Planetary Boundary Layer variability over New Delhi, India, during EUCAARI project

Abstract

Abstract. Ground-based lidar measurements were performed at Gual Pahari measurement station, approximately 20 km South of New Delhi, India, from March 2008 to March 2009. The height of the Planetary Boundary Layer (PBL) was retrieved with a portable Raman lidar system, utilizing the modified Wavelet Covariance Transform (WCT) method. The lidar derived PBL heights were compared to radiosonde data, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite observations and two atmospheric models. The results were also analyzed on a seasonal basis. To examine the difficulties of PBL lidar detection under different meteorological and aerosol load conditions we focused on three case studies of PBL diurnal evolution. In the presence of a multiple aerosol layer structure, the WCT method exhibited high efficiency in PBL height determination. Good agreement with the European Center for Medium-range Weather Forecasts (ECMWF) and the Weather Research and Forecasting (WRF) estimations was found (r=0.69 and r=0.74, respectively) for a cumulus convection case. In the aforementioned cases, temperature, relative humidity and potential temperature radiosonde profiles were well compared to the respective WRF profiles. The Bulk Richardson Number scheme, which was applied to radiosonde profile data, was in good agreement with lidar data, especially during daytime (r=0.68). The overall comparison with CALIPSO satellite observations; namely, CALIOP Level 2 Aerosol Layer Product, was very satisfying (r=0.84), with CALIPSO Feature Detection Algorithms slightly overestimating PBL height. Lidar measurements revealed that the maximum PBL height was reached approximately three hours after the solar noon, whilst the daily evolution of the PBL was completed, on average, one hour earlier. The PBL diurnal cycle was also analyzed using ECMWF estimations, which produced a stronger cycle during the winter and pre-monsoon period. The seasonal analysis of lidar PBL heights yielded a less pronounced PBL cycle than the one expected from long term climate records. The lowest mean daytime PBL height (695 m) appeared in winter, while the highest mean daytime PBL height (1326 m) was found in the monsoon season as expected. PBL daily growth rates exhibited also a weak seasonal variability.

Air quality trends in rural India: analysis of NO2 pollution using satellite measurements

“…Major hotspots observed are Delhi NCR (DEL), Mumbai (MUM), Bangalore (BAN), Kolkata (KOL), Chennai (CHE), Hyderabad (HYD), Singrauli (SIN), Korba (KOR), Talcher (TAL), Chandrapur (CHA), Mundra (MUN) and Durgapur (DUR) due to the presence of power plants, oil refineries, steel plants, vehicular emissions and other industries there. 16 Nakoudi et al 57 reported a lower boundary layer height of about 677 m during winter in Delhi as compared to that in other seasons, which causes stagnation of pollutants in the lower troposphere. The lowest NO 2 in India is observed during the monsoon season, where the rural regions account for 42.7% of NO 2 pollution.…”

Space-time variability of ambient PM<sub>2.5</sub> diurnal pattern over India from 18-years (2000–2017) of MERRA-2 reanalysis data

“…In the rest of India, ambient PM2. height and PR show strong negative correlation with PM2.5 concentration because deeper PBL facilitates pollution dispersion (Nakoudi et al, 2018) and large PR leads to washout of pollution. The arid region in the western India barely receives rain large enough to influence PM2.5 diurnal pattern, which can be attributed to the observed poor correlation (r = ~-0.3).…”

Reply to the Comments of Anonymous Referee #3

“…4.1) in the context of three case studies, and the stability of the WCT algorithm is assessed as well (Appendix A). Additional cases, in which the importance of a proper threshold and cutoff zone are discussed, can be found in Nakoudi et al (2018). The WCT method also allows for the detection of clouds by means of a negative threshold.…”