Black Carbon Mass Concentration Research Articles

Impact of COVID-19 lockdown on the fine particulate matter concentration levels: Results from Bengaluru megacity, India

Leveraging the COVID-19 India-wide lockdown situation, the present study attempts to quantify the reduction in the ambient fine particulate matter concentrations during the lockdown (compared with that of the pre-lockdown period), owing to the highly reduced specific anthropogenic activities and thereby pollutant emissions. The study was conducted over Bengaluru (India), using PM2.5 (mass concentration of particulate matter having size less than or equal to 2.5 µm) and Black Carbon mass concentration (BC) data. Open-access datasets from pollution control board (PCB) were also utilised to understand the spatial variability and region-specific reduction in PM2.5 across the city. The highest percentage reduction was observed in BCff (black carbon attributable to fossil fuel combustion), followed by total BC and PM2.5. No decrease in BCbb (black carbon attributable to wood/biomass burning) was observed, suggesting unaltered wood-based cooking activities and biomass-burning (local/regional) throughout the study period. Results support the general understanding of multi-source (natural and anthropogenic) nature of PM2.5 in contrast to limited-source (combustion based) nature of BC. The diurnal amplitudes in BC and BCff were reduced, while they remained almost the same for PM2.5 and BCbb. Analysis of PCB data reveal the highest reduction in PM2.5 in an industrial cluster area. The current lockdown situation acted as a natural model to understand the role of a few major anthropogenic activities (viz., traffic, construction, industries related to non-essential goods, etc.) in enhancing the background fine particulate matter levels. Contemporary studies reporting reduction in surface fine particulate matter and satellite retrieved columnar Aerosol Optical Depth (AOD) during COVID-19 lockdown period are discussed.

Optical and hygroscopic properties of black carbon influenced by particle microphysics at the top of the anthropogenically polluted boundary layer

Abstract. Aerosols at the top of the planetary boundary layer (PBL) could modify its atmospheric dynamics by redistributing the solar radiation and start to be activated to form low-level cloud at this layer. Black carbon (BC), as an aerosol component efficiently absorbing solar radiation, can introduce heating and positive radiative effects at this sensitive layer, especially in the polluted PBL over the continent. This study presents continuous measurements of detailed BC properties at a mountain site located at the top of the polluted PBL over the North China Plain, during seasons (3 and 4 weeks of data during winter and summer, respectively) with contrasting emission structure and meteorology. The pollution level was persistently influenced by local surface anthropogenic emission on a daily basis through daytime convective mixing, but the concentration was also enhanced or diluted depending on air mass direction, defined as a neutral, polluted and diluted PBL, respectively. Winter was observed to have a higher BC mass fraction (4 %–8 %) than summer (2 %–7 %). By resolving the detailed particle size-resolved mixing state of BC in optical and hygroscopic models, we found an enhanced BC mass absorption cross section (MACBC) for the polluted PBL (up to 13 m2 g−1 at λ = 550 nm), which was 5 % higher during summer than winter due to a smaller BC core size. The higher BC mass fraction in winter corresponded to a lower single-scattering albedo by 0.03–0.09 than summer, especially the lowest for the diluted winter PBL (0.86 ± 0.02). The water supersaturation (SS) required to activate half the number of BC decreased from 0.21 % ± 0.08 % to 0.1 % ± 0.03 % for the winter diluted and polluted PBL and from 0.22 % ± 0.06 % to 0.17 % ± 0.05 % for summer. Notably, at the top of the anthropogenically polluted PBL in both seasons, the enlarged BC with enhanced absorption capacity could also be efficiently droplet activated; e.g. winter (summer) BC with an MAC of 9.84 ± 1.2 (10.7 ± 1) m2 g−1 could be half activated at SS = 0.13 % ± 0.06 % (0.18 % ± 0.05 %). This BC at the top of the PBL can more directly interact with the free troposphere and be transported to a wider region, exerting important direct and indirect radiative impacts.

Multi-year characterization of aerosol black carbon concentrations over a semiarid tropical site Udaipur.

