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Global impact of smoke aerosols from landscape fires on climate and the Hadley circulation

Each year landscape fires across the globe emit black and organic carbon smoke particles that can last in the atmosphere for days to weeks. We characterized the climate response to these aerosols using an Earth system model. We used remote sensing observations of aerosol optical depth (AOD) and simulations from the Community Atmosphere Model, version 5 (CAM5) to optimize satellite-derived smoke emissions for high biomass burning regions. Subsequent global simulations using the adjusted fire emissions produced AODs that were in closer agreement with surface and space-based measurements. We then used CAM5, which included radiative aerosol effects, to evaluate the climate response to the fire-aerosol forcing. We conducted two 52 yr simulations, one with four sets of monthly cycling 1997–2009 fire emissions and one without. Fire emissions increased global mean annual AOD by 10% (+0.02) and decreased net all-sky surface radiation by 1% (1.3 W m−2). Elevated AODs reduced global surface temperatures by 0.13 &pm; 0.01 °C. Though global precipitation declined only slightly, patterns of precipitation changed, with large reductions near the Equator offset by smaller increases north and south of the intertropical convergence zone (ITCZ). A combination of increased tropospheric heating and reduced surface temperatures increased equatorial subsidence and weakened the Hadley circulation. As a consequence, precipitation decreased over tropical forests in South America, Africa and equatorial Asia. These results are consistent with the observed correlation between global temperatures and the strength of the Hadley circulation and studies linking tropospheric heating from black carbon aerosols with tropical expansion.
Preindustrial to present-day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)

We have analysed time-slice simulations from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore changes in present-day (2000) hydroxyl radical (OH) concentration and methane (CH4) lifetime relative to preindustrial times (1850) and to 1980. A comparison of modeled and observation-derived methane and methyl chloroform lifetimes suggests that the present-day global multi-model mean OH concentration is overestimated by 5 to 10% but is within the range of uncertainties. The models consistently simulate higher OH concentrations in the Northern Hemisphere (NH) compared with the Southern Hemisphere (SH) for the present-day (2000; inter-hemispheric ratios of 1.13 to 1.42), in contrast to observation-based approaches which generally indicate higher OH in the SH although uncertainties are large. Evaluation of simulated carbon monoxide (CO) concentrations, the primary sink for OH, against ground-based and satellite observations suggests low biases in the NH that may contribute to the high north–south OH asymmetry in the models. The models vary widely in their regional distribution of present-day OH concentrations (up to 34%). Despite large regional changes, the multi-model global mean (mass-weighted) OH concentration changes little over the past 150 yr, due to concurrent increases in factors that enhance OH (humidity, tropospheric ozone, nitrogen oxide (NOx) emissions, and UV radiation due to decreases in stratospheric ozone), compensated by increases in OH sinks (methane abundance, carbon monoxide and non-methane volatile organic carbon (NMVOC) emissions). The large inter-model diversity in the sign and magnitude of preindustrial to present-day OH changes (ranging from a decrease of 12.7% to an increase of 14.6%) indicate that uncertainty remains in our understanding of the long-term trends in OH and methane lifetime. We show that this diversity is largely explained by the different ratio of the change in global mean tropospheric CO and NOx burdens (ΔCO/ΔNOx, approximately represents changes in OH sinks versus changes in OH sources) in the models, pointing to a need for better constraints on natural precursor emissions and on the chemical mechanisms in the current generation of chemistry-climate models. For the 1980 to 2000 period, we find that climate warming and a slight increase in mean OH (3.5 ± 2.2%) leads to a 4.3 ± 1.9% decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present-day climate change decreased the methane lifetime by about four months, representing a negative feedback on the climate system. Further, we analysed attribution experiments performed by a subset of models relative to 2000 conditions with only one precursor at a time set to 1860 levels. We find that global mean OH increased by 46.4 ± 12.2% in response to preindustrial to present-day anthropogenic NOx emission increases, and decreased by 17.3 ± 2.3%, 7.6 ± 1.5%, and 3.1 ± 3.0% due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.
On the scaling effect in global surface air temperature anomalies

