ALBERT

Description: We investigate the upper tropospheric distribution of methane (CH4) at low latitudes based on the analysis of air samples collected from aboard passenger aircraft. The distribution of CH4 exhibits spatial and seasonal differences, such as the pronounced seasonal cycles over tropical Asia and elevated mixing ratios over central Africa. Over Africa, the correlations of methane, ethane, and acetylene with carbon monoxide indicate that these high mixing ratios originate from biomass burning as well as from biogenic sources. Upper tropospheric mixing ratios of CH4 were modeled using a chemistry transport model. The simulation captures the large-scale features of the distributions along different flight routes, but discrepancies occur in some regions. Over Africa, where emissions are not well constrained, the model predicts a too steep interhemispheric gradient. During summer, efficient convective vertical transport and enhanced emissions give rise to a large-scale CH4 maximum in the upper troposphere over subtropical Asia. This seasonal (monsoonal) cycle is analyzed with a tagged tracer simulation. The model confirms that in this region convection links upper tropospheric mixing ratios to regional sources on the Indian subcontinent, subtropical East Asia, and Southeast Asia. This type of aircraft data can therefore provide information about surface fluxes.

Description: Carbon monoxide (CO) and other atmospheric trace constituents were measured from onboard an Airbus 340-600 passenger aircraft in the upper troposphere (UT) between south China and the Philippines during Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) flights from May 2005 until March 2008. A total of 132 events having CO enhancements were observed in the UT over the region during the 81 CARIBIC flights from Frankfurt, Germany, to Manila, Philippines, with a stopover in Guangzhou, China. Among these, 51 high-CO events with enhancements more than 50 ppb above background were observed. For these events enhancements ranged from 52.7 to 221.3 ppb and persisted for 3 to 78 min (∼40 to 1200 km), indicating an influence of strong pollution from biomass/biofuel/fossil fuel burning on the trace gas composition of the UT. Back trajectory analysis shows that south China, the Indochinese Peninsula, and the Philippines/Indonesia are the main source regions of the high-CO events. The composition of air parcels originating from south China was found to be primarily influenced by anthropogenic urban/industrial emissions, while emissions from biomass/biofuel burning contributed substantially to CO enhancements from the Indochinese Peninsula. During the Philippines/Indonesia events, air parcel composition suggests contributions from both biomass/biofuel burning and urban/industrial sources. Long-range transport of air parcels from northeast Asia and India also contributed to CO enhancements in the UT over the region. The general features of regional influence, typical cases, and the contributions of biomass/biofuel burning and anthropogenic emissions are presented and discussed to characterize the air parcels during the observed high-CO events.

Description: Flask samples from two sites in East Asia, Tae-Ahn Peninsula, Korea (TAP), and Shangdianzi, China (SDZ), were measured for trace gases including CO2, CO and fossil fuel CO2 (CO2ff, derived from Δ14CO2 observations). The five-year TAP record shows high CO2ff when local air comes from the Korean Peninsula. Most samples, however, reflect air masses from Northeastern China with lower CO2ff. Our small set of SDZ samples from winter 2009/2010 have strongly elevated CO2ff. Biospheric CO2 contributes substantially to total CO2 variability at both sites, even in winter when non-fossil CO2 sources (including photosynthesis, respiration, biomass burning and biofuel use) contribute 20–30% of the total CO2 enhancement. Carbon monoxide (CO) correlates strongly with CO2ff. The SDZ and TAP far-field (China influenced) samples have CO: CO2ff ratios (RCO:CO2ff) of 47 ± 2 and 44 ± 3 ppb/ppm respectively, consistent with recent bottom-up inventory estimates and other observational studies. Locally influenced TAP samples fall into two distinct data sets, ascribed to air sourced from South Korea and North Korea. The South Korea samples have low RCO:CO2ff of 13 ± 3 ppb/ppm, slightly higher than bottom-up inventories, but consistent with emission ratios for other developed nations. We compare our CO2ff observations with modeled CO2ff using the FLEXPART Lagrangian particle dispersion model convolved with a bottom-up CO2ff emission inventories. The modeled annual mean CO2ff mole fractions are consistent with our observations when the model inventory includes the reported 63% increase in Chinese emissions from 2004 to 2010, whereas a model version which holds Chinese emissions flat is unable to replicate the observations.

Description: A modified Ambient Ion Monitor - Ion Chromatograph was utilized to monitor the composition of water-soluble PM 2.5 and precursor gases at the Bakersfield, CA supersite during CalNex in May and June of 2010. The observations were used to investigate inorganic gas/particle partitioning, to derive an empirical relationship between ammonia emissions and temperature, and to assess the performance of the Community Multiscale Air Quality (CMAQ) model. The water-soluble PM 2.5 maximized in the morning and in the evening because of gas/particle partitioning and possibly regional transport. Among the PM 2.5 constituents, pNO 3 − was the dominant chemical species with campaign average mass loading of 0.80 µg m −3 , and the mass loadings of pNH 4 + and pSO 4 2− were 0.46 µg m −3 and 0.53 µg m −3 , respectively. The observed HNO 3(g) levels had an average of 0.14 ppb. Sub-ppb levels of SO 2(g) were measured, consistent with the absence of major emission sources in the region. Measured NH 3(g) had an average of 19.7 ppb over the campaign, and demonstrated a strong relationship with temperature. Observations of ammonia were used to derive an empirical enthalpy for volatilization of 30.8 ± 2.1 kJ mol −1 . The gas/particle partitioning of semi-volatile PM 2.5 composition was driven by meteorological factors, and limited by total nitrate (TN) in this region. CMAQ model output exhibited significant biases in the predicted concentrations of pSO 4 2− , NH 3(g), and TN. The largest model bias was in HNO 3(g) , with an overprediction of an order of magnitude, which may be due to missing HNO 3(g) sinks such as reactive uptake on dust in the CMAQ framework.

