Characterization of the seasonal cycle of south Asian aerosols: A regional‐scale modeling analysis

Abstract

[1] The sulfur transport and deposition model (STEM) is used to study the aerosol seasonality, distribution, and composition over south Asia from September 2004 to August 2005. Model predictions of sulfate, black carbon, primary organic carbon, other anthropogenic particulate matter, windblown mineral dusts, and sea salt are compared at two sites in south Asia where yearlong experimental observations are available from the Atmospheric Brown Cloud (ABC) project. The model predictions are able to capture both the magnitude and seasonality of aerosols over Hanimaadhoo Observatory, Maldives. However, the model is not able to explain the seasonality at the Kathmandu Observatory; but the model does capture Kathmandu's observed annual mean concentration. The absence of seasonal brick kiln emissions within Kathmandu valley in the current inventory is a probable reason for this problem. This model study reveals high-anthropogenic aerosol loading over the Ganges valley even in the monsoonal months, which needs to be corroborated by experimental observations. Modeling results also show a high dust loading over south Asia with a distinct seasonality. Model results of aerosol monthly composition are also presented at five cities in south Asia. Total and fine-mode monthly aerosol optical depth along with contribution from each aerosol species is presented; the results show that the anthropogenic fraction dominates in the postmonsoon and the early dry season with major contributions from sulfate and absorbing aerosols. Model sensitivity studies of dry deposition velocity and wet scavenging efficiency show that model improvements are needed in the treatment of carbonaceous aerosol dry and wet removal processes. Modeled SO 2 conversion rate constrained with sulfate observations at Hanimaadhoo suggests the need to increase model sulfate production rate during the dry season to account for probable sulfate production via heterogeneous pathways.

The impact of residential combustion emissions on atmospheric aerosol, human health, and climate

“…The baseline simulation generally captures this seasonality relatively well (correlation coefficient between observed and simulated monthly mean concentrations r > 0.5 at most sites), with minimal improvement with monthly varying anthropogenic emissions. This suggests that meteorological conditions such as enhanced wet deposition during the summer monsoon period are the dominant drivers for the observed and simulated seasonal variability, consistent with other modelling studies for the same region (Adhikary et al, 2007;Moorthy et al, 2013). Model simulations where residential emissions have been switched off show that residential combustion contributes about two-thirds of simulated BC and OC at these locations.…”

Chemical composition of rainwater at Maldives Climate Observatory at Hanimaadhoo (MCOH)

“…With the available observations in this study the source of this nss-Ca 2+ could not be well determined. Long-range transport of soil dust, as suggested by Adhikary et al (2007) is one possibility although it is not well supported by our trajectory analysis. We discuss other possibilities, including soil derived dust from nearby islands, local sea spray containing elevated levels of Ca 2+ from erosion of coral reefs and calcareous (CaCO 3 ) plankton debris and exopolymer gels emitted from the ocean surface.…”

“…Several studies have attributed Ca 2+ enrichment in marine aerosol and precipitation to transported crustal dust from distant continents. A model study over Indian Ocean (Adhikary et al, 2007) suggested that aerosol mass composition at Hanimadhoo during the monsoon season was dominated by mineral dust in both coarse and fine modes. The dust was transported eastwards from Africa and then brought into the Asian monsoon circulation.…”

Organic and inorganic markers and stable C-, N-isotopic compositions of tropical coastal aerosols from megacity Mumbai: sources of organic aerosols and atmospheric processing

“…This study revealed that aerosol chemical composition is region and location specific. Therefore, region-specific and wide-range data of aerosol chemical composition are needed for better constraining model input (Adhikary et al, 2007).…”

“…The chemical characterization of aerosol particles in these regions is especially important to better understand their sources, transport, and transformation, which are needed for regional models to predict aerosol distributions and effects, monsoon activity, and air quality (Adhikary et al, 2007).…”