http://www.atmos-chem-phys.net/Atmospheric Chemistry and Physics Latest Articleshttp://www.atmos-chem-phys.net/12/1397/2012/<b>Evolution of trace gases and particles emitted by a chaparral fire in California</b><br /><br />Atmospheric Chemistry and Physics, 12, 1397-1421, 2012<br /><br />Author(s): S. K. Akagi, J. S. Craven, J. W. Taylor, G. R. McMeeking, R. J. Yokelson, I. R. Burling, S. P. Urbanski, C. E. Wold, J. H. Seinfeld, H. Coe, M. J. Alvarado, and D. R. Weise<br /><br />Biomass burning (BB) is a major global source of trace gases and particles. Accurately representing the production and evolution of these emissions is an important goal for atmospheric chemical transport models. We measured a suite of gases and aerosols emitted from an 81 hectare prescribed fire in chaparral fuels on the central coast of California, US on 17 November 2009. We also measured physical and chemical changes that occurred in the isolated downwind plume in the first ~4 h after emission. The measurements were carried out onboard a Twin Otter aircraft outfitted with an airborne Fourier transform infrared spectrometer (AFTIR), aerosol mass spectrometer (AMS), single particle soot photometer (SP2), nephelometer, LiCor CO₂ analyzer, a chemiluminescence ozone instrument, and a wing-mounted meteorological probe. Our measurements included: CO₂; CO; NO_x; NH₃; non-methane organic compounds; organic aerosol (OA); inorganic aerosol (nitrate, ammonium, sulfate, and chloride); aerosol light scattering; refractory black carbon (rBC); and ambient temperature, relative humidity, barometric pressure, and three-dimensional wind velocity. The molar ratio of excess O₃ to excess CO in the plume (ΔO₃/ΔCO) increased from −5.13 (±1.13) × 10⁻³ to 10.2 (±2.16) × 10⁻² in ~4.5 h following smoke emission. Excess acetic and formic acid (normalized to excess CO) increased by factors of 1.73 ± 0.43 and 7.34 ± 3.03 (respectively) over the same time since emission. Based on the rapid decay of C₂H₄ we infer an in-plume average OH concentration of 5.27 (±0.97) × 10⁶ molec cm⁻³, consistent with previous studies showing elevated OH concentrations in biomass burning plumes. Ammonium, nitrate, and sulfate all increased over the course of 4 h. The observed ammonium increase was a factor of 3.90 ± 2.93 in about 4 h, but accounted for just ~36% of the gaseous ammonia lost on a molar basis. Some of the gas phase NH₃ loss may have been due to condensation on, or formation of, particles below the AMS detection range. NO_x was converted to PAN and particle nitrate with PAN production being about two times greater than production of observable nitrate in the first ~4 h following emission. The excess aerosol light scattering in the plume (normalized to excess CO₂) increased by a factor of 2.50 ± 0.74 over 4 h. The increase in light scattering was similar to that observed in an earlier study of a biomass burning plume in Mexico where significant secondary formation of OA closely tracked the increase in scattering. In the California plume, however, ΔOA/ΔCO₂ decreased sharply for the first hour and then increased slowly with a net decrease of ~20% over 4 h. The fraction of thickly coated rBC particles increased up to ~85% over the 4 h aging period. Decreasing OA accompanied by increased scattering/particle coating in initial aging may be due to a combination of particle coagulation and evaporation processes. Recondensation of species initially evaporated from the particles may have contributed to the subsequent slow rise in OA. We compare our results to observations from other plume aging studies and suggest that differences in environmental factors such as smoke concentration, oxidant concentration, actinic flux, and RH contribute significantly to the variation in plume evolution observations.2012-02-07T00:00:00+01:00http://www.atmos-chem-phys.net/12/1377/2012/<b>Spatial and seasonal variability of PM_2.5 acidity at two Chinese megacities: insights into the formation of secondary inorganic aerosols</b><br /><br />Atmospheric Chemistry and Physics, 12, 1377-1395, 2012<br /><br />Author(s): K. He, Q. Zhao, Y. Ma, F. Duan, F. Yang, Z. Shi, and G. Chen<br /><br />Aerosol acidity is one of the most important parameters influencing atmospheric chemistry and physics. Based on continuous field observations from January 2005 to May 2006 and thermodynamic modeling, we investigated the spatial and seasonal variations in PM_2.5 acidity in two megacities in China, Beijing and Chongqing. Spatially, PM_2.5 was generally more acidic in Chongqing than in Beijing, but a reverse spatial pattern was found within the two cities, with more acidic PM_2.5 at the urban site in Beijing whereas the rural site in Chongqing. Ionic compositions of PM_2.5 revealed that it was the higher concentrations of NO₃^&minus; at the urban site in Beijing and the lower concentrations of Ca²⁺ within the rural site in Chongqing that made their PM_2.5 more acidic. Temporally, PM_2.5 was more acidic in summer and fall than in winter, while in the spring of 2006, the acidity of PM_2.5 was higher in Beijing but lower in Chongqing than that in 2005. These were attributed to the more efficient formation of nitrate relative to sulfate as a result of the influence of Asian desert dust in 2006 in Beijing and the greater wet deposition of ammonium compared to sulfate and nitrate in 2005 in Chongqing. Furthermore, simultaneous increase of PM_2.5 acidity was observed from spring to early summer of 2005 in both cities. This synoptic-scale evolution of PM_2.5 acidity was accompanied by the changes in air masses origins, which were influenced by the movements of a subtropical high over the northwestern Pacific in early summer. Finally, the correlations between [NO₃^&minus;]/[SO₄^2&minus;] and [NH₄⁺]/[SO₄^2&minus;] suggests that under conditions of high aerosol acidity, heterogeneous reactions became one of the major pathways for the formation of nitrate at both cities. These findings provided new insights in our understanding of the spatial and temporal variations in aerosol acidity in Beijing and Chongqing, as well as those reported in other cities in China.2012-02-06T00:00:00+01:00http://www.atmos-chem-phys.net/12/1367/2012/<b>Seasonal variations of stable carbon isotopic composition and biogenic tracer compounds of water-soluble organic aerosols in a deciduous forest</b><br /><br />Atmospheric Chemistry and Physics, 12, 1367-1376, 2012<br /><br />Author(s): Y. Miyazaki, P. Q. Fu, K. Kawamura, Y. Mizoguchi, and K. Yamanoi<br /><br />To investigate the seasonal changes in biogenic water-soluble organic carbon (WSOC) aerosols in a boreal forest, aerosol samples were collected continuously in the canopy of a deciduous forest in northern Japan during 2009–2010. Stable carbon isotopic composition of WSOC (&delta;¹³C_WSOC) in total suspended particulate matter (TSP) exhibited a distinct seasonal cycle, with lower values from June through September (−25.5&plusmn;0.5 &permil;). This cycle follows the net CO₂ exchange between the forest ecosystem and the atmosphere, indicating that &delta;¹³C_WSOC likely reflects the biological activity at the forest site. WSOC concentrations showed the highest values in early summer and autumn. Positive matrix factorization (PMF) analysis indicated that the factor in which biogenic secondary organic aerosols (BSOAs) dominated accounted for ~40 % of the highest concentrations of WSOC, where BSOAs mostly consisted of α-/β-pinene SOA. In addition, primary biological aerosol particles (PBAPs) made similar contributions (~57 %) to the WSOC near the forest floor in early summer. This finding indicates that the production of both primary and secondary WSOC aerosols is important during the growing season in a deciduous forest. The methanesulfonic acid (MSA) maximum was also found in early summer and had a distinct vertical gradient with larger concentrations near the forest floor. Together with the similar vertical gradients found for WSOC and &delta;¹³C_WSOC as well as the α-/β-pinene SOA tracers, our results indicate that the forest floor, including ground vegetation and soil, acts as a significant source of WSOC in TSP within a forest canopy at the study site.2012-02-03T00:00:00+01:00http://www.atmos-chem-phys.net/12/1353/2012/<b>ClOOCl photolysis at high solar zenith angles: analysis of the RECONCILE self-match flight</b><br /><br />Atmospheric Chemistry and Physics, 12, 1353-1365, 2012<br /><br />Author(s): O. Sumińska-Ebersoldt, R. Lehmann, T. Wegner, J.-U. Grooß, E. Hösen, R. Weigel, W. Frey, S. Griessbach, V. Mitev, C. Emde, C. M. Volk, S. Borrmann, M. Rex, F. Stroh, and M. von Hobe<br /><br />The photolysis rate constant of dichlorine peroxide (ClOOCl, ClO dimer) <i>J</i>_ClOOCl is a critical parameter in catalytic cycles destroying ozone (O₃) in the polar stratosphere. In the atmospherically relevant wavelength region (λ > 310 nm), significant discrepancies between laboratory measurements of ClOOCl absorption cross sections and spectra cause a large uncertainty in <i>J</i>_ClOOCl. Previous investigations of the consistency of published <i>J</i>_ClOOCl with atmospheric observations of chlorine monoxide (ClO) and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis, and thus could only constrain the ratio of <i>J</i>_ClOOCl over the ClOOCl formation rate constant <i>k</i>_rec. Here, we constrain the atmospherically effective <i>J</i>_ClOOCl independent of <i>k</i>_rec, using ClO measured in the same air masses before and directly after sunrise during an aircraft flight that was part of the RECONCILE field campaign in the winter 2010 from Kiruna, Sweden. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the increase in ClO concentrations is significantly faster than expected from <i>J</i>_ClOOCl based on the absorption spectrum proposed by Pope et al. (2007), but does not warrant cross sections larger than recently published values by Papanastasiou et al. (2009). In particular, the existence of a significant ClOOCl absorption band longwards of 420 nm is not supported by our observations. The observed night-time ClO would not be consistent with a ClO/ClOOCl thermal equilibrium constant significantly higher than the one proposed by Plenge et al. (2005).2012-02-02T00:00:00+01:00http://www.atmos-chem-phys.net/12/1339/2012/<b>Does acetone react with HO₂ in the upper-troposphere?</b><br /><br />Atmospheric Chemistry and Physics, 12, 1339-1351, 2012<br /><br />Author(s): T. J. Dillon, A. Pozzer, L. Vereecken, J. N. Crowley, and J. Lelieveld<br /><br />Recent theoretical calculations showed that reaction with HO₂ could be an important sink for acetone (CH₃C(O)CH₃) and source of acetic acid (CH₃C(O)OH) in cold parts of the atmosphere (e.g. the tropopause region). This work details studies of HO₂ + CH₃C(O)CH₃ (CH₃)₂C(OH)OO (R1) in laboratory-based and theoretical chemistry experiments; the atmospheric significance of Reaction (R1) was assessed in a global 3-D chemical model. Pulsed laser-kinetic experiments were conducted, for the first time, at the low-temperatures representative of the tropopause. Reaction with NO converted HO₂ to OH for detection by laser induced fluorescence. Reduced yields of OH at <i>T</i> < 220 K provided indirect evidence for the sequestration of HO₂ by CH₃C(O)CH₃ with a forward rate coefficient greater than 2 &times; 10^&minus;12 cm³ molecule⁻¹ s⁻¹. No evidence for Reaction (R1) was observed at <i>T</i> > 230 K, probably due to rapid thermal dissociation back to HO₂ + CH₃C(O)CH₃. Numerical simulations of the data