The annual and the monthly mean values of the land-surface air temperature anomalies from 1880–2011, over both hemispheres, are used to investigate the existence of long-range correlations in their temporal evolution. The analytical tool employed is the detrended fluctuation analysis, which eliminates the noise of the non-stationarities that characterize the land-surface air temperature anomalies in both hemispheres. The reliability of the results obtained from this tool (e.g., power-law scaling) is investigated, especially for large scales, by using error bounds statistics, the autocorrelation function (e.g., rejection of its exponential decay) and the method of local slopes (e.g., their constancy in a sufficient range). The main finding is that deviations of one sign of the land-surface air temperature anomalies in both hemispheres are generally followed by deviations with the same sign at different time intervals. In other words, the land-surface air temperature anomalies exhibit persistent behaviour, i.e., deviations tend to keep the same sign. Taking into account our earlier study, according to which the land and sea surface temperature anomalies exhibit scaling behaviour in the Northern and Southern Hemisphere, we conclude that the difference between the scaling exponents mainly stems from the sea surface temperature, which exhibits a stronger memory in the Southern than in the Northern Hemisphere. Moreover, the variability of the scaling exponents of the annual mean values of the land-surface air temperature anomalies versus latitude shows an increasing trend from the low latitudes to polar regions, starting from the classical random walk (white noise) over the tropics. There is a gradual increase of the scaling exponent from low to high latitudes (which is stronger over the Southern Hemisphere).
Particle and gaseous emissions from individual diesel and CNG buses

In this study size-resolved particle and gaseous emissions from 28 individual diesel-fuelled and 7 compressed natural gas (CNG)-fuelled buses, selected from an in-use bus fleet, were characterised for real-world dilution scenarios. The method used was based on using CO2 as a tracer of exhaust gas dilution. The particles were sampled by using an extractive sampling method and analysed with high time resolution instrumentation EEPS (10 Hz) and CO2 with a non-dispersive infrared gas analyser (LI-840, LI-COR Inc. 1 Hz). The gaseous constituents (CO, HC and NO) were measured by using a remote sensing device (AccuScan RSD 3000, Environmental System Products Inc.). Nitrogen oxides, NOx, were estimated from NO by using default NO2/NOx ratios from the road vehicle emission model HBEFA3.1. The buses studied were diesel-fuelled Euro III–V and CNG-fuelled Enhanced Environmentally Friendly Vehicles (EEVs) with different after-treatment, including selective catalytic reduction (SCR), exhaust gas recirculation (EGR) and with and without diesel particulate filter (DPF). The primary driving mode applied in this study was accelerating mode. However, regarding the particle emissions also a constant speed mode was analysed. The investigated CNG buses emitted on average a higher number of particles but less mass compared to the diesel-fuelled buses. Emission factors for number of particles (EFPN) were EFPN, DPF = 4.4 ± 3.5 × 1014, EFPN, no DPF = 2.1 ± 1.0 × 1015 and EFPN, CNG = 7.8 ± 5.7 ×1015 kg fuel−1. In the accelerating mode, size-resolved emission factors (EFs) showed unimodal number size distributions with peak diameters of 70–90 nm and 10 nm for diesel and CNG buses, respectively. For the constant speed mode, bimodal average number size distributions were obtained for the diesel buses with peak modes of ~10 nm and ~60 nm. Emission factors for NOx expressed as NO2 equivalents for the diesel buses were on average 27 ± 7 g (kg fuel)−1 and for the CNG buses 41 ± 26 g (kg fuel)−1. An anti-relationship between EFNOx and EFPM was observed especially for buses with no DPF, and there was a positive relationship between EFPM and EFCO.
Classifying organic materials by oxygen-to-carbon elemental ratio to predict the activation regime of Cloud Condensation Nuclei (CCN)

