Role of dynamics on the formation and maintenance of the elevated aerosol layer during monsoon season over south-east peninsular India

Ratnam, M. Venkat; Prasad, P. Durga; Raman, M. Roja; Kiran, V. Ravi; Rao, S. Vijaya Bhaskara; Murthy, B. V. Krishna; Jayaraman, A.

doi:10.1016/j.atmosenv.2018.06.023

Cited by 45 publications

(20 citation statements)

References 37 publications

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“…dust, but the exact physical and chemical composition was unknown. Previous work in March and April has shown elevated aerosol layers extended above the boundary layer to an altitude of ∼ 5 km (Kulkarni et al, 2012;Sinha et al, 2013;Babu et al, 2016;Sarangi et al, 2016) and with monsoon arrival the elevated aerosol layer is removed (Ratnam et al, 2018;Vernier et al, 2018), but our in situ measurements were able to quantify the aerosol presence and its characteristics above these heights during the pre-monsoon and how they developed as the monsoon progressed. The maximum black carbon mass concentrations of ∼ 0.5-1 µg m −3 occurred at an altitude of ∼ 2 km in previous studies (Moorthy et al, 2009;Nair et al, 2013), consistent with the EAL in our data present between 2 and 7 km, with similar concentrations of ∼ 0.5 µg m −3 of BC in the elevated layers.…”

Section: Discussionmentioning

confidence: 75%

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons

et al. 2019

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Abstract. The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons. Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43&thinsp;%) followed by sulfate (29&thinsp;%), ammonium (14&thinsp;%), nitrate (7&thinsp;%) and black carbon (7&thinsp;%). However, outside the Indo-Gangetic Plain, sulfate was the dominant species, contributing 44&thinsp;% to the total submicron aerosol mass in the boundary layer, followed by organic matter (30&thinsp;%), ammonium (14&thinsp;%), nitrate (6&thinsp;%) and black carbon (6&thinsp;%). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2&thinsp;µg&thinsp;m−3) compared to outside (1&thinsp;µg&thinsp;m−3). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattered in nature, suggesting greater dust presence, especially in north-western India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by ∼50&thinsp;% as the rainfall arrived; the pre-monsoon average total mass concentration was 30&thinsp;µg&thinsp;m−3 compared to a monsoon average total mass concentration of 10–20&thinsp;µg&thinsp;m−3. However, this mass concentration decrease was less noteworthy (∼20&thinsp;%–30&thinsp;%) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere (&lt;1.5&thinsp;km), with sulfate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5&thinsp;km with maximum aerosol height ∼6&thinsp;km. The elevated concentration of dust at altitudes &gt;1.5&thinsp;km is a clear indication of dust transport from the Great Indian Desert, also called the Thar Desert, in north-western India. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to ∼2&thinsp;km. The dust and sulfate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.

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Section: Discussionmentioning

confidence: 75%

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons

et al. 2019

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“…These differences in the positions of the sharp decrease in SSA and decrease in the magnitude of the extinction coefficients are likely to be a result of washout by the extensive monsoon rains, aided by the sinking motion of air while reaching BoB. The role of the low-level monsoon jets across the Arabian Sea in maintaining the elevated absorbing aerosol layers over the whole of the southern peninsula has been reported recently by Ratnam et al (2018).…”

Section: Synoptic Meteorology During the Campaignmentioning

confidence: 68%

“…However, these do not consider the vertical heterogeneity, which is important. To examine this aspect, we estimated the atmospheric radiative forcing due to aerosols (difference between the surface and top-of-atmosphere forcing) using the measured SSA and extinction profiles along with the concurrent atmospheric parameters (measured during the experiment) using the SB-DART radiative transfer scheme (Ricchiazzi et al, 1998). We also evaluated the atmospheric forcing by replacing the SSA profile with a single SSA value, computed as the weighted average of the profile.…”

Section: Radiative Forcing Estimates Using Measured Extinction and Ssmentioning

confidence: 99%

Vertical profiles of submicron aerosol single scattering albedo over the Indian region immediately before monsoon onset and during its development: research from the SWAAMI field campaign

et al. 2020

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Abstract. Vertical structures of aerosol single scattering albedo (SSA), from near the surface through the free troposphere, have been estimated for the first time at distinct geographical locations over the Indian mainland and adjoining oceans, using in situ measurements of aerosol scattering and absorption coefficients aboard the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the South West Asian Aerosol Monsoon Interactions (SWAAMI) campaign from June to July 2016. These are used to examine the spatial variation of SSA profiles and also to characterize its transformation from just prior to the onset of Indian Summer Monsoon (June 2016) to its active phase (July 2016). Very strong aerosol absorption, with SSA values as low as 0.7, persisted in the lower altitudes (<3 km) over the Indo-Gangetic Plains (IGP), prior to the monsoon onset, with a west-to-east gradient; lower values occurred in the north-western arid regions, peaking in the central IGP and somewhat decreasing towards the eastern end. During the active phase of the monsoon, the SSA is found to increase remarkably, indicating far less absorption. Nevertheless, significant aerosol absorption persisted in the lower and middle troposphere over the IGP. Inputting these SSA and extinction profiles into a radiative transfer model, we examined the effects of using height-resolved information in estimating atmospheric heating rates due to aerosols, over similar estimates made using a single columnar value. It was noted that use of a single SSA value leads to an underestimation (overestimation) of the heating rates over regions with low (high) SSA, emphasizing the importance of height-resolved information. Further, the use of realistic profiles showed significant heating of the atmosphere by submicron aerosol absorption at the middle troposphere, which may have strong implications for clouds and climate.

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“…We have modified the statement to convey this clearly (page 10, line number 4 and page 12, line number 10). Our observations cannot be completely explained by the cases discussed in Ratnam et al, (2018) as 1) our measurements do not show the extinction profiles immediately after rain and 2) the measurements were made around the local noon when the local sources are active. Fig.…”

mentioning

confidence: 62%

“…Are you talking about rainout? In fact washout should happen throughout the altitude as mentioned in many recent papers (for example Venkat Ratnam et al, 2018).…”

mentioning

confidence: 95%

Responses to the comments of Reviewer #2

Manoj¹

2019

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Role of dynamics on the formation and maintenance of the elevated aerosol layer during monsoon season over south-east peninsular India

Cited by 45 publications

References 37 publications

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons

Vertical profiles of submicron aerosol single scattering albedo over the Indian region immediately before monsoon onset and during its development: research from the SWAAMI field campaign

Responses to the comments of Reviewer #2

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