IMPACT OF FOREST FIRE EMISSIONS ON AIR QUALITY OVER WESTERN HIMALAYA REGION

Emissions from forest fires give out huge amounts of greenhouse gases into the atmosphere degrading the surrounding air quality. Climate change results in prolonged summer days and less precipitation thus increasing fuel accumulation facilitating recurring forest fires in the Western Himalayan regions. Most of the forest fire cases are human ignited that damaging the forest irreparably. This study attempts to analyse the impact of the forest fire events on the tropospheric concentrations of five major gaseous pollutants aerosols, Carbon Monoxide (CO), Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2) and Formaldehyde (HCHO). A clear increase in daily average of the air pollutants considered in the study was observed as the number of forest fires were also increasing. The gases responded inversely with precipitation. Precipitation washed away majority of the air pollutants. The high-resolution air quality data were retrieved from the Google Earth Engine platform using Sentinel 5 Precursor datasets, daily active fire points from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument of Suomi National Polar-orbiting Partnership (SNPP) and daily precipitation data from CHIRPS Daily Version 2.0 Final. Four out of five gases i.e., CO, aerosols, NO2, HCHO pollutants gave moderate to high correlation values with the forest fire incidences. Air pollutants responded inversely with the precipitation pattern.


INTRODUCTION
Western Himalayan region is an ecologically fragile environment. There is a great variation in altitude which renders different vegetation types like alluvial grasslands, subtropical forests, conifer mountain forests and alpine meadows (Tewari et al., 2017). The variation in height and sharp seasonal contrasts speeds up the triggering by climate change which intensely disturb the ecosystem functioning and human environments. Forest fires have become a periodic event for the Himalayan states and the changing climate is making it even worse. Climate change is causing more summer days and less precipitation, making the vegetation dry and conducive to large forest fires. Which results in longer fire seasons and reductions in carbon stocks (Jolly et al., 2015). One of its severe consequences is the deterioration of the regional air quality. Forest fire emits a large amount of greenhouse gases like Carbon Dioxide, Sulphur Dioxide, Nitrogen Dioxide, etc. these gases pose a direct threat to human health and to the climate at large. Most of the fire incidences are intentional by human for agricultural residue burning, shifting cultivation, seed and flower collection, better growth of grass (Satendra and Kaushik, 2014). Major cause for recurrent fire events is the regional domination of fire prone tree species like Chir Pine which are dominantly found in Jammu and Kashmir, Himachal Pradesh and most of its forest cover in Uttarakhand (Fulé et al., 2021). Forests are the major source and sinks of carbon. Recurrent forest fires releases tonnes of harmful gases into the atmosphere. Aerosol particles in the atmosphere both scatter and absorb incoming solar radiation, get deposited on the leaf surface, affecting plant productivity (Izuta, 2017) .
Sulphur Dioxide (SO2) is the main component of acid rain that destroys foliage tissue and acidify water bodies. In humans it can create respiratory illness and aggravate existing heart and lung * Corresponding author conditions. SO2 creates radiative forcing by the formation of sulphate aerosols.
High NO2 concentration affects the foliage by decreasing the growth. CO contributes indirectly to climate change as it is a precursor of tropospheric ozone. It does not affect plants as it is rapidly oxidised to form carbon dioxide an important input for photosynthesis. Exposure to carbon monoxide cause dizziness, confusion, and irritability. Formaldehyde is an intermediate gas in nearly all oxidation chains of Non-Methane Volatile Organic Compounds (NMVOC), leading ultimately to CO2. When wild animals are exposed to this, it can make them sick, affect their appearance, breeding behaviour, and reduce their life spans (Duong et al., 2011).
Ground monitoring resources especially in the Western Himalayan regions have spatial gaps which can be solved by using satellite data that can fill in the information on air quality in areas without ground monitoring. Many satellites can give information on criteria air pollutants and greenhouse gases for example. When selecting satellite data to address specific air quality issues, accuracy, spatial and temporal resolution are important considerations. Several studies and monitoring stations have documented a rapid increase in the concentration of air pollutants in the Himalayan ranges (Giri et al., 2008;Putero et al., 2014;Xu et al., 2009).
Air pollution pose a threat to Himalayan region, affecting its ecosystem, health of inhabitants, the cryosphere and many more. It is the emission of these harmful gases and their impact on the vegetation and climate that necessitate special attention. In this study we aim to analyse the impact of forest fires incidences on different air quality parameters-tropospheric concentration of aerosol, NO2, SO2, CO, HCHO from Sentinel 5P for the years 2019, 2020 and 2021 for Western Himalayan regions-Ladakh, Jammu and Kashmir, Himachal Pradesh, Kumaon and Garhwal. We have also analyzed the response of different air quality parameters with the precipitation pattern. We have also done the correlation analysis of the fire incidences with the air quality parameters.

