Altitude distribution of aerosols over Southeast Arabian Sea coast during pre-monsoon season: Elevated layers,long-range transport and atmospheric radiative heating

Every year, during the pre-monsoon period (March–May), a pronounced increase in aerosol optical depth (AOD) is observed over the eastern Arabian Sea, which is attributed to the transport of continental aerosols. This paper presents the altitude distribution of tropospheric aerosols, characteristics of elevated aerosol layers and aerosol radiative heating of the atmosphere during the pre-monsoon season over Trivandrum (8.5°N, 77°E), a station located at the southwest coast of Indian peninsula which is covered by the eastern Arabian Sea plume. Altitude profiles of aerosol backscatter coefficient (β_a) and linear depolarization ratio (LDR) reveal two distinct aerosol layers persisting between 0–2 km and 2–4 km. The layer at 2–4 km, which contributes about 25% of the AOD during polluted conditions, contains significant amount of non-spherical aerosols. This layer is prominent only when the advection of dry airmass occurs from the northern parts of the Indian subcontinent and northern Arabian Sea. Role of long-range transport in the development of this aerosol layer is further confirmed using latitude–altitude cross-section of β_a observed by CALIPSO. Aerosol content in the layer below 2 km is large when advection of air occurs from the north and east Arabian Sea and is significantly small when it occurs from the southwest Arabian Sea or Indian Ocean. During the highly polluted conditions, aerosols tend to increase the diurnal mean atmospheric radiative heating rate by ～0.8 K day^?1 at 500 m and 0.3 K day^?1 at 3 km, which are about 80% and 30% of the respective radiative heating in the aerosol-free atmosphere.     

Influence of accidental fire on black carbon aerosol concentrations over an urban environment

Carbonaceous aerosols are emitted by combustion sources and may influence the climate by altering the radiation balance of the atmosphere. Carbonaceous particles exist mainly in the accumulation mode and thus may be transported over long distances. The present study deals with the impact of anthropogenic activity associated with accidental fires on the black carbon aerosol concentrations over an urban environment, namely Hyderabad, India. Black carbon aerosol loading in association with meteorological parameters on a normal day, an accident day and a post-accident day have been analysed. Diurnal variations of black carbon aerosols on a normal day suggest that black carbon aerosol concentrations increased by a factor of about 2 during morning and evening hours compared with afternoon hours. A drastic increase in black carbon aerosol loading was found during an accident day compared with a normal day. An immediate return to normal black carbon concentration was found during the post-accident day. Black carbon aerosol loading in relation to rainfall is also discussed in the paper.     

Effects of radiative forcing by black carbon aerosol on spring rainfall decrease over Southeast Asia

The effects of black carbon (BC) aerosol radiative forcing on spring rainfall in Southeast Asia are studied using numerical simulations with the NASA finite-volume General Circulation Model (fvGCM) forced with monthly varying three-dimensional aerosol distributions from the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART).During the boreal spring, March–April–May (MAM), BC from local emissions accumulates over Southeast Asia. The BC aerosol layer, which extends from the surface to higher elevation above planetary boundary layer (PBL), absorbs solar radiation and heats the mid-troposphere through a semi-direct effect over regions of large aerosol optical thickness (AOT) and thereby significantly perturbs large-scale and meridional circulations. Results show that anomalous precipitation patterns and associated large-scale circulations induced by radiative forcing by BC aerosol can explain observed precipitation reductions, especially over Southeast Asia. Therefore, BC aerosol forcing may be one of the important factors affecting the spring rainfall trend over Southeast Asia.     

On the contribution of black carbon to the composite aerosol radiative forcing over an urban environment

This paper discusses the extent of Black Carbon (BC) radiative forcing in the total aerosol atmospheric radiative forcing over Pune, an urban site in India. Collocated measurements of aerosol optical properties, chemical composition and BC were carried out for a period of six months (during October 2004 to May 2005) over the site. Observed aerosol chemical composition in terms of water soluble, insoluble and BC components were used in Optical Properties of Aerosols and Clouds (OPAC) to derive aerosol optical properties of composite aerosols. The BC fraction alone was used in OPAC to derive optical properties of BC aerosols. The aerosol optical properties for composite and BC aerosols were separately used in SBDART model to derive direct aerosol radiative forcing due to composite and BC aerosols. The atmospheric radiative forcing for composite aerosols were found to be +35.5, +32.9 and +47.6 Wm^?2 during post-monsoon, winter and pre-monsoon seasons, respectively. The average BC mass fraction found to be 4.83, 6.33 and 4 μg m^?3 during the above seasons contributing around 2.2 to 5.8% to the total aerosol load. The atmospheric radiative forcing estimated due to BC aerosols was +18.8, +23.4 and +17.2 Wm^?2, respectively during the above seasons. The study suggests that even though BC contributes only 2.2–6% to the total aerosol load; it is contributing an average of around 55% to the total lower atmospheric aerosol forcing due to strong radiative absorption, and thus enhancing greenhouse warming.     

