VARIABILITY OF BIOMASS BURNING AEROSOLS LAYERS AND NEAR GROUND

The aim of this study is to characterize aerosols from both chemical and optical point of view and to explore the conditions to sense the same particles in elevated layers and at the ground. Three days of continuous measurements using a multi-wavelength depolarization lidar(RALI) and a C-ToF-AMS aerosol mass spectrometer are analyzed. The presence of smoke particles was assessed in low level layers from RALI measurements. Chemical composition of submicronic volatile/semi-volatile aerosols at ground level was monitored by the CTOF AMS Several episodes of biomass burning aerosols have been identified by both techniques due to the presence of specific markers (f60, linear particle depolarization ratio, Ängström exponent).


INTRODUCTION
Aerosol size distributions, chemical composition, optical and microphysical retrievals provide essential information regarding local and long range transported aerosols [1].Frequently, the atmospheric layers contain combination of different aerosol types picked up during transport, which can be dry deposited on the ground or remain suspended at higher altitudes [2].
Numerous studies are discussing independently the chemical and physical properties of aerosols, as well as their origin and local component [3][4] [5].The lidar observations and aerosol mass spectrometer ground based measurements can be combined in order to assess the contribution of long range transported aerosols at ground level and to retrieve their chemical and optical properties, including the level of oxidation, dimension, shape, lidar ratio, Angstrom exponents, linear particle depolarization ratio etc. [6].
Multi-wavelength Raman lidars are suitable to track the levels of aerosols layers, their thickness and variability in time, as well as type dependent aerosols properties.Linear particle depolarization ratio can be used to differentiate between highand low-depolarizing particles, to assess a possible dust intrusion or particles from wild fires [2].The variability of smoke particles properties and vertical distribution have been intensively studied all over the world, discrepancies being observed even though sometimes the studies focused on the same origin and loads of aerosols [1][3][5].Several studies already demonstrated that during the aging process Ängström exponent of the biomass burning aerosols decreases from around 2 down to 1, highlighting an increase of the particle size [1].
Organic ions loadings and proportions of smoke markers can be used to demonstrate the presence of biomass burning aerosols at ground level [7][6].Besides organic component these aerosols contain high proportions of sulfate, nitrate and small amounts of ammonium and chloride fractions.Generally m/z 43, 44, 60 and 73 ions are used to characterize both the biomass burning presence and their aging process.The mechanism of aging process of biomass burning aerosols are still poorly described, but gives important insights of the chemical and physical evolution of organic aerosols.Depending on the level of oxidation the secondary aerosols, the oxygenated organic aerosols (OOA), could be separated into lowvolatility OOA (LV-OOA -the more oxidized component) and semi-volatile OOA (SV-OOA).In this paper several cases of biomass burning The multi-wavelength Raman lidar system is frequently used in the frame of EARLINET (European Aerosol Lidar Network) for tropospheric measurements [2].The lidar signal, at 3.75m vertical resolution is collected at 1064, 532p, 532s, 355, 607, 387 and 408 nm, most of the channels having both analog and photon counting detection.The backscatter coefficients (1064, 532, 355 nm), extinction coefficients (532, 355 nm) and linear particle depolarization ratio (532 nm) of aerosols between 0.8 and 10 km altitude are usually used to assess the aerosols presence and their optical properties [6].
The data analysis for the three days continuous measurements started with detection of aerosol layers near ground and those with low linear particle depolarization ratios.Hysplit (Hybrid Single Particle Lagrangian Integrated Trajectory) model have been used to check the possible regional source of the aerosols, while the MODIS (Moderate Resolution Imaging Spectroradiometer) global fire maps to assess a possible smoke particles presence.From C-ToF AMS the m/z 43, 44, 60 and 73 ions evolutions were considered to identify the biomass burning aerosols presence at ground and their oxidation level.