indicate that these experiments were sensitive to only (R1a) HO₂-CH₃C(O)CH₃ complex formation, the first step in (R1). Rearrangement (R1b) of the complex to form peroxy radicals, and hence the atmospheric significance of (R1) has yet to be rigorously verified by experiment. <br><br> Results from new quantum chemical calculations indicate that <i>K</i>₁ is characterised by large uncertainties of at least an order of magnitude at <i>T</i> < 220 K. The large predicted values from Hermans et al. lie at the top end of the range of values obtained from calculations at different (higher) levels of theory. Atmospheric modelling studies demonstrated that whilst (R1) chemistry may be a significant loss process for CH₃C(O)CH₃ near the tropopause, it cannot explain observations of CH₃C(O)OH throughout the troposphere.2012-02-02T00:00:00+01:00http://www.atmos-chem-phys.net/12/1327/2012/<b>Characterization of soluble bromide measurements and a case study of BrO observations during ARCTAS</b><br /><br />Atmospheric Chemistry and Physics, 12, 1327-1338, 2012<br /><br />Author(s): J. Liao, L. G. Huey, E. Scheuer, J. E. Dibb, R. E. Stickel, D. J. Tanner, J. A. Neuman, J. B. Nowak, S. Choi, Y. Wang, R. J. Salawitch, T. Canty, K. Chance, T. Kurosu, R. Suleiman, A. J. Weinheimer, R. E. Shetter, A. Fried, W. Brune, B. Anderson, X. Zhang, G. Chen, J. Crawford, A. Hecobian, and E. D. Ingall<br /><br />A focus of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission was examination of bromine photochemistry in the spring time high latitude troposphere based on aircraft and satellite measurements of bromine oxide (BrO) and related species. The NASA DC-8 aircraft utilized a chemical ionization mass spectrometer (CIMS) to measure BrO and a mist chamber (MC) to measure soluble bromide. We have determined that the MC detection efficiency to molecular bromine (Br₂), hypobromous acid (HOBr), bromine oxide (BrO), and hydrogen bromide (HBr) as soluble bromide (Br⁻) was 0.9±0.1, 1.06+0.30/−0.35, 0.4±0.1, and 0.95±0.1, respectively. These efficiency factors were used to estimate soluble bromide levels along the DC-8 flight track of 17 April 2008 from photochemical calculations constrained to in situ BrO measured by CIMS. During this flight, the highest levels of soluble bromide and BrO were observed and atmospheric conditions were ideal for the space-borne observation of BrO. The good agreement (<i>R</i>² = 0.76; slope = 0.95; intercept = −3.4 pmol mol⁻¹) between modeled and observed soluble bromide, when BrO was above detection limit (>2 pmol mol⁻¹) under unpolluted conditions (NO<10 pmol mol⁻¹), indicates that the CIMS BrO measurements were consistent with the MC soluble bromide and that a well characterized MC can be used to derive mixing ratios of some reactive bromine compounds. Tropospheric BrO vertical column densities (BrO^VCD) derived from CIMS BrO observations compare well with BrO_TROP^VCD from OMI on 17 April 2008.2012-02-02T00:00:00+01:00http://www.atmos-chem-phys.net/12/1307/2012/<b>Impact of the isoprene photochemical cascade on tropical ozone</b><br /><br />Atmospheric Chemistry and Physics, 12, 1307-1325, 2012<br /><br />Author(s): F. Paulot, D. K. Henze, and P. O. Wennberg<br /><br />Tropical tropospheric ozone affects Earth's radiative forcing and the oxidative capacity of the atmosphere. Considerable work has been devoted to the study of the processes controlling its budget. Yet, large discrepancies between simulated and observed tropical tropospheric ozone remain. Here, we characterize some of the mechanisms by which the photochemistry of isoprene impacts the budget of tropical ozone. At the regional scale, we use forward sensitivity simulation to explore the sensitivity to the representation of isoprene nitrates. We find that isoprene nitrates can account for up to 70% of the local NO_x = NO+NO₂ sink. The resulting modulation of ozone can be well characterized by their net modulation of NO_x. We use adjoint sensitivity simulations to demonstrate that the oxidation of isoprene can affect ozone outside of continental regions through the transport of NO_x over near-shore regions (e.g., South Atlantic) and the oxidation of isoprene outside of the boundary layer far from its emissions regions. The latter mechanism is promoted by the simulated low boundary-layer oxidative conditions. In our simulation, ~20% of the isoprene is oxidized above the boundary layer in the tropics. Changes in the interplay between regional and global effect are discussed in light of the forecasted increase in anthropogenic emissions in tropical regions.2012-02-02T00:00:00+01:00http://www.atmos-chem-phys.net/12/1287/2012/<b>Examination of aerosol distributions and radiative effects over the Bay of Bengal and the Arabian Sea region during ICARB using satellite data and a general circulation model</b><br /><br />Atmospheric Chemistry and Physics, 12, 1287-1305, 2012<br /><br />Author(s): R. Cherian, C. Venkataraman, S. Ramachandran, J. Quaas, and S. Kedia<br /><br />In this paper we analyse aerosol loading and its direct radiative effects over the Bay of Bengal (BoB) and Arabian Sea (AS) regions for the Integrated Campaign on Aerosols, gases and Radiation Budget (ICARB) undertaken during 2006, using satellite data from the MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites, the Aerosol Index from the Ozone Monitoring Instrument (OMI) on board the Aura satellite, and the European-Community Hamburg (ECHAM5.5) general circulation model extended by Hamburg Aerosol Module (HAM). By statistically comparing with large-scale satellite data sets, we firstly show that the aerosol properties measured during the ship-based ICARB campaign and simulated by the model are representative for the BoB and AS regions and