Inter-comparison of aerosol optical depth from the Multi-Wavelength Solar Radiometer with other radiometric measurements

364-371The Multi-Wavelength Solar Radiometer (MWR), designed and developed in the Space Physics Laboratory, has been extensively used by several research institutions across the country for estimating columnar spectral aerosol optical depth (AOD), the most important parameter needed for assessing the impact of aerosols on regional aerosol forcing. A network of > 30 MWRs is currently in operation under the Aerosol Radiative Forcing over India (ARFI) Project of Indian Space Research Organization’s Geosphere Biosphere Program (ISRO-GBP). This paper reports the results of an extensive inter-comparison of the AODs deduced using the MWR with those obtained from other commercially available instruments, such as a Multi-Filter Rotating Shadowband Radiometer (MFRSR), a calibrated Microtops Sun Photometer (MTOPS) and an EKO Sun Photometer (ESP). The results indicated very good agreement between the AOD derived from MWR with MFRSR, MTOPS and ESP with correlation coefficients of ~ 0.99, 0.88 and 0.92, respectively. This report is intended to serve as a reference document for researchers while using the MWR along with other commercial instruments

Aerosol number size distributions over a coastal semi urban location: seasonal changes and ultrafine particle bursts

Number-size distribution is one of the important microphysical properties of atmospheric aerosols that influence aerosol life cycle, aerosol-radiation interaction as well as aerosol-cloud interactions. Making use of one-year long measurements of aerosol particle number-size distributions (PNSD) over a broad size spectrum (~15–15,000 nm) from a tropical coastal semi-urban location Trivandrum (Thiruvananthapuram), the size characteristics, their seasonality and response to mesoscale and synoptic scale meteorology are examined. While the accumulation mode contributed mostly to the annual mean concentration, ultrafine particles (having diameter <100 nm) contributed as much as 45% to the total concentration, and thus constitute a strong reservoir, that would add to the larger particles through size transformation. The size distributions were, in general, bimodal with well-defined modes in the accumulation and coarse regimes, with mode diameters lying in the range 141 to 167 nm and 1150 to 1760 nm respectively, in different seasons. Despite the contribution of the coarse sized particles to the total number concentration being meager, they contributed significantly to the surface area and volume, especially during transport of marine air mass highlighting the role of synoptic air mass changes. Significant diurnal variation occurred in the number concentrations, geometric mean diameters, which is mostly attributed to the dynamics of the local coastal atmospheric boundary layer and the effect of mesoscale land/sea breeze circulation. Bursts of ultrafine particles (UFP) occurred quite frequently, apparently during periods of land-sea breeze transitions, caused by the strong mixing of precursor-rich urban air mass with the cleaner marine air mass; the resulting turbulence along with boundary layer dynamics aiding the nucleation. These ex-situ particles were observed at the surface due to the transport associated with boundary layer dynamics. The particle growth rates from ultrafine particles to accumulation sizes varied between 1 and 15 nm h−1, with mean growth rate of ~7.35±2.93 nm h−1

Aerosol number size distributions over a coastal semi urban location: Seasonal changes and ultrafine particle bursts

Number-size distribution is one of the important microphysical properties of atmospheric aerosols that influence aerosol life cycle, aerosol-radiation interaction as well as aerosol-cloud interactions. Making use of one-yearlong measurements of aerosol particle number-size distributions (PNSD) over a broad size spectrum similar to 15-15,000 nm) from a tropical coastal semi-urban location-Trivandrum (Thiruvananthapuram), the size characteristics, their seasonality and response to mesoscale and synoptic scale meteorology are examined. While the accumulation mode contributed mostly to the annual mean concentration, ultrafine particles (having diameter <100 nm) contributed as much as 45% to the total concentration, and thus constitute a strong reservoir, that would add to the larger particles through size transformation. The size distributions were, in general, bimodal with well-defined modes in the accumulation and coarse regimes, with mode diameters lying in the range 141 to 167 nm and 1150 to 1760 nm respectively, in different seasons. Despite the contribution of the coarse sized particles to the total number concentration being meager, they contributed significantly to the surface area and volume, especially during transport of marine air mass highlighting the role of synoptic air mass changes. Significant diurnal variation occurred in the number concentrations, geometric mean diameters, which is mostly attributed to the dynamics of the local coastal atmospheric boundary layer and the effect of mesoscale land/sea breeze circulation. Bursts of ultrafine particles (UFP) occurred quite frequently, apparently during periods of land-sea breeze transitions, caused by the strong mixing of precursor-rich urban air mass with the cleaner marine air mass; the resulting turbulence along with boundary layer dynamics aiding the nucleation. These ex-situ particles were observed at the surface due to the transport associated with boundary layer dynamics. The particle growth rates from ultrafine particles to accumulation sizes varied between 1 and 15 nm h(-1), with mean growth rate of similar to 7.35 +/- 2.93 nm h(-1). (c) 2016 Elsevier B.V. All rights reserved