STUDY AREA
The study area comprises 5 regions of western Himalaya -Uttarakhand further divided into Kumaon and Garhwal, Himachal Pradesh and the union territories of Jammu and Kashmir and Ladakh. The western Himalaya includes the Zaskar Range, Pir Panjal Range, Siwalik Range, Ladakh Range, Dhauladhar Range and the Great Himalaya and parts of the Shivalik Range. The elevation of the study area ranges from 180 to 7600m. According to the (ISFR, 2019), in Uttarakhand the average annual rainfall is 1,500 mm and the annual temperature varies from 0ºC to 43ºC. 63.41% of the area is under forests. In Himachal Pradesh, the tree growth is minimal due to harsh conditions. The average annual rainfall is about 1,800 mm. The temperature varies from sub-zero to 35°C. Forests cover 24.61% of the state's geographical area.
For Jammu & Kashmir and Ladakh the average annual rainfall varies from about 600 mm to about 800 mm and the average annual temperature from sub-zero to 40°C. Forests cover 56.65% of the geographical area of these two union territories. The Western Himalaya consists of different forest types like alpine forests, semi-evergreen, deciduous, sub-tropical broad-leaved hill forests, subtropical pine forests and sub-tropical montane temperate forests.

DATA AND METHODOLOGY
For analysis, fire seasons starting from February 1 to June 30 of three years 2019, 2020 and 2021 were considered. Google Earth Engine (GEE) was used to retrieve the daily air quality and precipitation data.
The VIIRS 375 m thermal anomalies / active fire product from the VIIRS sensor aboard the joint NASA/NOAA Suomi NPP was used for analysis of temporal and spatial distribution of fire events.
Data for entire fire season with all low, nominal and high level of confidence for fire detection, for the specified years was taken and clipped for those 5 regions. Air quality data was taken from SENTINEL 5P satellite. It is focused on air quality and composition-climate interaction with the main data products being O3, NO2, SO2, HCHO, CH4, and UVAI with a spatial resolution of 1.11km. For analysis daily average of aerosols, Carbon Monoxide, Formaldehyde, Nitrogen Dioxide and Sulphur Dioxide were taken for all the 5 regions.
Daily precipitation data were obtained from CHIRPS Daily: Climate Hazards Group Infrared Precipitation with Station Data (Version 2.0 Final). This provides more than 30 years of quasiglobal rainfall datasets having a spatial resolution of 0.05°. The precipitation data was then combined with the fire points and air pollutant data of those 5 western Himalayan regions. The air pollutants concentration was multiplied by 100000 for all the gases except AI and CO.

Figure 1. Study Area Map
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B3-2022 XXIV ISPRS Congress (2022 edition), 6-11 June 2022, Nice, France      Figure 5 shows graphs of some selected places and years depicting the daily fire, precipitation and emission data. Table 2 shows aerosols have weak to moderate positive correlation values for Kumaon, Garhwal and Himachal Pradesh. For Jammu and Kashmir aerosols showed weakly negative associations. For CO, there is moderately positive correlation for all the four regions. Similarly, NO2 and HCHO have moderate to strong positive correlation for the four regions except few negative correlations.
The year 2020 gave low correlation values because of the less fire activity.
Correlation of the air quality parameters with the fire points was calculated and correlation coefficient (r) was estimated for each parameter for every 5 regions. The r value describes the degree of association between two values. The value ranges from -1 to 1 showing negative and positive relations.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B3-2022 XXIV ISPRS Congress (2022 edition), 6-11 June 2022, Nice, France  Table 2. Correlation values for all the gases with four region's fire incidences

CONCLUSION
This analysis presented the impact of the fire events on the tropospheric concentrations of these 5 major gases CO, NO2, HCHO, SO2 and aerosol showed enhanced levels during fire events in almost all the five regions. Positive UV AI indicates the presence of UV-absorbing aerosols like dust and smoke. SO2 shows weakly negative to weakly correlation with fire incidences which indicates there may be some external sources of SO2 apart from the fire events for example emissions from vehicles and industries (Pierson et al., 1978). SO2 from biomass burning is very small compared to the vehicular and industrial emissions. Tropospheric concentration of SO2 climbed up during the tourism period, just before the lockdown started and during the easing of the lockdown restrictions. Precipitation helped to reduce the concentration of the gaseous pollutants. Not all the peaked emissions resulted from the biomass burning. The year 2020 saw lowest forest fire incidence as a result of the pandemic led lockdown which helped in improving the quality of air. Ladakh being a cold desert witnessed very small number of fire events which did not alter the tropospheric concentrations of these gas pollutants, region showed low to moderate correlation with all the five gaseous pollutants. Correlation coefficient of NO2 is moderately correlated with the fire incidences. Formaldehyde presented moderated to high correlation for all the regions.
Many previous studies confirmed the increase of average temperatures, early onset of summer, less and erratic rainfall in the Western Himalaya (Bhutiyani et al., 2007;Dimri and Dash, 2012;Jain et al., 2009). Winter temperatures have warmed up faster in these mountainous regions than in the rest of the world. Forest fire season starts from February to June. Summer precipitation does little to reduce these forest fires. Vegetation type and tree species available in these regions like Chir pine are inflammable. Though they have adapted to forest fires because of their thick fire-resistant bark, the drier condition of these region makes the forest a tinder box (Bargali et al., 2020).
Western Himalaya is drier and vulnerable to climate change than Eastern Himalaya. Anthropogenic fire disturbances are damaging the forests ecology and the climate. Climate change leads to recurrent forest fire events releasing tonnes of carbon emissions (J S .
One major limitation of the study is that the data availability is only from 2018, so long term analysis of the impact of forest fire on air quality cannot be done with such high spatial and temporal resolution. With such high spatial and temporal resolution, this study of the air quality parameters can be further extended to broader interdisciplinary research relating to health, environment and social context which is crucial to build robust policies for climate change adaptation and protection of ecosystem services.