Seasonal variation of black carbon aerosols over a tropical urban city of Pune,India

Black carbon (BC) aerosols were monitored continuously at Pune, a tropical urban location in southwest India, using aethalometer AE-42 model. Results of the data for the 1-year period (January to December 2005) have been discussed here. Seasonal and diurnal variations of BC in relation to changes in the regional meteorological conditions and local boundary layer characteristics have been studied along with the mass fraction of BC to the total suspended particulates (TSP) in different months. Also, using the Hysplit model, back-trajectories are studied to assess the sources for transported BC particles. The data collected during January to December 2005 indicated that annual average BC concentration (4.1 μg m⁻³) at Pune was comparable to that reported for other urban locations in southern Indian region. During winter season, BC concentrations were maximum (about 80% more than annual mean), mainly due to prevailing meteorological conditions like low wind speeds and low ventilation coefficients; as well as due to the transport from northeast regions. Minimum BC concentrations were observed during monsoon season (about 68% less than annual mean), which could be attributed to the wash-out effects due to precipitation as well as due to southwesterly winds coming from marine areas. Diurnal variation of BC showed two peaks, one in morning and another in the evening, which are mostly related to the daily changes in the local boundary layer. However, the intensity of local traffic emissions could have some impact on the magnitude of these peaks. BC aerosols formed about 2.3% of the total aerosol mass fraction at Pune.     

MODIS derived fire characteristics and aerosol optical depth variations during the agricultural residue burning season, north India

Agricultural residue burning is one of the major causes of greenhouse gas emissions and aerosols in the Indo-Ganges region. In this study, we characterize the fire intensity, seasonality, variability, fire radiative energy (FRE) and aerosol optical depth (AOD) variations during the agricultural residue burning season using MODIS data. Fire counts exhibited significant bi-modal activity, with peak occurrences during April-May and October-November corresponding to wheat and rice residue burning episodes. The FRE variations coincided with the amount of residues burnt. The mean AOD (2003-2008) was 0.60 with 0.87 (+1σ) and 0.32 (−1σ). The increased AOD during the winter coincided well with the fire counts during rice residue burning season. In contrast, the AOD-fire signal was weak during the summer wheat residue burning and attributed to dust and fossil fuel combustion. Our results highlight the need for ‘full accounting of GHG’s and aerosols’, for addressing the air quality in the study area.     

Local circulation and aerosol water-soluble ions--a case study in Taiwan during Mei-yu season

The Mei-yu (plum rain) season is a short but important period when the weather changes from spring to summer in Taiwan. In this study, size-segregated aerosols were collected alternately at 5 sampling sites in northwestern Taiwan from June 16 to 24, 1994. For the first time in Taiwan, this study revealed the aerosol mass spectra and water-soluble ions in the Mei-yu season. For all samples, a bi-modal aerosol mass spectra was found with modal diameters at 3.2 and 0.32 microm, respectively. The aerosol samples were able to be divided into different groups to show their mass and ion spectra according to the calculated 5-hr backward air trajectory. The utilization of enrichment factors showed that aerosol Cl-, Na+, and Mg2+ for all sizes, and super-micron SO4(2-) were related to the sea. Both the scheme of "chlorine loss" (Ohta and Okita, 1990) and a multivariate analysis (Thurston and Spengler, 1985) for categorizing water-soluble ions showed that sea-salts were major contributors in the prevalence of a sea breeze. In contrast, the secondary salts were significant for land breeze and a mix of land-sea breeze. In conclusion, the influence of local circulation on the distribution of aerosol mass and ionic species was found to be prominent.     

