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Corrigendum to "Size-resolved observations of refractory black carbon particles in cloud droplets at a marine boundary layer site" published in Atmos. Chem. Phys., 15, 1367–1383, 2015
Scattering and absorption properties of near-surface aerosol over Gangetic–Himalayan region: the role of boundary-layer dynamics and long-range transport

Light scattering and absorption properties of atmospheric aerosols are of vital importance for evaluating their types, sources and radiative forcing. This is of particular interest over the Gangetic–Himalayan (GH) region due to uplift of aerosol from the plains to the Himalayan range, causing serious effects on atmospheric heating, glaciology and monsoon circulation. In this respect, the Ganges Valley Aerosol Experiment (GVAX) was initiated in Nainital from June 2011 to March 2012 with the aim of examining the aerosol properties, source regions, uplift mechanisms and aerosol–radiation–cloud interactions. The present study examines the temporal (diurnal, monthly, seasonal) evolution of scattering (σ<sub<sp) and absorption (σap) coefficients, their wavelength dependence, and the role of the Indo-Gangetic plains (IGP), boundary-layer dynamics (BLD) and long-range transport (LRT) in aerosol evolution via the Atmospheric Radiation Measurement Mobile Facility. The analysis is separated for particles <10 μm and <1 μm in diameter in order to examine the influence of particle size on optical properties. The σsp and σap exhibit a pronounced seasonal variation between the monsoon low and post-monsoon (November) high, while the scattering wavelength exponent exhibits higher values during the monsoon, in contrast to the absorption Ångström exponent which maximizes in December–March. The elevated-background measuring site provides the advantage of examining the LRT of natural and anthropogenic aerosols from the IGP and southwest Asia and the role of BLD in the aerosol lifting processes. The results reveal higher aerosol concentrations at noontime along with an increase in mixing height, suggesting influence from IGP. The locally emitted aerosols present higher wavelength dependence of the absorption in October–March compared to the rather well-mixed and aged transported aerosols. Monsoon rainfall and seasonally changing air masses contribute to the alteration of the extensive and intensive aerosol properties.
Carbonaceous aerosols on the south edge of the Tibetan Plateau: concentrations, seasonality and sources

To quantitatively evaluate the effect of carbonaceous aerosols on the south edge of the Tibetan Plateau, aerosol samples were collected weekly from August 2009 to July 2010 at Qomolangma (Mt. Everest) Station for Atmospheric and Environmental Observation and Research (QOMS, 28.36° N, 86.95° E, 4276 m a.s.l.). The average concentrations of organic carbon (OC), elemental carbon (EC) and water-soluble organic carbon were 1.43, 0.25 and 0.77 μg m−3, respectively. The concentration levels of OC and EC at QOMS are comparable to those at high-elevation sites on the southern slopes of the Himalayas (Langtang and Nepal Climate Observatory at Pyramid, or NCO-P), but 3 to 6 times lower than those at Manora Peak, India, and Godavari, Nepal. Sulfate was the most abundant anion species followed by nitrate, accounting for 25 and 12% of total ionic mass, respectively. Ca2+ was the most abundant cation species (annual average of 0.88 μg m−3). The dust loading, represented by Ca2+ concentration, was relatively constant throughout the year. OC, EC and other ionic species (NH4+, K+, NO3− and SO42−) exhibited a pronounced peak in the pre-monsoon period and a minimum in the monsoon season, being similar to the seasonal trends of aerosol composition reported previously from the southern slope of the Himalayas, such as Langtang and NCO-P. The strong correlation of OC and EC in QOMS aerosols with K+ and levoglucosan indicates that they mainly originated from biomass burning. The fire spots observed by MODIS and backward air-mass trajectories further demonstrate that in pre-monsoon season, agricultural and forest fires in northern India and Nepal were most likely sources of carbonaceous aerosol at QOMS. Moreover, the CALIOP observations confirmed that air-pollution plumes crossed the Himalayas during this period. The highly coherent variation of daily aerosol optical depth (500 nm) between QOMS and NCO-P indicates that both slopes of the Himalayas share a common atmospheric environment regime. In addition to large-scale atmospheric circulation, the unique mountain/valley breeze system can also have an important effect on air-pollutant transport.
The influence of the North Atlantic Oscillation and El Niño–Southern Oscillation on mean and extreme values of column ozone over the United States

Continuous measurements of total ozone (by Dobson spectrophotometers) across the contiguous United States began in the early 1960s. Here, we analyze temporal and spatial variability and trends in total ozone from the five US sites with long-term records. While similar long-term ozone changes are detected at all five sites, we find differences in the patterns of ozone variability on shorter timescales. In addition to standard evaluation techniques, STL-decomposition methods (Seasonal Trend decomposition of time series based on LOESS (LOcally wEighted Scatterplot Smoothing)) are used to address temporal variability and "fingerprints" of dynamical features in the Dobson data. Methods from statistical extreme value theory (EVT) are used to characterize days with high and low total ozone (termed EHOs and ELOs, respectively) at each station and to analyze temporal changes in the frequency of ozone extremes and their relationship to dynamical features such as the North Atlantic Oscillation (NAO) and El Niño–Southern Oscillation. A comparison of the fingerprints detected in the frequency distribution of the extremes with those for standard metrics (i.e., the mean) shows that more fingerprints are found for the extremes, particularly for the positive phase of the NAO, at all five US monitoring sites. Results from the STL decomposition support the findings of the EVT analysis. Finally, we analyze the relative influence of low- and high-ozone events on seasonal mean column ozone at each station. The results show that the influence of ELOs and EHOs on seasonal mean column ozone can be as much as ±5 %, about as large as the overall long-term decadal ozone trends.
Modelling street level PM10 concentrations across Europe: source apportionment and possible futures