the pre-monsoon season. In a second step, the modelled aerosol distributions were evaluated by a comparison with the measurements from the ship-based sunphotometer, and the satellite retrievals during ICARB. It is found that the model broadly reproduces the observed spatial and temporal variability in aerosol optical depth (AOD) over BoB and AS regions. However, AOD was systematically underestimated during high-pollution episodes, especially in the BoB leg. We show that this underprediction of AOD is mostly because of the deficiencies in the coarse mode, where the model shows that dust is the dominant component. The analysis of dust AOD along with the OMI Aerosol Index indicate that missing dust transport that results from too low dust emission fluxes over the Thar Desert region in the model caused this deficiency. Thirdly, we analysed the spatio-temporal variability of AOD comparing the ship-based observations to the large-scale satellite observations and simulations. It was found that most of the variability along the track was from geographical patterns, with a minor influence by single events. Aerosol fields were homogeneous enough to yield a good statistical agreement between satellite data at a 1&deg; spatial, but only twice-daily temporal resolution, and the ship-based sunphotometer data at a much finer spatial, but daily-average temporal resolution. Examination of the satellite data further showed that the year 2006 is representative for the five-year period for which satellite data were available. Finally, we estimated the clear-sky solar direct aerosol radiative forcing (DARF). We found that the cruise represents well the regional-seasonal mean forcings. Constraining simulated forcings using the observed AOD distributions yields a robust estimate of regional-seasonal mean DARF of &minus;8.6, &minus;21.4 and +12.9 W m^&minus;2 at the top of the atmosphere (TOA), at the surface (SUR) and in the atmosphere (ATM), respectively, for the BoB region, and over the AS, of, &minus;6.8, &minus;12.8, and +6 W m^&minus;2 at TOA, SUR, and ATM, respectively.2012-02-01T00:00:00+01:00http://www.atmos-chem-phys.net/12/1255/2012/<b>Analysis of satellite-derived Arctic tropospheric BrO columns in conjunction with aircraft measurements during ARCTAS and ARCPAC</b><br /><br />Atmospheric Chemistry and Physics, 12, 1255-1285, 2012<br /><br />Author(s): S. Choi, Y. Wang, R. J. Salawitch, T. Canty, J. Joiner, T. Zeng, T. P. Kurosu, K. Chance, A. Richter, L. G. Huey, J. Liao, J. A. Neuman, J. B. Nowak, J. E. Dibb, A. J. Weinheimer, G. Diskin, T. B. Ryerson, A. da Silva, J. Curry, D. Kinnison, S. Tilmes, and P. F. Levelt<br /><br />We derive tropospheric column BrO during the ARCTAS and ARCPAC field campaigns in spring 2008 using retrievals of total column BrO from the satellite UV nadir sensors OMI and GOME-2 using a radiative transfer model and stratospheric column BrO from a photochemical simulation. We conduct a comprehensive comparison of satellite-derived tropospheric BrO column to aircraft in-situ observations of BrO and related species. The aircraft profiles reveal that tropospheric BrO, when present during April 2008, was distributed over a broad range of altitudes rather than being confined to the planetary boundary layer (PBL). Perturbations to the total column resulting from tropospheric BrO are the same magnitude as perturbations due to longitudinal variations in the stratospheric component, so proper accounting of the stratospheric signal is essential for accurate determination of satellite-derived tropospheric BrO. We find reasonably good agreement between satellite-derived tropospheric BrO and columns found using aircraft in-situ BrO profiles, particularly when satellite radiances were obtained over bright surfaces (albedo >0.7), for solar zenith angle <80° and clear sky conditions. The rapid activation of BrO due to surface processes (the bromine explosion) is apparent in both the OMI and GOME-2 based tropospheric columns. The wide orbital swath of OMI allows examination of the evolution of tropospheric BrO on about hourly time intervals near the pole. Low surface pressure, strong wind, and high PBL height are associated with an observed BrO activation event, supporting the notion of bromine activation by high winds over snow.2012-02-01T00:00:00+01:00http://www.atmos-chem-phys.net/12/1239/2012/<b>The role of carbonyl sulphide as a source of stratospheric sulphate aerosol and its impact on climate</b><br /><br />Atmospheric Chemistry and Physics, 12, 1239-1253, 2012<br /><br />Author(s): C. Brühl, J. Lelieveld, P. J. Crutzen, and H. Tost<br /><br />Globally, carbonyl sulphide (COS) is the most abundant sulphur gas in the atmosphere. Our chemistry-climate model (CCM) of the lower and middle atmosphere with aerosol module realistically simulates the background stratospheric sulphur cycle, as observed by satellites in volcanically quiescent periods. The model results indicate that upward transport of COS from the troposphere largely controls the sulphur budget and the aerosol loading of the background stratosphere. This differs from most previous studies which indicated that short-lived sulphur gases are also important. The model realistically simulates the modulation of the particulate and gaseous sulphur abundance in the stratosphere by the quasi-biennial oscillation (QBO). In the lowermost stratosphere organic carbon aerosol contributes significantly to extinction. Further, using a chemical radiative convective model and recent spectra, we compute that the direct radiative forcing efficiency by 1 kg of COS is 724 times that of 1 kg CO₂. Considering an anthropogenic fraction of 30% (derived from ice core data), this translates into an overall direct radiative forcing by COS