Continuous and multi-year (2008-2012) measurements of black carbon (BC) mass concentrations (MBC), carried out from the semiarid tropical site Udaipur (24.6° N, 74° E, 580ma.s.l.) near the western Indian desert, are analyzed for their region-specific features. MBC varied over a wide range during the period of study, with the hourly mean values occurring as low as 0.09 to as high as 49.1μgm-3, with the multi-year average ~ 4.5 ± 2.6μgm-3. Annual variations showed the highest concentrations during November and December (winter seasonal mean = 7.4 ± 3.3μgm-3) and the lowest in the monsoon months of July and August (monsoon seasonal mean = 2.5 ± 2.2μgm-3). MBC showed significant inverse relationship with the planetary boundary layer height (ρ ~ - 0.50) as well as ventilation coefficient (ρ ~ - 0.72). Alike aforesaid atmospheric dynamic parameters, T, WS, and RH also possessed statistically significant negative correlations with monthly MBC. Enhancement in annual as well as diurnal amplitude in MBC during deficient monsoon years relative to excessive monsoon years have given marked indication of BC sink mechanism due to precipitation. Roles of long-range regional air pollutant transport also have been identified. Identical and consistent seasonal variation in percentage contribution of MBC with PM2.5 (varying from 2.6 to 9.1%) and absorption Angstrom exponent (αabs, monthly mean values varying from 0.77 ± 0.04 to 1.16 ± 0.08) gives evidence of substantial amount of enhanced anthropogenic source activities of fossil fuel incomplete combustion in post-monsoon and winter period.

The Impact of Lockdown on eBC Mass Concentration over a Semi-arid Region in Southern India: Ground Observation and Model Simulations

ABSTRACT In the present study, we focused on the impact of lockdown on black carbon (eBC) mass concentrations and their associated radiative implications from 01st March to 30th June 2020, over a semi-arid station, i.e., in the district of Anantapur in Southern India. The mean eBC mass concentration was observed before lockdown (01st–24th March 2020) and during the lockdown (25th March–30th June 2020) period and was about 1.74 ± 0.36 and 1.11 ± 0.14 µg m–3, respectively. The sharp decrease (~35%) of eBC mass concentration observed during the lockdown (LD) period as compared with before lockdown (BLD) period, was mainly due to the reduction of anthropogenic activities and meteorology. Furthermore, during the entire LD period, the net composite forcing at the top of the atmosphere (TOA) and at the surface (SUR) varied from –4.52 to –6.19 Wm–2 and –22.91 to –29.35 Wm–2, respectively, whereas the net forcing in the atmosphere (ATM) varied from 17.27 to 23.16 Wm–2. Interestingly, the amount of energy trapped in the atmosphere due to eBC is 11.19 Wm–2 before LD and 8.56 Wm–2 during LD. It is concluded that eBC contributes almost 43–50% to the composite forcing. As a result, the eBC atmospheric heating rate decreased significantly (25%) when compared to before lockdown days to lockdown days.

Air pollutant variations in Suzhou during the 2019 novel coronavirus (COVID-19) lockdown of 2020: High time-resolution measurements of aerosol chemical compositions and source apportionment.

To control the spread of the 2019 novel coronavirus (COVID-19), China imposed rigorous restrictions, which resulted in great reductions in pollutant emissions. This study examines the characteristics of air pollutants, including PM2.5 (particles with aerodynamic diameters < 2.5 μm), gas pollutants, water-soluble ions (WSIs), black carbon (BC) and elements, as well as the source apportionment of PM2.5 in Suzhou before, during and after the Chinese New Year (CNY) holiday of 2020 (when China was under an unprecedented state of lockdown to restrict the COVID-19 outbreak). Compared to those before CNY, PM2.5, BC, SNA (sulfate, nitrate and ammonium), other ions, elements, and NO2 and CO mass concentrations decreased by 9.9%–64.0% during CNY. The lockdown policy had strong (weak) effects on the diurnal variations in aerosol chemical compositions (gas pollutants). Compared to those before CNY, source concentrations and contributions of vehicle exhaust during CNY decreased by 72.9% and 21.7%, respectively. In contrast, increased contributions from coal combustion and industry were observed during CNY, which were recorded to be 2.9 and 1.7 times higher than those before CNY, respectively. This study highlights that the lockdown policy that was imposed in Suzhou during CNY not only reduced the mass concentrations of air pollutants but also modified their diurnal variations and the source contributions of PM2.5, which revealed the complex responses of PM2.5 sources to the rare, low emissions of anthropogenic pollutants that occurred during the COVID-19 lockdown.

