Index Geophysics

Items

• Aerosol-precipitation interactions in the southern Appalachian Mountains

  There are many uncertainties associated with aerosol-precipitation interactions, particularly in mountain regions where a variety of processes at different spatial scales influence precipitation patterns. Aerosol-precipitation linkages were examined in the southern Appalachian Mountains, guided by the following research questions: (1) how do aerosol properties observed during precipitation events vary by season (e.g., summer vs. winter) and synoptic event type (e.g., frontal vs. non-frontal); and (2) what influence does air mass source region have on aerosol properties? Precipitation events were identified based on regional precipitation data and classified using a synoptic classification scheme developed for this study. Hourly aerosol data were collected at the Appalachian Atmospheric Interdisciplinary Research (AppalAIR) facility at Appalachian State University in Boone, NC (1110 m a.s.l., 36.215°, −81.680°). Backward air trajectories provided information on upstream atmospheric characteristics and source regions. During the warm season (June to September), greater aerosol loading dominated by larger particles was observed, while cool season (November to April) precipitation events exhibited overall lower aerosol loading with an apparent influence from biomass burning particles. Aerosol-induced precipitation enhancement may have been detected in each season, particularly during warm season non-frontal precipitation.

• The influence of boreal biomass burning emissions on the distribution of tropospheric ozone over North America and the North Atlantic during 2010

  We have analysed the sensitivity of the tropospheric ozone distribution over North America and the North Atlantic to boreal biomass burning emissions during the summer of 2010 using the GEOS-Chem 3-D global tropospheric chemical transport model and observations from in situ and satellite instruments. We show that the model ozone distribution is consistent with observations from the Pico Mountain Observatory in the Azores, ozonesondes across Canada, and the Tropospheric Emission Spectrometer (TES) and Infrared Atmospheric Sounding Instrument (IASI) satellite instruments. Mean biases between the model and observed ozone mixing ratio in the free troposphere were less than 10 ppbv. We used the adjoint of GEOS-Chem to show the model ozone distribution in the free troposphere over Maritime Canada is largely sensitive to NO_x emissions from biomass burning sources in Central Canada, lightning sources in the central US, and anthropogenic sources in the eastern US and south-eastern Canada. We also used the adjoint of GEOS-Chem to evaluate the Fire Locating And Monitoring of Burning Emissions (FLAMBE) inventory through assimilation of CO observations from the Measurements Of Pollution In The Troposphere (MOPITT) satellite instrument. The CO inversion showed that, on average, the FLAMBE emissions needed to be reduced to 89% of their original values, with scaling factors ranging from 12% to 102%, to fit the MOPITT observations in the boreal regions. Applying the CO scaling factors to all species emitted from boreal biomass burning sources led to a decrease of the model tropospheric distributions of CO, PAN, and NO_x by as much as −20 ppbv, −50 pptv, and −20 pptv respectively. The modification of the biomass burning emission estimates reduced the model ozone distribution by approximately −3 ppbv (−8%) and on average improved the agreement of the model ozone distribution compared to the observations throughout the free troposphere, reducing the mean model bias from 5.5 to 4.0 ppbv for the Pico Mountain Observatory, 3.0 to 0.9 ppbv for ozonesondes, 2.0 to 0.9 ppbv for TES, and 2.8 to 1.4 ppbv for IASI.

• Five-year record of atmospheric precipitation chemistry in urban Beijing, China

  To investigate the chemical characteristics of precipitation in the polluted urban atmosphere in Beijing and possible mechanisms influencing their variations, a total of 131 event-based precipitation samples were collected from March 2001 to August 2005. The concentrations of major ions in the samples were analyzed by using ion chromatography. Intermediate pH (6.1–7.3) was recorded in approximately two-thirds of the precipitation samples and acidic pH (4.2–5.6) in only 16% of the samples. However, the precipitation acidity was on the growth track and the process was likely being accelerated. SO₄^2−, NO₃^−, NH₄⁺, and Ca²⁺ were the most abundant ions in the precipitations, with their single volume-weighted mean (VWM) concentration all above 100 μeq l⁻¹. The two major anions and two major cations accounted for more than 80% of total anionic and cationic mass, respectively. The VWM SO₄^2− concentration decreased by 13% compared to that during 1995–1998, much less than the 58% reduction in the annual average SO₂ concentration from 1998 to 2005 in Beijing. What seems more counterintuitive is that the VWM NO₃^− concentration nearly doubled over the period although the annual average NO₂ concentration decreased by 5% from 1998 to 2005. These results imply that the conversion of gaseous precursors to acid compounds and/or the regional transport were reinforced over the decade. The average ratio of neutralizing potential to acidifying potential (i.e. NP/AP) was as high as 1.2 but experienced an evident decline trend. This was mainly ascribed to reduced input of NH₄⁺ and Ca²⁺ and increased input of NO₃^−. Furthermore, the equivalent mass ratio of NO₃^− to non-sea-salt SO₄^2− presented an increasing trend over the study period, suggesting that the contribution of NO₃^− to the precipitation acidity increased in recent years. However, the mean ratio was only 0.37 ± 0.11 in the study period, which is significantly lower than those reported in some metropolitan areas in developed countries. This shows that the precipitation acidity in Beijing was still dominantly from SO₂ while the SO₂ contribution was progressively substituted by NO_x.