of 0.003 W m⁻². The direct global warming potentials of COS over time horizons of 20 and 100 yr are GWP(20 yr) = 97 and GWP(100 yr) = 27, respectively (by mass). Furthermore, stratospheric aerosol particles produced by the photolysis of COS (chemical feedback) contribute to a negative direct solar radiative forcing, which in the CCM amounts to −0.007 W m⁻² at the top of the atmosphere for the anthropogenic fraction, more than two times the direct warming forcing of COS. Considering that the lifetime of COS is twice that of stratospheric aerosols the warming and cooling tendencies approximately cancel.2012-02-01T00:00:00+01:00http://www.atmos-chem-phys.net/12/1229/2012/<b>A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks</b><br /><br />Atmospheric Chemistry and Physics, 12, 1229-1238, 2012<br /><br />Author(s): D. A. Hegg, D. S. Covert, H. H. Jonsson, and R. K. Woods<br /><br />Aircraft–based measurements of cloud condensation nuclei (CCN), accumulation mode and Aitken mode number concentrations, cloud drop number concentration (CDNC), and selected ancillary measurements are presented for the three large, semi-permanent marine stratocumulus decks of the earth (in the Pacific offshore of California and Chile and in the Atlantic offshore of Namibia). Based on these data, a simple linear relationship between CDNC and the accumulation mode number concentration (AMNC) is derived via regression. The slope of the regression is 0.72 ± 0.04 with an <i>R</i>² of 0.90, higher than those found for CDNC-CCN linear regressions. Explanations of the relatively favorable CDNC-AMNC relationship and its utility for climate studies are discussed.2012-02-01T00:00:00+01:00http://www.atmos-chem-phys.net/12/1213/2012/<b>Short-lived brominated hydrocarbons – observations in the source regions and the tropical tropopause layer</b><br /><br />Atmospheric Chemistry and Physics, 12, 1213-1228, 2012<br /><br />Author(s): S. Brinckmann, A. Engel, H. Bönisch, B. Quack, and E. Atlas<br /><br />We conducted measurements of the five important short-lived organic bromine species in the marine boundary layer (MBL). Measurements were made in the Northern Hemisphere mid-latitudes (Sylt Island, North Sea) in June 2009 and in the tropical Western Pacific during the TransBrom ship campaign in October 2009. For the one-week time series on Sylt Island, mean mixing ratios of CHBr₃, CH₂Br₂, CHBr₂Cl and CH₂BrCl were 2.0, 1.1, 0.2, 0.1 ppt, respectively. We found maxima of 5.8 and 1.6 ppt for the two main components CHBr₃ and CH₂Br₂. Along the cruise track in the Western Pacific (between 41° N and 13° S) we measured mean mixing ratios of 0.9, 0.9, 0.2, 0.1 and 0.1 ppt for CHBr₃, CH₂Br₂, CHBrCl₂, CHBr₂Cl and CH₂BrCl. Air samples with coastal influence showed considerably higher mixing ratios than the samples with open ocean origin. Correlation analyses of the two data sets yielded strong linear relationships between the mixing ratios of four of the five species (except for CH₂BrCl). Using a combined data set from the two campaigns and a comparison with the results from two former studies, rough estimates of the molar emission ratios between the correlated substances were: 9/1/0.35/0.35 for CHBr₃/CH₂Br₂/CHBrCl₂/CHBr₂Cl. Additional measurements were made in the tropical tropopause layer (TTL) above Teresina (Brazil, 5° S) in June 2008, using balloon-borne cryogenic whole air sampling technique. Near the level of zero clear-sky net radiative heating (LZRH) at 14.8 km about 2.25 ppt organic bromine was bound to the five short-lived species, making up 13% of total organic bromine (17.82 ppt). CH₂Br₂ (1.45 ppt) and CHBr₃ (0.56 ppt) accounted for 90% of the budget of short-lived compounds in that region. Near the tropopause (at 17.5 km) organic bromine from these substances was reduced to 1.35 ppt, with 1.07 and 0.12 ppt attributed to CH₂Br₂ and CHBr₃, respectively.2012-02-01T00:00:00+01:00http://www.atmos-chem-phys.net/12/1203/2012/<b>Direct measurement of ozone production rates in Houston in 2009 and comparison with two estimation methods</b><br /><br />Atmospheric Chemistry and Physics, 12, 1203-1212, 2012<br /><br />Author(s): M. Cazorla, W. H. Brune, X. Ren, and B. Lefer<br /><br />Net ozone production rates, <i>P</i>(O₃), were measured directly using the Penn State Measurement of Ozone Production Sensor (MOPS) during the Study of Houston Atmospheric Radical Precursors (SHARP, 2009). Measured <i>P</i>(O₃) peaked in the late morning, with values between 15 ppbv h^&minus;1 and 100 ppbv h⁻¹, although values of 40–80 ppbv h⁻¹ were typical for higher ozone days. These measurements were compared against ozone production rates calculated using measurements of hydroperoxyl (HO₂), hydroxyl (OH), and nitric oxide (NO) radicals, called "calculated <i>P</i>(O₃)". The same comparison was done using modeled radicals obtained from a box model with the RACM2 mechanism, called "modeled <i>P</i>(O₃)". Measured and calculated <i>P</i>(O₃) had similar peak values but the calculated <i>P</i>(O₃) tended to peak earlier in the morning when NO values were higher. Measured and modeled <i>P</i>(O₃) had a similar dependence on NO, but the modeled <i>P</i>(O₃) was only half the measured <i>P</i>(O₃). The modeled <i>P</i>(O₃) is less than the calculated <i>P</i>(O₃) because the modeled HO₂ is less than the measured HO₂. While statistical analyses are not conclusive regarding the comparison between MOPS measurements and the two estimation methods, the calculated <i>P</i>(O₃) with measured HO₂ produces peak values similar to the measured <i>P</i>(O₃) when ozone is high. Although the MOPS is new and more testing is required to verify its observations, the measurements in the SHARP field campaign show the potential of this new technique for