Three-dimensional spatiotemporal evolution of wildfire-induced smoke aerosols: A case study from Liangshan, Southwest China

Carbonaceous aerosols and gaseous pollutants emitted from wildfires play a crucial role in both the global climate system and regional air quality. Here, using multisource satellite and ground-based observations combined with reanalysis data, we investigate the three-dimensional evolution of biomass-burning emissions from a forest wildfire event in Liangshan, Southwest China, which occurred from 29 March to 1 April 2020. The meteorological field analysis showed that the negative anomaly of relative humidity and precipitation, as well as the positive anomaly of near-surface wind speed, created favourable conditions for the occurrence and spread of this wildfire event. During the fire, satellite observations suggested a maximum fire radiation power of over 100 MW. In addition, there were significant short-term effects of fire activity on regional air quality, with downwind surface PM2.5 concentrations at the Xichang site reaching a maximum of approximately 470 μg·m −3 on March 31. Driven by a southwesterly airflow, large amounts of smoke aerosols were transported rapidly to downstream areas, significantly deteriorating air quality, with the maximum value of the aerosol optical depth (AOD) exceeding 2. Moreover, the quantitative evaluation based on Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis showed that the instantaneous maximum values of the column mass concentration of black carbon (BC) and organic carbon (OC) reached 9.8 g·m−2 and 1.8 g·m−2 during the fire respectively. Further analysis suggested that the interaction between the lower and upper atmosphere constrained the smoke aerosols to altitudes below approximately 5 km, which was also supported by the vertical distribution of elevated smoke aerosols observed by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP).

Characterization of black carbon aerosol at the summit of Mount Tai (1534 m) in central east China: Temporal variation, source appointment and transport

As a crucial absorption component of aerosols, black carbon (BC) plays a key role in modifying aerosol-planetary boundary layer (PBL) meteorology and hence aggravating haze pollution. With an altitude of 1534 m, Mt. Tai is close to the top of the PBL and located within the transportation channel from the North China Plain (NCP) to the Yangtze River Delta (YRD). In this study, BC aerosols were continuously observed using a seven-channel aethalometer (AE-33) at the summit of Mt. Tai from May 11 to June 25, 2017. The temporal variation, source and transport characteristics of BC in the upper PBL of central and eastern China in summer were then discussed. The average concentrations of BC, BCliquid and BCsolid were 1722.6, 1194.6 and 545.8 ng m−3, respectively, during the observation period. Liquid fuel sources were calculated and accounted for 64.7% of the BC mass concentration. The distributions of BC were mostly centralized in concentrations less than 600 ng m−3 (accounting for 21.9% of the total BC samples) and ranged from 1400 to 2200 ng m−3 (32.5%). Meteorological elements were determined to have great impacts on the distribution of BC concentrations, which increased with the decrease in visibility and wind speed and the increase in relative humidity (RH). The diurnal variations in BC presented high values during the daytime and low values during the nighttime, with peaks at 06:00, 11:00 and 18:00. Long-range transport from the YRD regions in summer, as well as the surrounding regions, played an important role in determining the BC concentrations at Mt. Tai with concentration weighted trajectory (CWT) values exceeding 2000 ng m−3. Air masses may move northward to the NCP after passing through Mt. Tai, and they contributed more than 1800 ng m−3 to the BC in the whole NCP region.

Hidden black carbon air pollution in hilly rural areas—a case study of Dinaric depression

Air pollution is not an exclusively urban problem as wood burning is a widespread practice in rural areas. As we lack information on the air quality situation in rural mountainous regions, our aim is to examine equivalent black carbon (eBC) pollution in a typical rural karst area in the settlement of Loški Potok (Slovenia). eBC mass concentrations were measured by Aethalometer (AE-33) at two sites in Retje karst depression. The rural village station was located at the bottom of the karst depression whereas the rural background station was positioned at the top of the hill. We show the diurnal variation of equivalent black carbon mass concentrations for different seasons. In the populated karst depression, the major source of eBC pollution are households using wood as a heating fuel reaching the highest mass concentrations in winter. Diurnal pattern of eBC from biomass burning and traffic differ due to different source activity and it is influenced by typical formation of a cold air pool from late afternoon until late morning, restricting the dispersion of local emissions. The large difference in mass concentrations between the lowest part of the village (rural station) and the top of the hill (rural background station) indicates that in a vertically stratified and stable atmosphere local sources of black carbon have a major impact on air quality conditions in the area studied. Since in Alpine and Dinaric regions there are many similar inhabited areas, we can expect similar air quality conditions also in other rural hilly areas with limited self-cleaning air capacity.

