimization and validation of a new simple method for the quantitative determination of water in atmospheric particulate matter (PM). The analyses are performed by using a coulometric Karl-Fisher system equipped with a controlled heating device; different water contributions are separated by the application of an optimized thermal ramp (three heating steps: 50–120 °C, 120–180 °C, 180–250 °C). <br><br> The analytical performance of the method was verified by using standard materials containing 5.55% and 1% by weight of water. The recovery was greater than 95%; the detection limit was about 20 μg. The method was then applied to NIST Reference Materials (NIST1649a, urban particulate matter) and to real PM<sub>10</sub> samples collected in different geographical areas. In all cases the repeatability was satisfactory (10–15%). <br><br> When analyzing the Reference Material, the separation of four different types of water was obtained. In real PM<sub>10</sub> samples the amount of water and its thermal profile differed as a function of the chemical composition of the dust. Mass percentages of 3–4% of water were obtained in most samples, but values up to about 15% were reached in areas where the chemical composition of PM is dominated by secondary inorganic ions and organic matter. High percentages of water were also observed in areas where PM is characterized by the presence of desert dust. <br><br> A possible identification of the quality of water released from the samples was tried by applying the method to some hygroscopic compounds that are likely contained in PM (pure SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, ammonium salts, carbohydrates and dicarboxylic acids) and by comparing the results with those obtained from field samples.