Despite increasing emission controls, particulate matter (PM) has remained a critical issue for European air quality in recent years. The various sources of PM, both from primary particulate emissions as well as secondary formation from precursor gases, make this a complex problem to tackle. In order to allow for credible predictions of future concentrations under policy assumptions, a modelling approach is needed that considers all chemical processes and spatial dimensions involved, from long-range transport of pollution to local emissions in street canyons. Here we describe a modelling scheme which has been implemented in the GAINS integrated assessment model to assess compliance with PM10 (PM with aerodynamic diameter <10 μm) limit values at individual air quality monitoring stations reporting to the AirBase database. The modelling approach relies on a combination of bottom up modelling of emissions, simplified atmospheric chemistry and dispersion calculations, and a traffic increment calculation wherever applicable. At each monitoring station fulfilling a few data coverage criteria, measured concentrations in the base year 2009 are explained to the extent possible and then modelled for the past and future. More than 1850 monitoring stations are covered, including more than 300 traffic stations and 80% of the stations which exceeded the EU air quality limit values in 2009. As a validation, we compare modelled trends in the period 2000–2008 to observations, which are well reproduced. The modelling scheme is applied here to quantify explicitly source contributions to ambient concentrations at several critical monitoring stations, displaying the differences in spatial origin and chemical composition of urban roadside PM10 across Europe. Furthermore, we analyse the predicted evolution of PM10 concentrations in the European Union until 2030 under different policy scenarios. Significant improvements in ambient PM10 concentrations are expected assuming successful implementation of already agreed legislation; however, these will not be large enough to ensure attainment of PM10 limit values in hot spot locations such as Southern Poland and major European cities. Remaining issues are largely eliminated in a scenario applying the best available emission control technologies to the maximal technically feasible extent.
Influence of satellite-derived photolysis rates and NOx emissions on Texas ozone modeling

Uncertain photolysis rates and emission inventory impair the accuracy of state-level ozone (O3) regulatory modeling. Past studies have separately used satellite-observed clouds to correct the model-predicted photolysis rates, or satellite-constrained top-down NOx emissions to identify and reduce uncertainties in bottom-up NOx emissions. However, the joint application of multiple satellite-derived model inputs to improve O3 state implementation plan (SIP) modeling has rarely been explored. In this study, Geostationary Operational Environmental Satellite (GOES) observations of clouds are applied to derive the photolysis rates, replacing those used in Texas SIP modeling. This changes modeled O3 concentrations by up to 80 ppb and improves O3 simulations by reducing modeled normalized mean bias (NMB) and normalized mean error (NME) by up to 0.1. A sector-based discrete Kalman filter (DKF) inversion approach is incorporated with the Comprehensive Air Quality Model with extensions (CAMx)–decoupled direct method (DDM) model to adjust Texas NOx emissions using a high-resolution Ozone Monitoring Instrument (OMI) NO2 product. The discrepancy between OMI and CAMx NO2 vertical column densities (VCDs) is further reduced by increasing modeled NOx lifetime and adding an artificial amount of NO2 in the upper troposphere. The region-based DKF inversion suggests increasing NOx emissions by 10–50% in most regions, deteriorating the model performance in predicting ground NO2 and O3, while the sector-based DKF inversion tends to scale down area and nonroad NOx emissions by 50%, leading to a 2–5 ppb decrease in ground 8 h O3 predictions. Model performance in simulating ground NO2 and O3 are improved using sector-based inversion-constrained NOx emissions, with 0.25 and 0.04 reductions in NMBs and 0.13 and 0.04 reductions in NMEs, respectively. Using both GOES-derived photolysis rates and OMI-constrained NOx emissions together reduces modeled NMB and NME by 0.05, increases the model correlation with ground measurement in O3 simulations, and makes O3 more sensitive to NOx emissions in the O3 non-attainment areas.
Corrigendum to "Recent trends in aerosol optical properties derived from AERONET measurements" published in Atmos. Chem. Phys., 14, 12271-12289, 2014
Aged boreal biomass-burning aerosol size distributions from BORTAS 2011