Impact of aerosols on surface ozone during COVID-19 pandemic in southern India: A multi-instrumental approach from ground and satellite observations, and model simulations

The World Health Organization (WHO) declared the coronavirus disease of 2019 (COVID-19) as a pandemic due to its widespread global infection. This has resulted in lockdown under different phases in many nations, including India, around the globe. In the present study, we report the impact of aerosols on surface ozone in the context of pre-lockdown (01st - 24th March 2020 (PLD)), lockdown phase1 (25th March to 14th April 2020 (LDP1)), and lockdown phase 2 (15th April to 03rd May 2020 (LDP2)) on clear days at a semi-arid site, Anantapur in southern India using both in situ observations and model simulations. Collocated measurements of surface ozone (O3), aerosol optical depth (AOD), black carbon mass concentration (BC), total columnar ozone (TCO), solar radiation (SR), and ultraviolet radiation (UV-A) data were collected using an Ozone analyzer, MICROTOPS sunphotometer, Ozonometer, Aethalometer, and net radiometer during the study period. The diurnal variations of O3 and BC exhibited an opposite trend during three phases. The concentrations of ozone were ~10.7% higher during LDP1 (44.8 ± 5.2 ppbv) than the PLD (40.5 ± 6.0 ppbv), which mainly due to an unprecedented reduction in NOx emissions leading to a lower O3 titration by NO. The prominent increase in the surface zone during LDP1 is reasonably consistent with the observed photolysis frequencies (j (O1D)) through Tropospheric Ultraviolet and Visible (TUV) model. The results show that a pronounced spectral and temporal variability in the AOD during three lockdown phases is mainly due to distinct aerosol sources. The increase in AOD during LDP2 due to long-range transport can bring large amounts of mineral dust and smoke aerosols from the west Asian region and central India, and which is reasonably consistent with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) air mass back trajectories and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts analysis over the measurement location. Overall, a drastic reduction in BC concentration (~8.4%) and AOD (10.8%) were observed in the semi-arid area during LDP1 with correspondence to PLD. The columnar aerosol size distributions retrieved from the spectral AODs followed power-law plus unimodal during three phases. The absorption angstrom exponent (AAE) analysis reveals a predominant contribution to the BC from biomass burning activities during the lockdown period over the measurement location.

Resolving aerosol mixing state increases accuracy of black carbon respiratory deposition estimates

Summary Black carbon (BC) particles, also known as soot, deteriorate air quality and threaten public health. BC is emitted from the incomplete combustion of fossil fuel, which is a major energy source in the world today. Therefore, evaluating the impact of BC on global public health is vital, especially in the context of growing global population and energy demand. Since the atmospheric lifetime and deposition efficiency of BC after inhalation are governed by the size and chemical composition, realistic representation of those properties in models is required for such evaluation, yet is currently lacking. Here, we show that a simple but commonly found representation of size and chemical composition of BC particles in global models results in globally prevalent underestimation of the respiratory deposited BC mass concentration. The results emphasize that realistic representation of BC particles is indispensable to the evaluation and hence the mitigation of BC impacts on global public health.

A Major Combustion Aerosol Event Had a Negligible Impact on the Atmospheric Ice‐Nucleating Particle Population

AbstractClouds containing supercooled water are important for both climate and weather, but our knowledge of which aerosol particle types nucleate ice in these clouds is far from complete. Combustion aerosols have strong anthropogenic sources, and if these aerosol types were to nucleate ice in clouds, they might exert a climate forcing. Here, we quantified the atmospheric ice‐nucleating particle (INP) concentrations during the United Kingdom's annual Bonfire Night celebrations, which are characterized by large amounts of combustion aerosol from bonfires and fireworks. We used three immersion mode techniques covering more than 6 orders of magnitude in INP concentration over the temperature range from −10°C to homogeneous freezing. We found no observable systematic change in the INP concentration on three separate nights, despite more than a factor of 10 increase in aerosol number concentrations, up to a factor of 10 increase in PM10 concentration, and more than a factor of 100 increase in black carbon (BC) mass concentration relative to pre‐event levels. This implies that BC and other combustion aerosol such as ash did not compete with the INPs present in the background air. Furthermore, the upper limit of the ice‐active site surface density, ns(T), of BC generated in these events was shown to be consistent with several other recent laboratory studies, showing a very low ice‐nucleating activity of BC. We conclude that combustion aerosol particles similar to those emitted on Bonfire Night are at most of secondary importance for the INP population relevant for mixed‐phase clouds in typical midlatitude terrestrial locations.