Rapid response of atmospheric BC to anthropogenic sources: observational evidence

The response of Black Carbon (BC) concentrations to a near cut-off of vehicular and industrial emissions favoured by a regional ‘strike call’ by organized trade unions, that brought to a halt of all industrial activities and commercial road traffic for 2 days is presented. In a dramatic, spectacular response to the seizure of emissions, BC concentrations dropped rapidly to ∼22% of the average concentrations that prevailed before the strike depicting the rapidity with which the concentrations deplete and the ‘rather slow’ replenishment as the emissions restart. These observations support the argument that controlling BC emissions yields quick results

Spring-time enhancement in aerosol burden over a high-altitude location in western trans-Himalaya: results from long-term observations

Long-term measurements (from August 2009 to December 2014) of aerosol black carbon mass concentration (MBC) and spectral aerosol optical depth (AOD) were carried out from a high-altitude location, Hanle in western trans-Himalaya as part of the Regional Aerosol Warming Experiment. Both MBC and AOD showed distinct annual pattern with a clear spring-time enhancement (April-June) with significant inter-annual variability associated with the changes in source processes. The potential source regions contributing to the spring-time enhancement in aerosol loading are the dust-dominated west Asian region as well as biomass burning from NW India. The overall annual mean value of MBC over Hanle is extremely low compared to many other Himalayan locations, including the Ganges Valley Aerosol Experiment campaign site at Nainital, which also showed spring-time (pre-monsoon) enhancement. The vertical extents of elevated aerosol layers, which contribute to the spring-time enhancement, are found to be in the range 5-7 km amsl from the analysis of vertical profiles of extinction coefficients from CALIPSO data

Formation pathways of organic aerosols over a tropical coastal atmosphere

The characteristics of non-refractory submicron aerosols (NR-PM1), focusing on organic aerosols, are examined over a tropical coastal location, Thumba, in southern peninsular India. The measurements were carried out using an aerosol chemical speciation monitor during 2017–2021. The results indicated organic aerosols as the major component (67–85%) in the NR-PM1 mass loading during all seasons. Positive matrix factorization (PMF) analysis on the OA mass spectra yielded two factors resolving Hydrocarbon-like organic aerosols (HOA) and Oxygenated organic aerosols (OOA), with the major contribution to OA coming from OOA (mass fraction &gt;50%) during all the seasons. Examination of the diurnal variations revealed the daytime enhancement in OOA and sulfate mass fractions, indicating photochemical production overwhelming the dilution at the surface due to local atmospheric boundary layer expansion. The OOA mass concentration showed a good correlation (R &gt; 0.5) with odd oxygen (O3+NO2) and with an increase in oxygen-to-carbon ratio (ΔO:C = 0.11 to 0.35 with Δodd oxygen = 60 μg m−3 for different seasons) (i.e., increased oxidation degree of OA) signifying the dominance of photochemistry and subsequent enhanced atmospheric oxidative capacity in OOA formation. Interestingly, the aerosol liquid water content (ALWC) doubled during the nighttime (&gt;40 μg m−3) compared to the daytime (&lt;20 μg m−3) in different seasons, indicating the combined impact of varying submicron aerosol composition and ambient relative humidity. Since highly humid (relative humidity &gt;85%) conditions prevail over this tropical coastal station, especially during the nighttime, the role of aqueous-phase formation of OA is also examined. It was found that less oxidized/less-hygroscopic OA were prevalent during the nighttime, resulting in lower aqueous-phase OA processing. During all the seasons, the hygroscopicity of organics (кorganics) reached as high as &gt; 0.22 and was mainly driven by photochemical processing rather than aqueous-phase chemistry. These results highlight the importance of the OA formation pathways in affecting their hygroscopic properties over the tropical atmosphere.</p