Biomass-burning aerosols contribute to aerosol radiative forcing on the climate system. The magnitude of this effect is partially determined by aerosol size distributions, which are functions of source fire characteristics (e.g. fuel type, MCE) and in-plume microphysical processing. The uncertainties in biomass-burning emission number–size distributions in climate model inventories lead to uncertainties in the CCN (cloud condensation nuclei) concentrations and forcing estimates derived from these models. The BORTAS-B (Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellite) measurement campaign was designed to sample boreal biomass-burning outflow over eastern Canada in the summer of 2011. Using these BORTAS-B data, we implement plume criteria to isolate the characteristic size distribution of aged biomass-burning emissions (aged ~ 1–2 days) from boreal wildfires in northwestern Ontario. The composite median size distribution yields a single dominant accumulation mode with Dpm = 230 nm (number-median diameter) and σ = 1.5, which are comparable to literature values of other aged plumes of a similar type. The organic aerosol enhancement ratios (ΔOA / ΔCO) along the path of Flight b622 show values of 0.09–0.17 μg m−3 ppbv−1 (parts per billion by volume) with no significant trend with distance from the source. This lack of enhancement ratio increase/decrease with distance suggests no detectable net OA (organic aerosol) production/evaporation within the aged plume over the sampling period (plume age: 1–2 days), though it does not preclude OA production/loss at earlier stages. A Lagrangian microphysical model was used to determine an estimate of the freshly emitted size distribution corresponding to the BORTAS-B aged size distributions. The model was restricted to coagulation and dilution processes based on the insignificant net OA production/evaporation derived from the ΔOA / ΔCO enhancement ratios. We estimate that the young-plume median diameter was in the range of 59–94 nm with modal widths in the range of 1.7–2.8 (the ranges are due to uncertainty in the entrainment rate). Thus, the size of the freshly emitted particles is relatively unconstrained due to the uncertainties in the plume dilution rates.
Technical Note: On the possibly missing mechanism of 15 μm emission in the mesosphere–lower thermosphere (MLT)

Accurate knowledge of the rate as well as the mechanism of excitation of the bending mode of CO2 is necessary for reliable modeling of the mesosphere–lower thermosphere (MLT) region of the atmosphere. Assuming the excitation mechanism to be thermal collisions with atomic oxygen, the rate coefficient derived from the observed 15 μm emission by space-based experiments (kATM = 6.0 × 10−12 cm3s−1) differs from the laboratory measurements (kLAB =(1.5-2.5) × 10−12 cm3s−1) by a factor of 2–4. The general circulation models (GCMs) of Earth, Venus, and Mars have chosen to use a median value of kGCM = 3.0 × 10−12 cm3s−1 for this rate coefficient. As a first step to resolve the discrepancies between the three rate coefficients, we attempt to find the source of disagreement between the first two. It is pointed out that a large magnitude of the difference between these two rate coefficients (kx &equiv; kATM - kLAB) requires that the unknown mechanism involve one or both major species: N2, O. Because of the rapidly decreasing volume mixing ratio (VMR) of CO2 with altitude, the exciting partner must be long lived and transfer energy efficiently. It is shown that thermal collisions with N2, mediated by a near-resonant rotation-to-vibration (RV) energy transfer process, while giving a reasonable rate coefficient kVR for de-excitation of the bending mode of CO2, lead to vibration-to-translation kVT rate coefficients in the terrestrial atmosphere that are 1–2 orders of magnitude larger than those observed in the laboratory. It is pointed out that the efficient near-resonant rotation-to-vibration (RV) energy transfer process has a chance of being the unknown mechanism if very high rotational levels of N2, produced by the reaction of N and NO and other collisional processes, have a super-thermal population and are long lived. Since atomic oxygen plays a critical role in the mechanisms discussed here, it suggested that its density be determined experimentally by ground- and space-based Raman lidars proposed earlier.
Characterization of forest fire smoke event near Washington, DC in summer 2013 with multi-wavelength lidar

The multi-wavelength lidar technique was applied to the study of a smoke event near Washington, DC on 26–28 August 2013. Satellite observations combined with transport model predictions imply that the smoke plume originated mainly from Wyoming/Idaho forest fires and its transportation to Washington, DC took approximately 5 days. The NASA Goddard Space Flight Center (GSFC) multi-wavelength Mie–Raman lidar was used to measure the smoke particle intensive parameters such as extinction and backscatter Ångström exponents together with lidar ratios at 355 and 532 nm wavelengths. For interpretation of the observed vertical profiles of the backscatter Ångström exponents γβ at 355–532 and 532–1064 nm, numerical simulation was performed. The results indicate that, for fine-mode dominant aerosols, the Ångström exponents γβ(355–532) and γβ(532–1064) have essentially different dependence on the particle size and refractive index. Inversion of 3 β + 2 α lidar observations on 27–28 August provided vertical variation of the particle volume, effective radius and the real part of the refractive index through the planetary boundary layer (PBL) and the smoke layer. The particle effective radius decreased with height from approximately 0.27 μm inside the PBL to 0.15 μm in the smoke layer, which was situated above the PBL. Simultaneously the real part of the refractive index in the smoke layer increased to mR ≈ 1.5. The retrievals demonstrate also that the fine mode is predominant in the particle size distribution, and that the decrease of the effective radius with height is due to a shift of the fine mode toward smaller radii.