A mutual response between surface temperature and black carbon mass concentration during the daytime

The mutual response between surface temperature and the mass concentration of regional black carbon (BC) aerosols has still remained far from understanding due to its complex nature. A detailed analysis presented in this study using long-term data indicates a significant pattern of mutual response between surface temperature and BC in restricted background weather conditions (water vapor, cloud cover and wind speeds). The analysis shows that a fall in surface temperature which naturally occurs daily after the sunrise, leads to the development of a stronger inversion in the near-surface level and this, in turn, contributes to the enhancement of BC fumigation peak. Further, the enhanced fumigation peak (especially during pre-monsoon) is found positively influencing the mid-day temperature rise possibly due to the immediate impact of the direct radiative forcing of BC aerosols. These observations lead us to consider a hypothesis that ‘an extra fall in the morning hour surface temperature contributes to the enhancement of BC fumigation peak and can degrade the morning hour air quality which gives positive feedback to the mid-day temperature rise over a region’. A substantial in situ data [over Gadanki (13.5°N, 79.2°E)] along with MERRA-2 and ERA-5 data are used in this methodical analysis. Moreover, the validity of the hypothesis has been tested over other locations. Regional weather and seasonal cycle are found to have apparent interference with the feature of the observed mutual response pattern. The results from this study clearly indicate that the approach used, can be executed location independently.

Role of semi-volatile particulate matter in gas-particle partitioning leading to change in oxidative potential.

Atmospheric semi-volatile organic compounds (SVOCs) are complex in their chemical and toxicological characteristics with sources from both primary combustion emissions and secondary oxygenated aerosol formation processes. In this study, thermal desorption of PM2.5 in association with online measurement of reactive oxygen species (ROS) was carried out to study the role of SVOCs in its gas-particle partitioning. The mass concentrations of PM2.5, black carbon (BC) and p-PAHs downstream of a thermodenuder were measured online at different temperature settings (25, 50, 100, and 200°C) to characterize PM physico-chemical properties. While the mass concentrations of PM2.5 and p-PAHs reduced to ∼34% at 200°C compared to that in ambient temperature, BC mass concentration has decreased by 30% at the highest temperature. Furthermore, the submicron particle size distribution showed reduced particle number concentration in Aitken mode at 200°C heating. The ROS, measured by Particle-into-Nitroxide-Quencher, also showed reduction and followed a similar trend with PM measurements, where the total ROS decreased by 12%, 31%, and 53% at 50°C, 100°C, and 200°C, respectively, compared to the ambient sample. When a HEPA filter was included in the upstream of samples, 39% of gas phase ROS reduction was observed at 200°C. This provided a good estimate of the contribution of SVOCs in ROS production in PM2.5, where decreased SVOCs concentration at 200°C increased the percentage of particle surface area. This concludes that the surface chemistry of these organic coatings on the particles is important for assessing the health impacts of PM.

Spatial Distribution of Atmospheric Aerosol Physicochemical Characteristics in the Russian Sector of the Arctic Ocean

The results from studies of aerosol in the Arctic atmosphere are presented: the aerosol optical depth (AOD), the concentrations of aerosol and black carbon, as well as the chemical composition of the aerosol. The average aerosol characteristics, measured during nine expeditions (2007–2018) in the Eurasian sector of the Arctic Ocean, had been 0.068 for AOD (0.5 µm); 2.95 cm−3 for particle number concentrations; 32.1 ng/m3 for black carbon mass concentrations. Approximately two–fold decrease of the average characteristics in the eastern direction (from the Barents Sea to Chukchi Sea) is revealed in aerosol spatial distribution. The average aerosol characteristics over the Barents Sea decrease in the northern direction: black carbon concentrations by a factor of 1.5; particle concentrations by a factor of 3.7. These features of the spatial distribution are caused mainly by changes in the content of fine aerosol, namely: by outflows of smokes from forest fires and anthropogenic aerosol. We considered separately the measurements of aerosol characteristics during two expeditions in 2019: in the north of the Barents Sea (April) and along the Northern Sea Route (July–September). In the second expedition the average aerosol characteristics turned out to be larger than multiyear values: AOD reached 0.36, particle concentration up to 8.6 cm−3, and black carbon concentration up to 179 ng/m3. The increased aerosol content was affected by frequent outflows of smoke from forest fires. The main (99%) contribution to the elemental composition of aerosol in the study regions was due to Ca, K, Fe, Zn, Br, Ni, Cu, Mn, and Sr. The spatial distribution of the chemical composition of aerosols was analogous to that of microphysical characteristics. The lowest concentrations of organic and elemental carbon (OC, EC) and of most elements are observed in April in the north of the Barents Sea, and the maximal concentrations in Far East seas and in the south of the Barents Sea. The average contents of carbon in aerosol over seas of the Asian sector of the Arctic Ocean are OC = 629 ng/m3, EC = 47 ng/m3.