• Ionic and carbonaceous compositions of PM10, PM2.5 and PM1.0 at Gosan ABC Superstation and their ratios as source signature

  PM_1.0, PM_2.5, and PM₁₀ were sampled at Gosan ABC Superstation on Jeju Island from August 2007 to September 2008. The carbonaceous aerosols were quantified with the thermal/optical reflectance (TOR) method, which produced five organic carbon (OC) fractions, OC1, OC2, OC3, OC4, and pyrolyzed organic carbon (OP), and three elemental carbon (EC) fractions, EC1, EC2, and EC3. The mean mass concentrations of PM_1.0, PM_2.5, and PM₁₀ were 13.7 μg m⁻³, 17.2 μg m⁻³, and 28.4 μg m⁻³, respectively. The averaged mass fractions of OC and EC were 23.0% and 10.4% for PM_1.0, 22.9% and 9.8% for PM_2.5, and 16.4% and 6.0% for PM₁₀. Among the OC and EC sub-components, OC2 and EC2+3 were enriched in the fine mode, but OC3 and OC4 in the coarse mode. The filter-based PM_1.0 EC agreed well with black carbon (BC) measured by an Aethalometer, and PM₁₀ EC was higher than BC, implying less light absorption by larger particles. EC was well correlated with sulfate, resulting in good relationships of sulfate with both aerosol scattering coefficient measured by Nephelometer and BC concentration. Our measurements of EC confirmed the definition of EC1 as char-EC emitted from smoldering combustion and EC2+3 as soot-EC generated from higher-temperature combustion such as motor vehicle exhaust and coal combustion (Han et al., 2010). In particular, EC1 was strongly correlated with potassium, a traditional biomass burning indicator, except during the summer, when the ratio of EC1 to EC2+3 was the lowest. We also found the ratios of major chemical species to be a useful tool to constrain the main sources of aerosols, by which the five air masses were well distinguished: Siberia, Beijing, Shanghai, Yellow Sea, and East Sea types. Except Siberian air, the continental background of the study region, Beijing plumes showed the highest EC1 (and OP) to sulfate ratio, which implies that this air mass had the highest net warming by aerosols of the four air masses. Shanghai-type air, which was heavily influenced by southern China, showed the highest sulfate enhancement. The highest EC2+3/EC1 ratio was found in aged East Sea air, demonstrating a significant influence of motor vehicle emissions from South Korea and Japan and less influence from industrial regions of China. The high ratio results from the longer residence time and less sensitivity to wet scavenging of EC2+3 compared to EC1, indicating that soot-EC could have greater consequence in regional-scale warming.

• Short-lived climate forcers from current shipping and petroleum activities in the Arctic

  Emissions of short-lived climate forcers (SLCF) in the Arctic region are expected to increase, notably from shipping and petroleum extraction. We here discuss changes in atmospheric SLCF concentrations and resulting radiative forcing (RF) from present day shipping and petroleum activities in the Arctic. The three-dimensional chemistry transport OsloCTM2 and a state of the art radiative forcing model are used, based on a coherent dataset of present day Arctic emissions. We find that the net RF of SLCF of shipping in the Arctic region is negative, mainly due to the direct and indirect RF effects of sulphate emissions, while the net RF of SLCF of petroleum extraction is positive, mainly due to the effects of black carbon aerosols in the air and deposited on snow. Strong seasonal variations of the sensitivities to emissions are found. In terms of annual mean values we find that the Arctic sensitivities to SLCF is similar to global average sensitivities. One exception to this is the stronger snow/ice albedo effect from BC emissions.