contributing to the understanding of ozone-producing chemistry and to the monitoring of ozone's response to future air quality regulatory actions.2012-01-30T00:00:00+01:00http://www.atmos-chem-phys.net/12/1189/2012/<b>Deposition nucleation on mineral dust particles: a case against classical nucleation theory with the assumption of a single contact angle</b><br /><br />Atmospheric Chemistry and Physics, 12, 1189-1201, 2012<br /><br />Author(s): M. J. Wheeler and A. K. Bertram<br /><br />Deposition nucleation on two mineral species, kaolinite and illite, was studied using a flow cell coupled to an optical microscope. The results show that the <i>S</i>_ice conditions when ice first nucleated, defined as the onset <i>S</i>_ice (<i>S</i>_ice,onset), is a strong function of the surface area available for nucleation, varying from 100% to 125% at temperatures between 242 and 239 K. The surface area dependent data could not be described accurately using classical nucleation theory and the assumption of a single contact angle (defined here as the single-α model). These results suggest that caution should be applied when using contact angles determined from <i>S</i>_ice,onset data and the single-α model. In contrast to the single-α model, the active site model, the deterministic model, and a model with a distribution of contact angles fit the data within experimental uncertainties. Parameters from the fits to the data are presented.2012-01-30T00:00:00+01:00http://www.atmos-chem-phys.net/12/1173/2012/<b>Link between local scale BC emissions in the Indo-Gangetic Plains and large scale atmospheric solar absorption</b><br /><br />Atmospheric Chemistry and Physics, 12, 1173-1187, 2012<br /><br />Author(s): P. S. Praveen, T. Ahmed, A. Kar, I. H. Rehman, and V. Ramanathan<br /><br />Project Surya has documented indoor and outdoor concentrations of black carbon (BC) from traditional biomass burning cook stoves in a rural village located in the Indo-Gangetic Plains (IGP) region of N. India from November 2009–September 2010. In this paper, we systematically document the link between local scale aerosol properties and column averaged regional aerosol optical properties and atmospheric radiative forcing. We document observations from the first phase of Project Surya and estimate the source dependent (biomass and fossil fuels) aerosol optical properties from local to regional scale. Data were collected using surface based observations of BC, organic carbon (OC), aerosol light absorption, scattering coefficient at the Surya village (SVI_1) located in IGP region and integrated with satellite and AERONET observations at the regional scale (IGP). The daily mean BC concentrations at SVI_1 showed a large increase of BC during the dry season (December to February) with values reaching 35 μg m⁻³. Space based LIDAR data revealed how the biomass smoke was trapped within the first kilometer during the dry season and extended to above 5 km during the pre-monsoon season. As a result, during the dry season, the variance in the daily mean single scattering albedo (SSA), the ratio of scattering to extinction coefficient, and column aerosol optical properties at the local IGP site correlated (with slopes in the range of 0.85 to 1.06 and <i>R</i>²>0.4) well with the "IGP_AERONET" (mean of six AERONET sites). The statistically significant correlation suggested that in-situ observations can be used to derive spatial mean forcing, at least for the dry season. The atmospheric forcing due to BC and OC exceeded 20 Wm⁻² during all months from November to May, supporting the deduction that elimination of cook stove smoke emissions through clean cooking technologies will likely have a major positive impact not only on human health but also on regional climate.2012-01-30T00:00:00+01:00http://www.atmos-chem-phys.net/12/1151/2012/<b>Assessment of some parameterizations of heterogeneous ice nucleation in cloud and climate models</b><br /><br />Atmospheric Chemistry and Physics, 12, 1151-1172, 2012<br /><br />Author(s): J. A. Curry and V. I. Khvorostyanov<br /><br />Several different types of parameterization of heterogeneous ice nucleation for cloud and climate models have been developed over the past decades, ranging from empirically-derived expressions to parameterizations of ice crystal nucleation rates derived from theory, including the parameterization developed by the authors that includes simultaneous dependence on the temperature and saturation ratio, hereafter referred to as KC. Parameterizations schemes that address the deliquescence-heterogeneous-freezing (DHetF), which combines the modes of condensation freezing and immersion freezing, are assessed here in the context of thermodynamic constraints, laboratory measurements, and recent field measurements. It is shown that empirical schemes depending only on the ice saturation ratio or only on temperature can produce reasonable crystal concentrations, but ice crystal nucleation is thermodynamically prohibited in certain regions of the temperature-saturation ratio phase space. Some recent empirical parameterizations yield clouds that are almost entire liquid at temperatures as low as −35 °C in contrast to cloud climatology. Reasonable performance of the KC ice nucleation scheme is demonstrated by comparison with numerous data from several recent field campaigns, laboratory data, climatology of cloud phase-state. Several mis-applications of the KC parameterization that appeared recently in the literature are described and corrected. It is emphasized here that a correct application of the KC scheme requires integration of the individual nucleation rates over the measured size spectrum of ice nuclei that represent a fraction or several fractions of the environmental aerosol