Impact of local and regional emission sources on air quality in foothills of the Himalaya during spring 2016: An observation, satellite and modeling perspective

摘要

Chitwan National Park in south-central Nepal is one of the few remaining protected forest ecosystems in South Asia with rich flora and fauna. However, in recent years, southern Nepal including the region of the National park area have experienced deteriorating air quality, which has raised concerns about its impact on ecosystem and public health. The region experiences forest fires during pre-monsoon season and thus, provide an ideal opportunity not only to study its impact on air quality but also to provide source specific diagnostic ratios. In this study, we report measurements of black carbon (BC), trace gases as well as particulate matter (PM10, PM2.5 and PM1) from Chitwan Air Quality Monitoring Station (CAQMS) during pre-monsoon season (February-May 2016). Average concentrations of BC, PM10, PM2.5, PM1 were found to be 10.9 +/- 6.9 mu g m(-3), 156.4 +/- 68.2 mu g m(-3), 95.9 +/- 49.0 mu g m(-3), 80.1 +/- 45.5 mu g m(-3) which are much higher than the prescribed 24 h limits of World Health Organization (25 mu g m(-3) for PM10 and 10 mu g m(-3) for PM2.5). Furthermore, average concentrations of trace gases, CO, CO2 and CH4 were 713.4 +/- 476.6 ppbv, 446.0 +/- 15.3 ppm and 2.182 +/- 0.117 ppm respectively which are also higher than those in the surrounding regions. We identified two pollution events (Event #1 from 16 to 28 March 2016 and Event #2 from 6 to 17 April 2016) and found that surface concentrations of all measured species increased by a factor of 2-3 during these events. The BC/CO ratio of 16.3 +/- 2.0, 11.1 +/- 3.7 and 9.9 +/- 4.2 ng m(-3) ppbv(-1) was observed during Event#1, Event#2 and during non-event period. Based on MODIS active fire counts and Angstr.m absorption exponent (AAE values of 1.8 and 1.9 during Event #1 and #2 respectively), we found that biomass burning (BB) activities were an important source during these events. This is further substantiated by relatively higher contribution of BB to observed BC concentration during both the events (73% and 75% during Event#1 and #2 respectively). Furthermore, a Langrangian modeling framework was used to quantify the contribution of local and regional fires to BC concentration. The contribution of local fires to modelled BC at CAQMS during Event #1 and #2 is estimated to be similar to 70% and similar to 78%, respectively. The satellite retrievals of AOD, NO2 and CO also exhibit a significant increase in columnar concentration during these pollution events. The CALIOP retrieved vertical profile of extinction coefficient further confirmed that these pollutants are not only confined near the surface but are lifted up to 2 km altitude, which can subsequently be transported to high-altitude pristine sites and can impose serious implications on the cryosphere.