重庆市核心区黑碳气溶胶浓度特征以及影响因素分析

2013年4月至2014年2月期间利用重庆市大气超级站的黑碳气溶胶(black carbon,BC)、气态污染物(SO2、NOx和O3)和颗粒物观测数据,分析了重庆市BC浓度的变化特征及与能见度、颗粒物以及SO2、NOx和O3气态污染物的相关性。观测期间BC年日均值为(4.86±2.37)μg/m3,浓度范围为1.32~11.54μg/m3。秋冬季BC日均浓度及相对偏差比春夏季高。BC和能见度呈负相关性。4个季度的BC与PM10、PM2.5和PM1日均值显著正相关,相关系数最小在夏季,最大在秋季。BC与O3日均值呈负相关性。BC与SO2,NOx日均值显著正相关,表明重庆市BC与SO2,NOx来源相近,即为燃煤和机动车尾气排放。     

Field validation of a semi-continuous method for aerosol black carbon (aethalometer) and temporal patterns of summertime hourly black carbon measurements in southwestern PA

Two methods for measuring aerosol elemental carbon (EC) are compared. Three-hour integrated carbon samples were collected on quartz filters during the summer of 1990 in Uniontown, PA, primarily during episodes of elevated particulate pollution levels. These samples were analyzed for EC and organic carbon (OC) using a thermo/optical reflectance (TOR) method. Aerosol black carbon (BC) was measured using an Aethalometer, a semi-continuous optical absorption method. The optical attenuation factor for ambient BC was supplied by the instrument manufacturer. Three-hour average concentrations were calculated from the semi-continuous BC measurements to temporally match the EC/OC integrated quartz filter samples. BC and EC concentrations are highly correlated over the study period (R²=0.925). The regression equation is BC (μg m^-3)=0.95 (±0.04) EC−0.2 (±0.4). The means of 3 h average measurements for EC and BC are 2.3 and 2.0 μg m^-3, respectively, average concentrations of EC and BC ranged from 0.6 to 9.4 and 0.5 to 9.0 μg m^-3 respectively. TOR OC and EC concentrations were not highly correlated (R²=0.22). The mean OC/EC ratio was 1.85.The 10-week Aethalometer hourly dataset was analyzed for daily and weekly temporal patterns. A strong diurnal BC pattern was observed, with peaks occurring between 7 a.m. and 9 a.m. local time. This is consistent with the increase in emissions from ground level combustion sources in the morning, coupled with poor dispersion before daytime vertical mixing is established. There was also some indication of a day-of-week effect on BC concentrations, attributed to activity of local ground level anthropogenic sources. Comparison of BC concentrations with co-located measurements of coefficient of haze in a separate field study in Philadelphia, PA, during the summer of 1992 showed good correlation between the two measurements (R²=0.82).     

The size distribution of marine atmospheric aerosol with regard to primary biological aerosol particles over the South Atlantic Ocean

The marine atmosphere is characterized by particles which originate from the ocean and by those which reached the air by advection from the continent. The bubble-burst mechanism produces both sea salt as well as biological particles. The following article describes the determination of the size distribution of marine aerosol particles with special emphasis on the biological particles. Th data were obtained on three cruises with the German Research Vessel “METEOR” crossing the South Atlantic Ocean. The measurements showed that biological particles amount to 17% in number and 10% in volume concentration. Another type of particle became obvious in the marine atmosphere, the biologically contaminated particle, i.e. particles which consist partly (approximately up to one-third) of biological matter. Their concentration in the evaluated size class (r>2 μm) is higher than the concentration of the pure biological particles. The concentrations vary over about one to two orders of magnitude during all cruises.     

Role of organic and black carbon in the chemical composition of atmospheric aerosol at European background sites

The mass concentrations of inorganic ions, water-soluble organic carbon, water-insoluble organic carbon and black carbon were determined in atmospheric aerosol collected at three European background sites: (i) the Jungfraujoch, Switzerland (high-alpine, PM_2.5 aerosol); (ii) K-puszta, Hungary (rural, PM_1.0 aerosol); (iii) Mace Head, Ireland (marine, total particulate matter). At the Jungfraujoch and K-puszta the contribution of carbonaceous compounds to the aerosol mass was higher than that of inorganic ions by 33% and 94%, respectively. At these continental sites about 60% of the organic carbon was water soluble, 55–75% of the total carbon proved to be refractory and a considerable portion of the water soluble, refractory organic matter was composed of humic-like substances. At Mace Head the mass concentration of organic matter was found to be about twice than that of nonsea-salt ions, 40% of the organic carbon was water soluble and the amount of highly refractory carbon was low. Humic-like substances were not detected but instead low molecular weight carboxylic acids were responsible for about one-fifth of the water-soluble organic mass. These results imply that the influence of carbonaceous compounds on aerosol properties (e.g. hygroscopic, optical) might be significant.     