• Modelling future changes in surface ozone: a parameterized approach

  This study describes a simple parameterization to estimate regionally averaged changes in surface ozone due to past or future changes in anthropogenic precursor emissions based on results from 14 global chemistry transport models. The method successfully reproduces the results of full simulations with these models. For a given emission scenario it provides the ensemble mean surface ozone change, a regional source attribution for each change, and an estimate of the associated uncertainty as represented by the variation between models. Using the Representative Concentration Pathway (RCP) emission scenarios as an example, we show how regional surface ozone is likely to respond to emission changes by 2050 and how changes in precursor emissions and atmospheric methane contribute to this. Surface ozone changes are substantially smaller than expected with the SRES A1B, A2 and B2 scenarios, with annual global mean reductions of as much as 2 ppb by 2050 vs. increases of 4–6 ppb under SRES, and this reflects the assumptions of more stringent precursor emission controls under the RCP scenarios. We find an average difference of around 5 ppb between the outlying RCP 2.6 and RCP 8.5 scenarios, about 75% of which can be attributed to differences in methane abundance. The study reveals the increasing importance of limiting atmospheric methane growth as emissions of other precursors are controlled, but highlights differences in modelled ozone responses to methane changes of as much as a factor of two, indicating that this remains a major uncertainty in current models.

• Methanol-CO correlations in Mexico City pollution outflow from aircraft and satellite during MILAGRO

  The correlation between methanol (CH₃OH) and carbon monoxide (CO) is of particular interest for characterizing biogenic and anthropogenic emission sources of CH₃OH and other chemical species. Here, the CH₃OH/CO enhancement ratio (ΔCH₃OH/ΔCO) in the lower to middle troposphere is examined using coincident CH₃OH and CO observations from aircraft (NCAR C-130 and NASA DC-8) and from the Tropospheric Emission Spectrometer (TES) satellite during the MegaCity Initiative: Local and Global Research Observations (MILAGRO) in the Mexico City region in March 2006. ΔCH₃OH/ΔCO ratios from the two in-situ aircraft measurements are far higher than previously reported CH₃OH emission ratios relative to CO from US cities. This may reflect combustion of different fuel types in this area, and possibly photochemical production of CH₃OH in Mexico City outflow. TES CH₃OH and CO retrievals over the MILAGRO domain show relatively high sensitivity in the 600–800 hPa range, associated with Mexico City pollution outflow. The TES derived ΔCH₃OH/ΔCO ratios during MILAGRO are 18–24 ppt ppb⁻¹, which are similar to those observed from the DC-8 (26–39 ppt ppb⁻¹), but lower than the C-130 observations (41–55 ppt ppb⁻¹). Differences between the ΔCH₃OH/ΔCO ratios measured aboard the two aircraft preclude an absolute validation of the TES-derived ratios for this dataset. The ΔCH₃OH/ΔCO ratios observed from TES over this domain reflect bulk enhancements of CH₃OH and CO in Mexico City outflow. Although the TES measurements are not expected to resolve small-scale variability in the ΔCH₃OH/ΔCO ratio downwind of the strong source region of Mexico City, it is demonstrated that TES can clearly distinguish differences in the ΔCH₃OH/ΔCO ratio due to different source categories of CH₃OH. An example of this is shown by contrasting measurements over Mexico City (strong anthropogenic emissions) with those over the Amazon Basin (strong biogenic emissions). The results from this case study show the potential to gain insight into global sources of CH₃OH and related species from satellite observations, especially for regions and time periods where no in situ measurements are available.

• Assessing filtering of mountaintop CO2 mole fractions for application to inverse models of biosphere-atmosphere carbon exchange

  There is a widely recognized need to improve our understanding of biosphere-atmosphere carbon exchanges in areas of complex terrain including the United States Mountain West. CO₂ fluxes over mountainous terrain are often difficult to measure due to unusual and complicated influences associated with atmospheric transport. Consequently, deriving regional fluxes in mountain regions with carbon cycle inversion of atmospheric CO₂ mole fraction is sensitive to filtering of observations to those that can be represented at the transport model resolution. Using five years of CO₂ mole fraction observations from the Regional Atmospheric Continuous CO₂ Network in the Rocky Mountains (Rocky RACCOON), five statistical filters are used to investigate a range of approaches for identifying regionally representative CO₂ mole fractions. Test results from three filters indicate that subsets based on short-term variance and local CO₂ gradients across tower inlet heights retain nine-tenths of the total observations and are able to define representative diel variability and seasonal cycles even for difficult-to-model sites where the influence of local fluxes is much larger than regional mole fraction variations. Test results from two other filters that consider measurements from previous and following days using spline fitting or sliding windows are overly selective. Case study examples showed that these windowing-filters rejected measurements representing synoptic changes in CO₂, which suggests that they are not well suited to filtering continental CO₂ measurements. We present a novel CO₂ lapse rate filter that uses CO₂ differences between levels in the model atmosphere to select subsets of site measurements that are representative on model scales. Our new filtering techniques provide guidance for novel approaches to assimilating mountain-top CO₂ mole fractions in carbon cycle inverse models.