The governing highly soluble, slightly soluble, or insoluble activation regime of organic compounds as cloud condensation nuclei (CCN) was examined as a function of oxygen-to-carbon elemental ratio (O : C). New data were collected for adipic, pimelic, suberic, azelaic, and pinonic acids. Secondary organic materials (SOMs) produced by α-pinene ozonolysis and isoprene photo-oxidation were also included in the analysis. The saturation concentrations C of the organic compounds in aqueous solutions served as the key parameter for delineating regimes of CCN activation, and the values of C were tightly correlated to the O : C ratios. The highly soluble, slightly soluble, and insoluble regimes of CCN activation were found to correspond to ranges of [O : C] > 0.6, 0.2 < [O : C] < 0.6, and [O : C] < 0.2, respectively. These classifications were evaluated against CCN activation data of isoprene-derived SOM (O : C = 0.69–0.72) and α-pinene-derived SOM (O : C = 0.38–0.48). Isoprene-derived SOM had highly soluble activation behavior, consistent with its high O : C ratio. For α-pinene-derived SOM, although CCN activation can be modeled as a highly soluble mechanism, this behavior was not predicted by the O : C ratio, for which a slightly soluble mechanism was anticipated. Complexity in chemical composition, resulting in continuous water uptake and the absence of a deliquescence transition that can thermodynamically limit CCN activation, might explain the difference in the behavior of α-pinene-derived SOM compared to that of pure organic compounds. The present results suggest that atmospheric particles dominated by hydrocarbon-like organic components do not activate (i.e., insoluble regime) whereas those dominated by oxygenated organic components activate (i.e., highly soluble regime) for typical atmospheric cloud life cycles.
An evaluation of the CMAQ reproducibility of satellite tropospheric NO2 column observations at different local times over East Asia

Despite the importance of the role of nitrogen dioxide (NO2) in tropospheric chemistry, the causes leading to the discrepancy between satellite-derived and modeled tropospheric NO2 vertical column densities (VCDs) over East Asia remain unclear. Here the reproducibility of satellite tropospheric NO2 VCD data by a regional chemical transport model (CMAQ) with the Regional Emission inventory in ASia (REAS) Version 2 is evaluated from the viewpoint of the diurnal variation of tropospheric NO2 VCDs, where satellite observations at different local times (SCIAMACHY/ENVISAT, OMI/Aura, and GOME-2/Metop-A) are utilized considering literature validation results. As a case study, we concentrate on June and December 2007 for a detailed evaluation based on various sensitivity simulations, for example with different spatial resolutions (80, 40, 20, and 10 km) for CMAQ. For June, CMAQ generally reproduces absolute values of satellite NO2 VCDs and their diurnal variations over all 12 selected diagnostic regions in East Asia. In contrast, a difficulty arises in interpreting the significant disagreement between satellite and CMAQ values over most of the diagnostic regions in December. The disagreement cannot be explained by any of the sensitivity simulations performed in this study. To address this, more investigations, including further efforts for satellite validations in wintertime, are needed.
Determination of gaseous and particulate carbonyls (glycolaldehyde, hydroxyacetone, glyoxal, methylglyoxal, nonanal and decanal) in the atmosphere at Mt. Tai

Gaseous and particulate semi-volatile carbonyl compounds were determined every three hours in the atmosphere of Mount Tai (elevation, 1534 m) in the North China Plain during 2–5, 23–24 and 25 June 2006 under clear sky conditions. Using a two-step filter cartridge in a series, particulate carbonyls were first collected on a quartz filter and then gaseous carbonyls were collected on a quartz filter impregnated with O-benzylhydroxylamine (BHA). After the two-step derivatization with BHA and N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA), carbonyl derivatives were measured using a gas chromatography. The gaseous concentrations were obtained as follow: glycolaldehyde (range 0–826 ng m−3, average 303 ng m−3), hydroxyacetone (0–579 ng m−3, 126 ng m−3), glyoxal (46–1200 ng m−3, 487 ng m−3), methylglyoxal (88–2690 ng m−3, 967 ng m−3), n-nonanal (0–500 ng m−3, 89 ng m−3), and n-decanal (0–230 ng m−3, 39 ng m−3). These concentrations are among the highest ever reported in the urban and forest atmosphere. We found that gaseous α-dicarbonyls (glyoxal and methylglyoxal) are more than 20 times more abundant than particulate carbonyls and that glycolaldehyde is one order of magnitude more abundant than in aerosol phase. In contrast, hydroxyacetone and normal aldehydes (nonanal and decanal) are equally present in both phases. Time-resolved variations of carbonyls did not show any a clear diurnal pattern, except for hydroxyacetone. We found that glyoxal, methylglyoxal and glycolaldehyde positively correlated with levoglucosan (a tracer of biomass burning), suggesting that a contribution from field burning of agricultural wastes (wheat crops) is significant for the bifunctional carbonyls in the atmosphere of Mt. Tai. Upward transport of the pollutants to the mountaintop from the low lands in the North China Plain is a major process to control the distributions of carbonyls in the upper atmosphere over Mt. Tai.
Corrigendum to "Pre-industrial to end 21st century projections of tropospheric ozone from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)" published in Atmos. Chem. Phys., 13, 2063–2090, 2013
Net radiative forcing and air quality responses to regional CO emission reductions