with specific ice nucleation properties. The concentration in these fractions can be substantially smaller than that of the total aerosol, but greater than the crystal concentration measured by an experimental device. Simulations with temperature-dependent active site area or with several IN fractions having different properties show that ice nucleation in the KC scheme occurs in a wide temperature range of 10–20 °C, which depends on IN properties. Simulation with a spectral bin model and correct application of KC scheme adequately describes ice nucleation via the DHetF mode and yields crystal concentrations and phase state close to those measured in the single-layer stratocumulus cloud observed in the Mixed Phase Arctic Cloud Experiment (MPACE). An assessment of some deficiencies in current parcel modeling methods and cloud chamber observations and their impact on parameterization development and evaluation is provided.2012-01-27T00:00:00+01:00http://www.atmos-chem-phys.net/12/1135/2012/<b>Impact of the deep convection of isoprene and other reactive trace species on radicals and ozone in the upper troposphere</b><br /><br />Atmospheric Chemistry and Physics, 12, 1135-1150, 2012<br /><br />Author(s): E. C. Apel, J. R. Olson, J. H. Crawford, R. S. Hornbrook, A. J. Hills, C. A. Cantrell, L. K. Emmons, D. J. Knapp, S. Hall, R. L. Mauldin III, A. J. Weinheimer, A. Fried, D. R. Blake, J. D. Crounse, J. M. St. Clair, P. O. Wennberg, G. S. Diskin, H. E. Fuelberg, A. Wisthaler, T. Mikoviny, W. Brune, and D. D. Riemer<br /><br />Observations of a comprehensive suite of inorganic and organic trace gases, including non-methane hydrocarbons (NMHCs), halogenated organics and oxygenated volatile organic compounds (OVOCs), obtained from the NASA DC-8 over Canada during the ARCTAS aircraft campaign in July 2008 illustrate that convection is important for redistributing both long- and short-lived species throughout the troposphere. Convective outflow events were identified by the elevated mixing ratios of organic species in the upper troposphere relative to background conditions. Several dramatic events were observed in which isoprene and its oxidation products were detected at hundreds of pptv at altitudes higher than 8 km. Two events are studied in detail using detailed experimental data and the NASA Langley Research Center (LaRC) box model. One event had no lightning NO_x (NO + NO₂) associated with it and the other had substantial lightning NO_x (LNO_x > 1 ppbv). When convective storms transport isoprene from the boundary layer to the upper troposphere and no LNO_x is present, OH is reduced due to scavenging by isoprene, which serves to slow the chemistry, resulting in longer lifetimes for species that react with OH. Ozone and PAN production is minimal in this case. In the case where isoprene is convected and LNO_x is present, there is a large effect on the expected ensuing chemistry: isoprene exerts a dominant impact on HO_x and nitrogen-containing species; the relative contribution from other species to HO_x, such as peroxides, is insignificant. The isoprene reacts quickly, resulting in primary and secondary products, including formaldehyde and methyl glyoxal. The model predicts enhanced production of alkyl nitrates (ANs) and peroxyacyl nitrate compounds (PANs). PANs persist because of the cold temperatures of the upper troposphere resulting in a large change in the NO_x mixing ratios which, in turn, has a large impact on the HO_x chemistry. Ozone production is substantial during the first few hours following the convection to the UT, resulting in a net gain of approximately 10 ppbv compared to the modeled scenario in which LNO_x is present but no isoprene is present aloft.2012-01-27T00:00:00+01:00http://www.atmos-chem-phys.net/12/1121/2012/<b>Depositional ice nucleation onto crystalline hydrated NaCl particles: a new mechanism for ice formation in the troposphere</b><br /><br />Atmospheric Chemistry and Physics, 12, 1121-1134, 2012<br /><br />Author(s): M. E. Wise, K. J. Baustian, T. Koop, M. A. Freedman, E. J. Jensen, and M. A. Tolbert<br /><br />Sea-salt aerosol (SSA) particles are ubiquitous in the marine boundary layer and over coastal areas. Therefore SSA have ability to directly and indirectly affect the Earth's radiation balance. The influence SSA have on climate is related to their water uptake and ice nucleation characteristics. In this study, optical microscopy coupled with Raman spectroscopy was used to detect the formation of a crystalline NaCl hydrate that could form under atmospheric conditions. NaCl_(s) particles (~1 to 10 μm in diameter) deliquesced at 75.7 ± 2.5% RH which agrees well with values previously established in the literature. NaCl_(aq) particles effloresced to a mixture of hydrated and non-hydrated particles at temperatures between 236 and 252 K. The aqueous particles effloresced into the non-hydrated form at temperatures warmer than 252 K. At temperatures colder than 236 K all particles effloresced into the hydrated form. The deliquescence relative humidities (DRH) of hydrated NaCl_(s) particles ranged from 76.6 to 93.2% RH. Based on the measured DRH and efflorescence relative humidities (ERH), we estimate crystalline NaCl particles could be in the hydrated form 40–80% of the time in the troposphere. Additionally, the ice nucleating abilities of NaCl_(s) and hydrated NaCl_(s) were determined at temperatures ranging from 221 to 238 K. Here, depositional ice nucleation is defined as the onset of ice nucleation and represents the conditions at which the first particle on the substrate nucleated ice. Thus the values reported here represent the lower limit of depositional ice nucleation. NaCl_(s) particles depositionally nucleated ice at an average <i>S</i>_ice