• SOA formation from the atmospheric oxidation of 2-methyl-3-buten-2-ol and its implications for PM2.5

  The formation of secondary organic aerosol (SOA) generated by irradiating 2-methyl-3-buten-2-ol (MBO) in the presence and/or absence of NO_x, H₂O₂, and/or SO₂ was examined. Experiments were conducted in smog chambers operated in either dynamic or static mode. A filter/denuder sampling system was used for simultaneously collecting gas- and particle-phase products. The structural characterization of gas and particulate products was investigated using BSTFA, BSTFA + PFBHA, and DNPH derivatization techniques followed by GC-MS and liquid chromatography analysis. This analysis showed the occurrence of more than 68 oxygenated organic compounds in the gas and particle phases, 28 of which were tentatively identified. The major components observed include 2,3-dihydroxyisopentanol (DHIP), 2-hydroxy-2-oxoisopentanol, 2,3-dihydroxy-3-methylbutanal, 2,3-dihydroxy-2-methylsuccinic acid, 2-hydroxy-2-methylpropanedioic acid, acetone, glyoxal, methylglyoxal, glycolaldehyde, and formaldehyde. Most of these oxygenated compounds were detected for the first time in this study. <br><br> While measurements of the gas-phase photooxidation products have been made, the focus of this work has been an examination of the particle phase. SOA from some experiments was analyzed for the organic mass to organic carbon ratio (OM/OC), the effective enthalpy of vaporization (&Delta;H_vap^eff), and the aerosol yield. Additionally, aerosol size, volume, and number concentrations were measured by a Scanning Mobility Particle Sizer coupled to a Condensation Particle Counter system. The OM/OC ratio was 2.1 in the MBO/H₂O₂ system. The ΔH_vap^eff was 41 kJ mol⁻¹, a value similar to that of isoprene SOA. The laboratory SOA yield measured in this study was 0.7% in MBO/H₂O₂ for an aerosol mass of 33 μg m⁻³. Secondary organic aerosol was found to be negligible under conditions with oxides of nitrogen (NO_x) present. Time profiles and proposed reaction schemes are provided for selected compounds. <br><br> The contribution of SOA products from MBO oxidation to ambient PM_2.5 was investigated by analyzing a series of ambient PM_2.5 samples collected in several places around the United States. In addition to the occurrence of several organic compounds in both field and laboratory samples, DHIP was found to originate only from the oxidation of MBO, and therefore this compound could potentially serve as a tracer for MBO SOA. Initial attempts have been made to quantify the concentrations of DHIP and other compounds based on surrogate compound calibrations. The average concentrations of DHIP in ambient PM_2.5 samples from Duke Forest in North Carolina ranged from zero during cold seasons to approximately 1 ng m⁻³ during warm seasons. This appears to be the first time that DHIP has been detected in ambient PM_2.5 samples. The occurrence of several other compounds in both laboratory and field samples suggests that SOA originating from MBO can contribute under selected ambient conditions to the ambient aerosol mainly in areas where MBO emissions are high.

• Analysis of the vertical structure and size distribution of dust aerosols over the semi-arid region of the Loess Plateau in China

  Using measurements of dual-wavelength polarisation lidar, particle sizer, and nephelometer from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), the properties of dust aerosol extinction coefficient, optical depth, depolarisation ratio, colour ratio, size distribution, and concentration over the semi-arid region of the Loess Plateau in north-western China are analysed in a case study of dust storms from 16–18 March 2010. The results show that dust aerosols are distributed mostly within the lower layer (below 3.0 km), with the dust aerosol extinction coefficient ranging from 0.1 to 1.0 km⁻¹. The average optical depth and depolarisation ratio are near 0.6 and 0.3, respectively, while the colour ratio ranges from 0.8 to 1.0. The mass size distribution of dust aerosols has two peaks at 0.7 μm and 5.0 μm, respectively, while the number size distribution of dust aerosols is log-normal with a maximum near 0.8 μm. Particles in the fine mode (<I>r</I> ≤ 2.5 μm) are predominant in the dust storm. Their number concentration decreases while those of particles in the moderate (2.5 μm < <I>r</I> ≤ 10.0 μm) and coarse (10.0 μm < <I>r</I> ≤ 20.0 μm) modes increase. <br><br> Based on Mie theory and the number size distribution of the aerosol, the dust aerosol scattering coefficient and its variation with particle size are calculated and analysed. A fairly close correlation is found with that measured by the nephelometer, for which the correlation coefficients are 0.89 and 0.94, respectively, at 520 and 700 nm. It shows a Gaussian distribution of dust aerosol scattering coefficient against effective diameter, with a fitting coefficient of 0.96 and centre diameter of 5.5 μm. The contribution percentages of aerosol within fine, moderate, and coarse modes to dust aerosol scattering coefficient are 20.95%, 62.93%, and 16.12%, respectively, meaning that PM₁₀ is a dominant factor in the dust aerosol scattering properties.