Carbon monoxide (CO) emissions influence global and regional air quality and global climate change by affecting atmospheric oxidants and secondary species. We simulate the influence of halving anthropogenic CO emissions globally and individually from 10 regions on surface and tropospheric ozone, methane, and aerosol concentrations using a global chemical transport model (MOZART-4 for the year 2005). Net radiative forcing (RF) is then estimated using the GFDL (Geophysical Fluid Dynamics Laboratory) standalone radiative transfer model. We estimate that halving global CO emissions decreases global annual average concentrations of surface ozone by 0.45 ppbv, tropospheric methane by 73 ppbv, and global annual net RF by 36.1 mW m−2, nearly equal to the sum of changes from the 10 regional reductions. Global annual net RF per unit change in emissions and the 100 yr global warming potential (GWP100) are estimated as −0.124 mW m−2 (Tg CO)−1 and 1.34, respectively, for the global CO reduction, and ranging from −0.115 to −0.131 mW m−2 (Tg CO)−1 and 1.26 to 1.44 across 10 regions, with the greatest sensitivities for regions in the tropics. The net RF distributions show widespread cooling corresponding to the O3 and CH4 decreases, and localized positive and negative net RFs due to changes in aerosols. The strongest annual net RF impacts occur within the tropics (28° S–28° N) followed by the northern midlatitudes (28° N–60° N), independent of reduction region, while the greatest changes in surface CO and ozone concentrations occur within the reduction region. Some regional reductions strongly influence the air quality in other regions, such as East Asia, which has an impact on US surface ozone that is 93% of that from North America. Changes in the transport of CO and downwind ozone production clearly exceed the direct export of ozone from each reduction region. The small variation in CO GWPs among world regions suggests that future international climate agreements could adopt a globally uniform metric for CO with little error, or could use different GWPs for each continent. Doing so may increase the incentive to reduce CO through coordinated policies addressing climate and air quality.
Comparing the cloud vertical structure derived from several methods based on measured atmospheric profiles and active surface measurements

The cloud vertical distribution and especially the cloud base height, which is linked to cloud type, is an important characteristic in order to describe the impact of clouds in a changing climate. In this work several methods to estimate the cloud vertical structure (CVS) based on atmospheric sounding profiles are compared, considering number and position of cloud layers, with a ground based system which is taken as a reference: the Active Remote Sensing of Clouds (ARSCL). All methods establish some conditions on the relative humidity, and differ on the use of other variables, the thresholds applied, or the vertical resolution of the profile. In this study these methods are applied to 125 radiosonde profiles acquired at the ARM Southern Great Plains site during all seasons of year 2009 and endorsed by GOES images, to confirm that the cloudiness conditions are homogeneous enough across their trajectory. The overall agreement for the methods ranges between 44–88%; four methods produce total agreements around 85%. Further tests and improvements are applied on one of these methods. In addition, we attempt to make this method suitable for low resolution vertical profiles, which could be useful in atmospheric modeling. The total agreement, even when using low resolution profiles, can be improved up to 91% if the thresholds for a moist layer to become a cloud layer are modified to minimize false negatives with the current data set, thus improving overall agreement.