value of 1.11 ± 0.07. Hydrated NaCl_(s) particles depositionally nucleated ice at an average <i>S</i>_ice value of 1.02 ± 0.04. When a mixture of hydrated and anhydrous NaCl_(s) particles was present in the same sample, ice preferentially nucleated on the hydrated particles 100% of the time. While both types of particles are efficient ice nuclei, hydrated NaCl_(s) particles are better ice nuclei than NaCl_(s) particles.2012-01-27T00:00:00+01:00http://www.atmos-chem-phys.net/12/1101/2012/<b>Statistical analysis of an LES shallow cumulus cloud ensemble using a cloud tracking algorithm</b><br /><br />Atmospheric Chemistry and Physics, 12, 1101-1119, 2012<br /><br />Author(s): J. T. Dawe and P. H. Austin<br /><br />A technique for the tracking of individual clouds in a Large Eddy Simulation (LES) is presented. We use this technique on an LES of a shallow cumulus cloud field based upon the Barbados Oceanographic and Meteorological Experiment (BOMEX) to calculate statistics of cloud height, lifetime, and other physical properties for individual clouds in the model. We also examine the question of nature versus nurture in shallow cumulus clouds: do properties at cloud base determine the upper-level properties of the clouds (nature), or are cloud properties determined by the environmental conditions they encounter (nurture). We find that clouds which ascend through an environment that has been pre-moistened by previous cloud activity are no more likely to reach the inversion than clouds that ascend through a drier environment. Cloud base thermodynamic properties are uncorrelated with upper-level cloud properties, while mean fractional entrainment and detrainment rates display moderate correlations with cloud properties up to the inversion. Conversely, cloud base area correlates well with upper-level cloud area and maximum cloud height. We conclude that cloud thermodynamic properties are primarily influenced by entrainment and detrainment processes, cloud area and height are primarily influenced by cloud base area, and thus nature and nurture both play roles in the dynamics of BOMEX shallow cumulus clouds.2012-01-26T00:00:00+01:00http://www.atmos-chem-phys.net/12/1083/2012/<b>The composition and variability of atmospheric aerosol over Southeast Asia during 2008</b><br /><br />Atmospheric Chemistry and Physics, 12, 1083-1100, 2012<br /><br />Author(s): W. Trivitayanurak, P. I. Palmer, M. P. Barkley, N. H. Robinson, H. Coe, and D. E. Oram<br /><br />We use a nested version of the GEOS-Chem global 3-D chemistry transport model to better understand the composition and variation of aerosol over Borneo and the broader Southeast Asian region in conjunction with aircraft and satellite observations. Our focus on Southeast Asia reflects the importance of this region as a source of reactive organic gases and aerosols from natural forests, biomass burning, and food and fuel crops. We particularly focus on July 2008 when the UK BAe-146 research aircraft was deployed over northern Malaysian Borneo as part of the ACES/OP3 measurement campaign. During July 2008 we find using the model that Borneo (defined as Borneo Island and the surrounding Indonesian islands) was a net exporter of primary organic aerosol (42 kT) and black carbon aerosol (11 kT). We find only 13% of volatile organic compound oxidation products partition to secondary organic aerosol (SOA), with Borneo being a net exporter of SOA (15 kT). SOA represents approximately 19% of the total organic aerosol over the region. Sulphate is mainly from aqueous-phase oxidation (68%), with smaller contributions from gas-phase oxidation (15%) and advection into the regions (14%). We find that there is a large source of sea salt, as expected, but this largely deposits within the region; we find that dust aerosol plays only a relatively small role in the aerosol burden. In contrast to coincident surface measurements over Northern Borneo that find a pristine environment with evidence for substantial biogenic SOA formation we find that the free troposphere is influenced by biomass burning aerosol transported from the northwest of the Island and further afield. We find several transport events during July 2008 over Borneo associated with elevated aerosol concentrations, none of which coincide with the aircraft flights. We use MODIS aerosol optical depths (AOD) data and the model to put the July campaign into a longer temporal perspective. We find that Borneo is where the model has the least skill at reproducing the data, where the model has a negative bias of 76% and only captures 14% of the observed variability. This model performance reflects the small-scale island-marine environment and the mix of aerosol species, with the model showing more skill at reproducing observed AOD over larger continental regions such as China where AOD is dominated by one aerosol type. The model shows that AOD over Borneo is approximately evenly split between organic and sulphate aerosol with sea salt representing 10–20% during May–September; we find a similar breakdown over continental Southeast Asia but with less sea salt aerosol and more dust aerosol. In contrast, East China AOD is determined mainly by sulphate aerosol and a seasonal source of dust aerosol, as expected. Realistic sensitivity runs, designed to test our underlying assumptions about emissions and chemistry over Borneo, show that model AOD is most sensitive to isoprene emissions and organic gas-phase partitioning but all fail to improve significantly upon the control model calculation. This emphasises the multi-faceted dimension of the problem and the need for concurrent and coordinated development of BVOC emissions, and BVOC chemistry and organic aerosol formation mechanisms.2012-01-26T00:00:00+01:00