Seasonal variability and source apportionment of black carbon over a rural high-altitude and an urban site in western India

Black carbon (BC) mass concentration observations were carried out using Aethalometer (AE-33) at an urban site Pune and at a high altitude forest site Mahabaleshwar during April 2018 to February 2019. The annual averaged BC mass concentration over Pune was observed to be ~2.8 times higher than Mahabaleshwar. Daily, seasonal and diurnal variations of BC over these two different environments were compared. The effect of meteorological parameters on the BC was also evaluated. Planetary boundary layer (PBL) plays a pivotal role in diluting the atmospheric pollutant BC during daytime at Pune, whereas, contribution of combustion sources mostly dominated over PBL dilution at Mahabaleshwar. Higher wind speed regime also tends to dilute the atmosphere whereas higher BC concentration was observed during RH condition depicting the significance of low dispersion under these conditions. The contribution of different sources namely biomass burning (BB) and fossil fuel (FF), wavelength dependent source apportionment study was also evaluated. Traffic emission contributed 72%–75% of the BC loading at Mahabaleshwar, whereas at Pune it was estimated to be 82%–94% during all the seasons indicating BB contributed more at Mahabaleshwar. Cluster analysis and concentration weighted trajectory (CWT) analysis were also performed to visualize the importance of regional transport for both the locations. The study confers the significance of regional transport in addition to local emissions over both the sites.

Snow Surface Albedo Sensitivity to Black Carbon: Radiative Transfer Modelling

The broadband surface albedo of snow can greatly be reduced by the deposition of light-absorbing impurities, such as black carbon on or near its surface. Such a reduction increases the absorption of solar radiation and may initiate or accelerate snowmelt and snow albedo feedback. Coincident measurements of both black carbon concentration and broadband snow albedo may be difficult to obtain in field studies; however, using the relationship developed in this simple model sensitivity study, black carbon mass densities deposited can be estimated from changes in measured broadband snow albedo, and vice versa. Here, the relationship between the areal mass density of black carbon found near the snow surface to the amount of albedo reduction was investigated using the popular snow radiative transfer model Snow, Ice, and Aerosol Radiation (SNICAR). We found this relationship to be linear for realistic amounts of black carbon mass concentrations, such as those found in snow at remote locations. We applied this relationship to measurements of broadband albedo in the Chilean Andes to estimate how vehicular emissions contributed to black carbon (BC) deposition that was previously unquantified.

Ozone variability: Influence by its precursors and meteorological parameters- an investigation

Surface observations on trace gases such as ozone (O3), oxides of nitrogen (August 2014–May 2017) and carbon monoxide (January 2016–May 2017) were used to investigate O3 chemistry. Study shows O3 concentration is high during winter with a mean value of 35.54 ± 7.16 ppbv and low concentration in monsoon with a mean value of 12.70 ± 3.55 ppbv. Oxides of nitrogen (NOx = NO + NO2) concentrations are high during pre-monsoon followed by winter, post-monsoon and monsoon. Mean carbon monoxide (CO) concentrations were observed to be 201.21 ± 36.90 ppbv, 222.61 ± 37.31 ppbv, 170.14 ± 14.34 ppbv, 249.59 ± 52.03 ppbv respectively during winter, pre monsoon, monsoon and post-monsoon. Study suggests that irrespective of seasonal changes, strong correlation coefficients (0.97–0.99) were observed between O3 and air temperature. Solar radiation showed an observable influence on surface O3 concentrations in the lower troposphere. The rate of change of O3 concentration during morning and evening hours at the study site were 3.6 ppbv/h and −3.9 ppbv/h respectively characterizing the site as sub-urban category. The mass concentrations of black carbon (BC) measurements (January–February 2016) were also used to understand its influence on CO and O3 concentrations. The BC showed a strong positive correlation (0.90) with CO whereas strong negative correlation (0.90) with O3. Study also indicates a strong influence of atmospheric boundary layer height on O3 and BC concentrations. Study revealed that emissions from crop residue burning during winter and forest fire during pre-monsoon have significant influence on enhancing CO concentrations at study site.