Aerosol mass size distribution and black carbon over a high altitude location in western Trans-Himalayas: impact of a dust episode

The information on the aerosol properties from remote locations provides insights into the background and natural conditions against which anthropogenic impacts could be compared. Measurements of the near surface aerosol mass size distribution from the high altitude remote site help us to understand the natural processes, such as, the association between Aeolian and fluvial processes that have a direct bearing on the mass concentrations, especially in the larger size ranges. In the present study, the total mass concentration and mass-size distribution of the near surface aerosols, measured using a 10-channel Quartz Crystal Microbalance (QCM) Impactor from a high altitude location-Hanle (32.78°N, 78.95°E, 4520 m asl) in the western Trans-Himalayas, have been used to characterize the composite aerosols. Also the impact of a highly localized, short-duration dust storm episode on the mass size distribution has been examined. In general, though the total mass concentration (Mt) remained very low (∼0.75 ± 0.61 μg m−3), interestingly, coarse mode (super-micron) aerosols contributed almost 72 ± 6% to the total aerosol mass loading near the surface. The mass-size distribution showed 3 modes, a fine particle mode (∼0.2 μm), an accumulation mode at ∼0.5 μm, and a coarse mode at ∼3 μm. During a localized short duration dust storm episode, Mt reached as high as ∼13.5 μg m−3 with coarse mode aerosols contributing to nearly 90% of it. The mass size distribution changed significantly, with a broad coarse mode so that the accumulation mode became inconspicuous. Concurrent measurements of aerosol black carbon (BC) using twin wavelength measurements of the aethalometer showed an increase in the wavelength index of absorption, from the normal values of ∼1 to 1.5 signifying the enhanced absorption at the short wavelength (380 nm) by the dust

Columnar aerosol extinction characteristics: measurements from a free-tropospheric observatory in Western-Himalayas

Continuous measurements of spectral aerosol optical depths (AOD) were made over a very high altitude aerosol observatory at Hanle (32.76°N, 78.95°E, 4520 m amsl), during the summer and autumn of 2009 and 2010, by using a 10-channel multi-wavelength radiometer (MWR). The results depicted very low values of AOD (with a mean for the measurement period as ~ 0.071 ± 0.003 at 500 nm), but with a spectral dependence that changed with the months from ~ 0.8 to 1.3, with a mean value of ~ 1.01 ± 0.06. The AODs though were in general comparable to those seen over pristine Antarctic environments, are quite significant when we consider the altitude of the station. The details are presented

The formation and growth of ultrafine particles in two contrasting environments: a case study

Formation of ultrafine particles and their subsequent growth have been examined during new particle formation (NPF) events in two contrasting environments under varying ambient conditions, one for a tropical semi-urban coastal station, Trivandrum, and the other for a high-altitude free-tropospheric Himalayan location, Hanle. At Trivandrum, NPF bursts took place in the late evening/night hours, whereas at Hanle the burst was a daytime event. During the nucleation period, the total number concentration reached levels as high as ~ 15 900 cm−3 at Trivandrum, whereas at Hanle, the total number concentration was ~ 2700 cm−3, indicating the abundant availability of precursors at Trivandrum and the pristine nature of Hanle. A sharp decrease was associated with NPF for the geometric mean diameter of the size distribution, and a large increase in the concentration of the particles in the nucleation regime (Dp 25 nm). Once formed, these (secondary) aerosols grew from nucleation (diameter Dp 25 nm) to Aitken (25 ≤ Dp ≤ 100 nm) regime and beyond, to the accumulation size regimes (100 ≤ Dp ≤ 1000 nm), with varying growth rates (GR) for the different size regimes at both the locations. A more rapid growth ~ 50 nm h−1 was observed at Trivandrum, in contrast to Hanle where the growth rate ranged from 0.1 to 20 nm h−1 for the transformation from the nucleation to accumulation – a size regime that can potentially act as cloud condensation nuclei (CCN). The faster coagulation led to lifetimes of 1 h for nucleation mode particles