Southern Himalayas rainfall as a key driver of interannual variation of pre-monsoon aerosols over the Tibetan Plateau

The Tibetan Plateau (TP), a climate-sensitive region crucial for Asian rivers and glaciers, receives significant aerosol influx from neighboring regions, particularly during the pre-monsoon season. These aerosols, originating from sources like biomass burning in South Asia and dust plumes, play a vital role in TP's environment. While previous research has highlighted the importance of wind in aerosol transport, the significant interannual variability in aerosol loading on the TP requires a more nuanced understanding. This study investigates the relative contributions of wind and precipitation in driving this variability, particularly focusing on the influence of rainfall over the Southern Himalayas and its interplay with biomass burning events and aerosol transport. Understanding this complex relationship is crucial for predicting future changes in aerosol loading on the TP and its downstream consequences.

Prior research has established the role of biomass burning as a primary driver of pre-monsoon aerosol variability over the TP. Studies have emphasized the impact of large-scale westerlies and valley/glacier winds in transporting aerosols to the TP. A common assumption is that enhanced wind speed directly increases aerosol loading. However, the interannual correlation between these factors has not been fully examined. Li et al. (2020) linked Arctic sea ice loss to increased aerosol transport via intensified subtropical westerlies. Existing studies largely relied on statistical correlation analysis, neglecting the complex interactions between biomass burning, meteorological fields, and their combined influence on aerosol loading. This study addresses this gap by exploring the synergistic effects of meteorological fields on pollutant sources and transport.

This study combines observational data and numerical simulations to analyze interannual variations in aerosols, fire events, and meteorological fields around the TP from 2003. Data sources included MODIS active fire data, MERRA2 reanalysis for meteorological fields and aerosol optical depth (AOD), and APHRODITE precipitation data. Analysis focused on two stations, QOMS and Nam Co. AOD from MERRA2 was used due to the discontinuous nature of AERONET data. The study examined correlations between AOD, fire density (Southern Himalayas), and near-surface wind speed. A fire regression model, incorporating rainfall and near-surface temperature, was developed to predict fire events. Partial correlation coefficients and convergent cross-mapping were employed to assess relationships between variables. Numerical experiments using WRF-Chem v4.0 with 1° resolution were conducted for 2015 and 2016, contrasting years with differing wind speeds and AOD values. Sensitivity experiments varied meteorological fields and biomass burning emissions to quantify their individual impacts. The contribution of each physical and chemical process (transport, emission, dry/wet deposition, PBL mixing, chemistry) to column-integrated PM2.5, biomass burning, and dust aerosols in the southern TP was also analyzed. Additional sensitivity experiments using data from 2011/2012 and 2017/2018 were performed to further support the findings. The model used Morrison two-moment microphysics, Community Land Model, Mellor-Yamada-Nakanishi-Niino PBL scheme, and RRTMG for radiation. The CBM-Z photochemical mechanism and eight-bin aerosol model were used for chemical simulations.

The study found that rainfall over the Southern Himalayas had a much greater impact on interannual aerosol variation over the TP compared to wind speed. Rainfall strongly negatively correlates with fire density (-0.75), modulating biomass burning emissions. The interannual correlation between fire density and AOD was significant (0.85 and 0.47 for QOMS and Nam Co respectively). In contrast, the correlation between near-surface wind speed and AOD was weaker. A negative correlation (-0.58) was found between rainfall and near-surface wind speed in the Southern Himalayas. This co-variability, along with the negative correlation between rainfall and fire events, contributed to the positive correlation between wind and AOD, suggesting that the wind effect may be indirect. Numerical experiments showed that while higher wind speed occurred in 2016 (higher AOD), the southerlies were weaker, reducing aerosol transport. The experiments also revealed that meteorological conditions in 2015 (weaker wind, higher precipitation) increased PM2.5 via enhanced transport, while reducing it via wet deposition compared to 2016 conditions. The analysis of the contribution of each physical and chemical process at the QOMS station indicated that transport was the dominant factor for PM2.5 followed by wet deposition. Additional experiments consistently showed that increased near-surface wind in the Southern Himalayas is associated with lower aerosol loading in the southern TP, particularly in years with increased fire density. The regression model accurately predicted interannual variability in fire density (correlation coefficient 0.67, R² = 0.45).

The findings challenge the conventional view that stronger wind speeds directly increase aerosol transport to the TP. Instead, the study demonstrated the critical role of Southern Himalayan rainfall in regulating aerosol loading by controlling biomass burning emissions and through wet scavenging. The confounding effects of wind and rainfall highlight the importance of mechanistic analysis in understanding complex interactions. The significant impact of rainfall on fire events (explaining 56% of interannual variance) underscores its influence on aerosol production. The projected increase in Southern Himalayan rainfall suggests a potential mitigation of future aerosol transport to the TP.

This research highlights the paramount role of Southern Himalayan rainfall in modulating the interannual variability of pre-monsoon aerosols over the Tibetan Plateau. Rainfall's influence on fire events and wet scavenging outweighs the effect of wind speed. Future research should investigate the feedback mechanisms between aerosol transport and TP precipitation, refine the model's resolution to improve accuracy, and consider human activities influence on fire events.

The study's use of a relatively coarse horizontal resolution (1°) in the numerical simulations might impact the quantitative assessment of aerosol transport. The impact of human activities on fire events, which can deviate from model predictions, was not fully accounted for, and the model used a simplified representation of these processes. Future studies should address these limitations.