Absorption closure in highly aged biomass burning smoke

Abstract. The optical properties of black carbon (BC) are a major source of uncertainty in regional and global climate studies. In the past, detailed investigation of BC absorption has been hampered by systematic biases in the measurement instrumentation. We present airborne measurements of aerosol absorption and black carbon microphysical properties in highly aged biomass burning plumes measured 4–8 d from their source over the southeast Atlantic Ocean during CLARIFY-2017, using a suite of novel photoacoustic spectrometers to measure aerosol absorption at 405, 514, and 655 nm and a single-particle soot photometer to measure the BC mass concentration, size, and mixing state. These measurements are of sufficient quality and detail to provide constraint on optical schemes used in climate models for the first time in biomass burning plumes far from their source – an aerosol environment that is one of the most important climatically. The average absorption Ångström exponents (AAE) were 1.38 over the wavelength range from 405 to 514 nm and 0.88 over the range from 514 to 655 nm, suggesting that brown carbon (BrC) contributed to 11±2 % of absorption at 405 nm. The effective organic aerosol (OA) mass absorption coefficient (MAC) was 0.31±0.09 m2 g−1 at 405 nm. The BC particles were universally thickly coated, and almost no externally mixed BC particles were detected. The average MAC of BC was 20±4, 15±3, and 12±2 m2g−1 at wavelengths of 405, 514, and 655 nm respectively, with equivalent absorption enhancements of around 1.85±0.45 at all three wavelengths, suggesting that the thick coatings acted as a lens that enhanced light absorption by the BC. We compared the measured MAC and AAE values with those calculated using several optical models and absorption parameterisations that took the measured BC mass and mixing state as inputs. Homogeneous grey-sphere Mie models were only able to replicate MAC for some low (real and imaginary) values of the complex BC refractive index (mBC) at the shortest wavelength, but they would have to use unrealistically low values of mBC to accurately replicate the AAE. A core–shell Mie model was able to generate good agreement for MAC in the green–red end of the visible spectrum for most values of mBC. However, there are no possible values of mBC that produce MAC values that agree with our observations at all three wavelengths, due to a wavelength-dependent underestimation of the MAC of the underlying BC core. Four semiempirical parameterisations from the literature were also tested, linking the BC mixing state to either the MAC or absorption enhancement. Two of these schemes produced results that agreed within a few percent with the measured MAC at all three wavelengths, and the AAE agreed well when discounting the effects of BrC. Our results uniquely demonstrate the validity of absorption parameterisations, as well as the failings of Mie calculations, in this highly aged environment. We recommend that future work should conduct similar analyses in environments where BC has different properties; future studies should also investigate the impact of implementing these types of schemes within climate models as well as the impact of developing equivalent schemes for light scattering by soot particles at visible wavelengths.

Multi-year ACSM measurements at the central European research station Melpitz (Germany) – Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter

