Impact of low clouds and boundary layer height on regional air quality over Delhi-NCR

In this study, Ceilometer measurements of attenuated backscatter coefficient over Delhi-NCR during monsoon (June-September) and post-monsoon (October-November) of 2022 are analyzed to determine cloud base height (CBH) and atmospheric boundary layer (ABL) height. The derived CBH and ABL height are used to assess the impact of low clouds (CBH < 2km) and ABL height on regional air quality. The Ceilometer measurements are augmented by ERA5 reanalysis dataset of hourly averaged CBH and ABL height and radiosonde measurements. The aerosol loading over Delhi-NCR is derived using Moderate Resolution Imaging Spectroradiometer (MODIS) Terra aerosol optical depth (AOD) measurements and Dusttrack retrieved particulate matter with aerodynamic diameter ≤ 10 µm (PM₁₀) concentrations. We also used Ceilometer measurements for non-cloudy days to interpolate missing PM₁₀ values during the study period. To evaluate the dilution and diffusion of pollutants, we calculated ventilation coefficient from ABL height and wind speed data.

The results reveal that out of the total measurements during monsoon season, 41% cloud occurrence was observed, out of which 24% of clouds were low-level clouds. In post-monsoon season, cloud occurrence was low (nearly 12%), out of which 40% of clouds were low-level clouds. The ABL results show that during monsoon season, average ABL height was 0.85±0.6 km and during post-monsoon, it was 0.53±0.45 km. The seasonal difference is not only noted in the average values but also in the growth of ABL with full growth of ABL happening 2 hours later in post-monsoon season than monsoon season. The MODIS derived AOD results show average AOD values of 0.72±0.29 and 0.97±0.54 in monsoon and post-monsoon season respectively.

The comparison of PM₁₀ & AOD values indicate that during the cloudy days, both PM₁₀ values and AOD values were higher suggesting the abundance of cloud nuclei which could facilitate low cloud formation. For cloudy days, the correlation of observed CBH of low clouds with PM₁₀ and AOD shows a strong negative correlation (-0.78 and -0.83 respectively) suggesting that under same atmospheric thermodynamic conditions, CBH lowers under polluted conditions. The seasonal characteristics show that this tendency is predominant in post-monsoon than monsoon which might require further investigation. We observed strong negative correlation of ABL height with PM₁₀ and AOD (-0.84 and -0.89 respectively) during the study period. The derived ventilation coefficient shows a strong negative correlation with PM₁₀ and AOD values (-0.67 and -0.69 respectively). Both seasons showed similar characteristics indicating that the dissipation of pollutants depends more on ABL during both the seasons. However, substantiation of the diffusion and dilution processes over Delhi-NCR may require further investigation with different meteorological conditions. This will be added to this study along with the impact of clouds and ABL on different size distributions of aerosols. In conclusion, we used advanced instrumentation to study the interlinkages of atmospheric vertical structure with air quality. Our findings are relevant for the Indo Gangetic Plain (IGP) having population more than 400 million and can be applied to other places in the global south experiencing high pollution episodes often linked to unfavourable meteorology.