Description: [1]  National Ambient Air Quality Standards (NAAQS) have been set for PM 2.5 due to its association with adverse health effects. PM 2.5 design values in the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) of California exceed NAAQS levels, and NH 4 + and NO 3 - make up the largest fraction of total PM 2.5 mass on polluted days. Here we evaluate fine-scale simulations of PM 2.5 NH 4 + and NO 3 - with the Community Multiscale Air Quality model using measurements from routine networks and the CalNex-2010 campaign. The model correctly simulates broad spatial patterns of NH 4 + and NO 3 - including the elevated concentrations in eastern SoCAB. However, areas for model improvement have been identified. NH 3 emissions from livestock and dairy facilities appear to be too low, while those related to waste disposal in western SoCAB may be too high. Analyses using measurements from flights over SoCAB suggest that problems with NH 3 predictions can influence NO 3 - predictions there. Offline ISORROPIA II calculations suggest that over-predictions of NH x in Pasadena cause excessive partitioning of total nitrate to the particle phase overnight, while under-predictions of Na + cause too much partitioning to the gas phase during the day. Also, the model seems to under-estimate mixing during the evening boundary layer transition leading to excessive nitrate formation on some nights. Overall, the analyses demonstrate fine-scale variations in model performance within and across the air basins. Improvements in inventories and spatial allocations of NH 3 emissions and in parameterizations of sea-spray emissions, evening mixing processes, and heterogeneous ClNO 2 chemistry could improve model performance.

Description: We present spatial and temporal variations of methyl chloride (CH 3 Cl) in the upper troposphere (UT) observed mainly by the CARIBIC passenger aircraft for the years 2005–2011. The CH 3 Cl mixing ratio in the UT over Europe was higher than that observed at a European surface baseline station throughout the year, indicative of a persistent positive vertical gradient at NH mid latitudes. A series of flights over Africa and South Asia show that CH 3 Cl mixing ratios increase toward tropical latitudes, and the observed UT CH 3 Cl level over these two regions and the Atlantic was higher than that measured at remote surface sites. Strong emissions of CH 3 Cl in the tropics combined with meridional air transport through the UT may explain such vertical and latitudinal gradients. Comparisons with carbon monoxide (CO) data indicate that non-combustion sources in the tropics dominantly contribute to forming the latitudinal gradient of CH 3 Cl in the UT. We also observed elevated mixing ratios of CH 3 Cl and CO in air influenced by biomass burning in South America and Africa, and the enhancement ratios derived for CH 3 Cl to CO in those regions agree with previous observations. In contrast, correlations indicate a high CH 3 Cl to CO ratio of 2.9 ± 0.5 ppt ppb -1 in the Asian summer monsoon anticyclone and domestic biofuel emissions in South Asia are inferred to be responsible. We estimated the CH 3 Cl emission in South Asia to be 134 ± 23 Gg Cl yr -1 , which is higher than a previous estimate due to the higher CH 3 Cl to CO ratio observed in this study.

Description: The structural evolution of low-density snow under a high temperature gradient over a short period usually takes place in the surface layers on clear, cold nights. In this paper, X-ray computed microtomography (micro-CT) was combined with numerical simulations to investigate the temperature gradient metamorphism (TGM) on different types of snow. Precipitation particles (PP), small rounded particles (RGsr) and large rounded particles (RGlr) were each observed in high temperature gradients (100–500 K m −1 ) at a mean temperature of −4 °C. The specific surface area (SSA) was used to characterize the TGM, which were influenced by both the magnitude of the temperature gradient and the initial snow structures. PP samples experienced a logarithmic decrease of SSA with time, and the depth hoar structures created under high TGM (500 K m −1 ) have higher SSA compared to those under lower TGM. Unlike previous observations, for initial rounded and connected structures, like RGlr samples, the SSA increased during TGM. Simulated normal vapor flux distributions for different snow types were used to help understand the structural evolution under TGM. Understanding the SSA increase is important in order to predict the enhanced uptake of chemical species by snow or increase in snow albedo.

Description: [1]  The relationship between snow microstructure and its mechanical properties is crucial for modeling the microstructural cause of snow avalanches. In this paper, the knowledge gap between snow microstructural evolution and overburden is bridged by combining compression tests with X-ray computed micro-tomography (micro CT). Continuous compression tests and interrupted compression tests were performed on three types of snow (fresh low temperature snow, fresh high temperature snow, and sintered low temperature snow with densities ranging from 100-350 kg/m 3 ) to investigate the effects of initial structure on both the stress-displacement curve and structural changes during the compression test. 3D micro CT images and SEM images showed that well-connected structures of sintered snow contribute to a more rapid stress increase during deformation. By combining the analyses of the specific surface area, structure model index, and structure thickness, it is shown that compression tests at a fixed temperature, loading rate, and density snow metamorphism can be regarded as an accelerated pressure sintering process. However, when extremely low-density snow (50 kg/m 3 ) was subjected to compression testing, the same evolution of structure parameters was again found, but fracture of snowflakes was also observed. The overall process of low-density snow deformation can be explained as a combination of fracturing and sintering, but with the sintering predominating.