RESULTS
During the intensive lidar measurements the layers were generally present above 2 km and below 6 km altitude, but the descent of air masses from the free troposphere to the ground was favorable.The lofted layers reached the ground after downward mixing in the planetary boundary layer for several hours during the continuous lidar measurement.Three predominant layers near ground were typically sensed (altitude lower than 1.5 km, between 1.5-2.5 km and above 2.5km).The tendency of the first two aerosols lidar layers to mix with the planetary boundary layer can be observed in the time series of the average height of the layers (Figure 1).More spherical particle were observed in higher layers, while particles with increased depolarization values were evidenced for lower layers above measurement site during the investigated period (Figure 2).The higher linear depolarization ratio (~9%) could corresponds to smoke mixed with urban aerosols, while smaller depolarization values (~3%) for lofted layers to smoke or industrial type.On 11 th of July 2012 conversion from small to higher values of depolarization, from higher to lower layers can be seen.This could be a case of downward mixing into the planetary boundary layer.
The Ängström backscatter-related coefficient values are higher than 1.2 for more than 50% of the three days data, which also indicate the presence of small particles; usually the presence of smoke.Two periods of Ängström backscatterrelated coefficient values higher than 1.2 were noticed: in the afternoon of 10 th and from 11 th to 13 th of July.Organics, sulfate, ammonium and nitrate fractions presented high loads during the three days continuous measurements.The organics species represented more than 50% from the total mass of the non-refractory submicronic aerosols (Figure 3).Higher concentrations were evidenced during night time due to the low planetary boundary layer height.
The m/z 60 fragment represent a marker for "levoglucosan like" species -based organics, indicating the presence of biomass burning aerosols.Its concentration varied same as m/z 73, high values of f60 (ratio of m/z 60 to total organic fragment loadings) and f73 being retrieved for the entire campaign.The f60 is present for all AMS measurements, a threshold value indicates the biomass burning influence or non-biomass burning sources.During our intensive measurements period the f60 exceeded very often the background level (0.003) found by Cubison paper [4], indicating the presence of biomass burning aerosols.Doubled than the background level f60 values were noticed for between 11 th and 13 th of July, when Hysplit back-trajectories showed air masses travelling from the West of Europe [10] and Balkans where MODIS fire map revealed a high density of forest fires.

Figure 3 PM1 chemical fractions and total concentration time series as measured by CTOF AMS
The f44 versus f60 plot suggested the increasing oxidation state of the organic aerosols (Figure 4).The regional LV-OOA represents the more oxidized component, higher f 44 (ratio of m/z 44 to total organic fragment loadings) values is present for 70% of the data.The less oxidized organic aerosols component (SV-OOA), characterized by lower f44, is highlighted mostly for 9 th -10 th of July and few hours on the 12 th .The aging process of aerosols can be also identified using the Cubison f44 versus f60 plot, the higher f44 and lower f60 values corresponding to aged biomass burning aerosols.
The AMS biomass burning marker showed a strong variability of aerosol properties well correlated with Lidar mean layers altitude for the second part of the measurements period.During the aging process the biomass burning aerosols Ängström exponent decreased from 2 to 1, while the f60 ranged between 0.009 and 0.007.

CONCLUSIONS
The concentration variations of few specific organic ions have been used to identify biomass burning aerosols and to evidence their evolution.Significant chemical markers of biomass burninglike particles have been noticed for around 40% of the measurements.Also the Ängström backscatter related coefficient and linear particle depolarization ratio values from lidar continuous measurements pin pointed the presence of low depolarizing and small diameter particles.
The 11 th -13 th July period has been characterized by highest f60 values when also Hysplit backtrajectories and MODIS fire indicate biomass burning presence.Correlations of aerosols mean layer height and mass spectrometry biomass burning markers are noticed for half of the measurement period, when conversion from small to higher values of depolarization, from higher to lower layers can be seen.The higher f44 and lower f60 values corresponding to aged biomass burning aerosols are identified for first hours of 12 th of July.

Figure 2
Figure 2 Average linear particle depolarization at 532nm of the three Lidar layers, arrows indicate the conversion from low to higher depolarizing particles

Figure 4
Figure 4 f44 versus f60 for biomass burning aerosols