Characteristic number size distribution of aerosol during Asian dust period in Korea

The size-separated number concentrations of aerosols ranging from 0.3 to 25 μm were observed in Seoul and Anmyon Island in the west coast of Korea during Asian dust period in Spring 1998. During the heavy dust period, the number size distributions of aerosols observed in both places were characterized by decreases in small size<0.5 μm and increase in large size between 1.35 and 10 μm. For particles in this range, there was a good correlation between number concentrations observed in both two places. The number of coarse particles >10 μm showed a distinct diurnal variation without a significant change in amplitude, which was more pronounced in Seoul. It suggests that coarse particles were more affected by local sources. Trajectories back in time showed that the air collected in Korea during dust period originated from desert regions in the central part of China. From these results, it was evident that increased particles in the range of 1.35–10 μm during dust source period represented mineral components, which originated possibly from the dust source regions.     

Some specific features of the aerosol particle concentrations during the dry season and during a bushfire event in West Africa

This study shows the results of concentration measurements of large particles (D ⩾0.3μm), CCN and Aitken nuclei (CN) in two different sites of the Ivory Coast—Lamto and Abidjan—during the middle of the dry season. A comparison is established over a period of 24 h; it clearly indicates that the aerosol is the same in the two stations if we consider the large particles and nuclei activated at 0.25% supersaturation. However, it is more abundant in smaller panicles in the Abidjan area, mainly in the early morning, and its emission is reinforced during the rains of the monsoon.     

Elemental composition of aerosol in the Nam Co region,Tibetan Plateau,during summer monsoon season

In order to investigate the chemical characteristics of atmospheric aerosols in a remote region of the Tibetan Plateau, total suspended particles were collected continuously at the Nam Co Comprehensive Observation and Research Station from July to October 2005. The PIXE analytical results showed that Si, Ca, Fe, Al, K, and S were the major components of aerosols, ranging from 82 (K)–550 (Si) ng m⁻³. The mean elemental concentrations were comparable with those from other remote sites and significantly lower than those from megacities (e.g. Beijing). The very low presence of anthropogenic elements demonstrates that the Nam Co region is an ideal background site for atmospheric monitoring. Crustal enrichment factor (EF) calculation indicated that several anthropogenic heavy metals (Cr, Ni, Cu, Zn, As) are transported long distances atmospherically. The backward air mass trajectory analysis suggests that South Asia may be the source region of those pollutants.     

The effect of aerosol optical depth on rainfall with reference to meteorology over metro cities in India

Rainfall is a key link in the global water cycle and a proxy for changing climate; therefore, proper assessment of the urban environment’s impact on rainfall will be increasingly important in ongoing climate diagnostics and prediction. Aerosol optical depth (AOD) measurements on the monsoon seasons of the years 2008 to 2010 were made over four metro regional hotspots in India. The highest average of AOD was in the months of June and July for the four cities during 3 years and lowest was in September. Comparing the four regions, Kolkata was in the peak of aerosol contamination and Chennai was in least. Pearson correlation was made between AOD with climatic parameters. Some changes in the parameters were found during drought year. Temperature, cloud parameters, and humidity play an important role for the drought conditions. The role of aerosols, meteorological parameters, and their impacts towards the precipitation during the monsoon was studied.     

The vertical profile of atmospheric heating rate of black carbon aerosols at Kanpur in northern India

Altitude profiles of the mass concentrations of aerosol black carbon (BC) and composite aerosols were obtained from the collocated measurements of these quantities onboard an aircraft, over the urban area of Kanpur, in the Ganga basin of northern India during summer, for the first time in India. The enhancement in the mean BC concentration was observed at ∼1200 m in the summer, but the vertical gradient of BC concentration is less than the standard deviation at that altitude. The difference in the BC altitude profile and columnar concentration in the winter and summer is attributed to the enhanced turbulent mixing within the boundary layer in summer. This effect is more conspicuous with BC than the composite aerosols, resulting in an increase in the BC mass fraction (F_BC) at higher levels in summer. This high BC fraction results in an increase in the lower atmospheric heating rate in both the forenoon, FN and afternoon, AN, but with contrasting altitude profile. The FN profile shows fluctuating trend with highest value (2.1 K day⁻¹) at 300 m and a secondary peak at 1200 m altitudes, whereas the AN profile shows increasing trend with highest value (1.82 K day⁻¹) at 1200 m altitude.     