Abstract. The aerosol chemical speciation monitor (ACSM) is nowadays widely used to identify and quantify the main components of fine particles in ambient air. As such, its deployment at observatory platforms is fully incorporated within the European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Regular intercomparisons are organized at the Aerosol Chemical Monitoring Calibration Center (ACMCC; part of the European Center for Aerosol Calibration, Paris, France) to ensure the consistency of the dataset, as well as instrumental performance and variability. However, in situ quality assurance remains a fundamental aspect of the instrument's stability. Here, we present and discuss the main outputs of long-term quality assurance efforts achieved for ACSM measurements at the research station Melpitz (Germany) since 2012 onwards. In order to validate the ACSM measurements over the years and to characterize seasonal variations, nitrate, sulfate, ammonium, organic, and particle mass concentrations were systematically compared against the collocated measurements of daily offline high-volume PM1 and PM2.5 filter samples and particle number size distribution (PNSD) measurements. Mass closure analysis was made by comparing the total particle mass (PM) concentration obtained by adding the mass concentration of equivalent black carbon (eBC) from the multi-angle absorption photometer (MAAP) to the ACSM chemical composition, to that of PM1 and PM2.5 during filter weighing, as well as to the derived mass concentration of PNSD. A combination of PM1 and PM2.5 filter samples helped identifying the critical importance of the upper size cutoff of the ACSM during such exercises. The ACSM–MAAP-derived mass concentrations systematically deviated from the PM1 mass when the mass concentration of the latter represented less than 60 % of PM2.5, which was linked to the transmission efficiency of the aerodynamic lenses of the ACSM. The best correlations are obtained for sulfate (slope =0.96; R2=0.77) and total PM (slope =1.02; R2=0.90). Although, sulfate did not exhibit a seasonal dependency, total PM mass concentration revealed a small seasonal variability linked to the increase in non-water-soluble fractions. The nitrate suffers from a loss of ammonium nitrate during filter collection, and the contribution of organo-nitrate compounds to the ACSM nitrate signal make it difficult to directly compare the two methods. The contribution of m∕z 44 (f44) to the total organic mass concentration was used to convert the ACSM organic mass (OM) to organic carbon (OC) by using a similar approach as for the aerosol mass spectrometer (AMS). The resulting estimated OCACSM was compared with the measured OCPM1 (slope =0.74; R2=0.77), indicating that the f44 signal was relatively free of interferences during this period. The PM2.5 filter samples use for the ACSM data quality might suffer from a systematic bias due to a size truncation effect as well as to the presence of chemical species that cannot be detected by the ACSM in coarse mode (e.g., sodium nitrate and sodium sulfate). This may lead to a systematic underestimation of the ACSM particle mass concentration and/or a positive artifact that artificially decreases the discrepancies between the two methods. Consequently, ACSM data validation using PM2.5 filters has to be interpreted with extreme care. The particle mass closure with the PNSD was satisfying (slope =0.77; R2=0.90 over the entire period), with a slight overestimation of the mobility particle size spectrometer (MPSS)-derived mass concentration in winter. This seasonal variability was related to a change on the PNSD and a larger contribution of the supermicrometer particles in winter. This long-term analysis between the ACSM and other collocated instruments confirms the robustness of the ACSM and its suitability for long-term measurements. Particle mass closure with the PNSD is strongly recommended to ensure the stability of the ACSM. A near-real-time mass closure procedure within the entire ACTRIS–ACSM network certainly represents an optimal quality control and assurance of both warranting the quality assurance of the ACSM measurements as well as identifying cross-instrumental biases.

Vertical profiles of light absorption and scattering associated with black carbon particle fractions in the springtime Arctic above 79° N

Abstract. Despite the potential importance of black carbon (BC) for radiative forcing of the Arctic atmosphere, vertically resolved measurements of the particle light scattering coefficient (σsp) and light absorption coefficient (σap) in the springtime Arctic atmosphere are infrequent, especially measurements at latitudes at or above 80∘ N. Here, relationships among vertically distributed aerosol optical properties (σap, σsp and single scattering albedo or SSA), particle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9 and 83.4∘ N from near the surface to 500 hPa. Airborne data collected during April 2015 are combined with ground-based observations from the observatory at Alert, Nunavut and simulations from the Goddard Earth Observing System (GEOS) model, GEOS-Chem, coupled with the TwO-Moment Aerosol Sectional (TOMAS) model (collectively GEOS-Chem–TOMAS; Kodros et al., 2018) to further our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for σsp less than 15 Mm−1, which represent 98 % of the observed σsp, because the single scattering albedo (SSA) has a tendency to be lower at lower σsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g−1, the average BC mass absorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two averaged modelled MAC values (13.6 and 9.1 m2 g−1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC, the presence of small amounts of dust and/or possible differences in BC microphysics and morphologies between the observations and model. In comparing the observations and simulations, we present σap and SSA, as measured, and σap∕2 and the corresponding SSA to encompass the lower modelled MAC that is more consistent with accepted MAC values. Median values of the measured σap, rBC and the organic component of particles all increase by a factor of 1.8±0.1, going from near-surface to 750 hPa, and values higher than the surface persist to 600 hPa. Modelled BC, organics and σap agree with the near-surface measurements but do not reproduce the higher values observed between 900 and 600 hPa. The differences between modelled and observed optical properties follow the same trend as the differences between the modelled and observed concentrations of the carbonaceous components (black and organic). Model-observation discrepancies may be mostly due to the modelled ejection of biomass burning particles only into the boundary layer at the sources. For the assumption of the observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of σsp&lt;15 Mm−1. The large uncertainties in measuring optical properties and BC, and the large differences between measured and modelled values here and in the literature, argue for improved measurements of BC and light absorption by BC and more vertical profiles of aerosol chemistry, microphysics and other optical properties in the Arctic.