Total scatter-to-backscatter ratio of aerosol derived from aerosol size distribution measurement

Based on in-situ aerosol size-distribution measurements and Mie scattering theory, total scattering coefficients and backscattering coefficients were calculated to derived wavelength dependent lidar ratio S for 355 nm and 532 nm. Effective radius and C/F ratio of aerosol are also calculated to study the relationships between lidar ratio and particle size dependences. The results show backscatter-related scattering properties are more sensitive to coarse mode particle than total scattering. The mean values of lidar ratio for 355 nm and 532 nm are 31.9 ± 6.2 sr and 40.5 ± 6.1 sr respectively, and S₃₅₅ and S₅₃₂ are linear correlated for S₃₅₅ < 50 sr. S₃₅₅ is highly correlated with effective radius of aerosol, and S₅₃₂ is highly correlated with volume C/F ratio.     

Characterization of aerosol over the Northern South China Sea during two cruises in 2003

Atmospheric transport of trace elements has been found to be an important pathway for their input to the ocean. TSP, PM10, and PM2.5 aerosol samples were collected over the Northern South China Sea in two cruises in 2003 to estimate the input of aerosol from continent to the ocean. About 23 elements and 14 soluble ions in aerosol samples were measured. The average mass concentration of TSP in Cruise I in January (78 μg m⁻³) was ∼twice of that in Cruise II in April (37 μg m⁻³). Together with the crustal component, heavy metals from pollution sources over the land (especially from the industry and automobiles in Guangzhou) were transported to and deposited into the ocean. The atmospheric MSA concentrations in PM2.5 (0.048 μg m⁻³ in Cruise I and 0.043 μg m⁻³ in Cruise II) over Northern South China Sea were comparable to those over other coastal regions. The ratio of non-sea-salt (NSS)-sulfate to MSA is 103-655 for Cruise I and 15-440 for Cruise II in PM2.5 samples, which were much higher than those over remote oceans. The estimated anthropogenic sulfate accounts for 83–98% in Cruise I and 63–95% in Cruise II of the total NSS-sulfate. Fe (II) concentration in the aerosols collected over the ocean ranged from 0.1 to 0.9 μg m⁻³, accounting for 16–82% of the total iron in the aerosol, which could affect the marine biogeochemical cycle greatly.     

Sulphate aerosol size distributions at Mumbai,India, during the INDOEX-FFP (1998)

Sulphate size distributions were measured at the coastal station of Mumbai (formerly Bombay) through 1998, during the Indian ocean experiment (INDOEX) first field phase (FFP), to fill current gaps in size-resolved aerosol chemical composition data. The paper examines meteorological, seasonal and source-contribution effects on sulphate aerosol and discusses potential effects of sulphate on regional climate. Sulphate size-distributions were largely trimodal with a condensation mode (mass median aerodynamic diameter or MMAD 0.6 μm), a droplet mode (MMAD 1.9–2.4 μm) and a coarse mode (MMAD 5 μm). Condensation mode sulphate mass-fractions were highest in winter, consistent with the high meteorological potential for gas-to-particle conversion along with low relative humidity (RH). The droplet mode concentrations and MMADs were larger in the pre-monsoon and winter than in monsoon, implying sulphate predominance in larger sized particles within this mode. In these seasons the high RH, and consequently greater aerosol water in the droplet mode, would favour aerosol-phase partitioning and reactions of SO₂. Coarse mode sulphate concentrations were lowest in the monsoon, when continental contribution to sulphate was low and washout was efficient. In winter and pre-monsoon, coarse mode sulphate concentrations were somewhat higher, likely from SO₂ gas-to-particle conversion. Low daytime sulphate concentrations with a large coarse fraction, along with largely onshore winds, indicated marine aerosol predominance. High nighttime sulphate concentrations and a coincident large fine fraction indicated contributions from anthropogenic/industrial sources or from gas-to-particle conversion. Monthly mean sulphate concentrations increased with increasing SO₂ concentrations, RH and easterly wind direction, indicating the importance of gas-to-particle conversion and industrial sources located to the east. Atmospheric chemistry effects on sulphate size distributions in Mumbai, indicated by this data, must be further examined.