Source apportionment and diurnal variability of autumn-time black carbon in a coastal city of Salé, Morocco+

: This research aims to understand the temporal variation of concentrations of equivalent black carbon (eBC) and to calculate the fossil fuel (BCff) and biomass combustion (BCwb) contribution of to eBC during the 2020 autumn season. In-situ measurements of eBC and NO2 were performed for this aim in Sale, Morocco. The contribution of BCff and BCwb was assigned based on the spectrum dependence of BC absorption. The average eBC concentration was 1.9±2.2µg/m3 with contirbution of 13% for BCwb. The eBC was strongly correlated with NO2 (R²=0.63). Fossil fuel combustion is the most significant contributor to eBC and NO2 concentrations.


Introduction
Air pollution, mainly from traffic combustion processes, is a serious environmental problem in Salé, the second densest city in Morocco (8163 hab/km2), as in other major urban areas worldwide. Air pollution in cities leads to increased atmospheric concentrations of combustion products such as nitrogen dioxide (NO2) and particulate matter (PM). The latter includes primary particles generated by combustion, secondary particles, and mineral dust particles. Black carbon (BC), is identified as a large amount of carbonaceous of aerosols and consequently the fine PM2. 5 [1], comes mainly from combustion of fossil fuels and biomass [2].
Studies [3], [4] stated that BC concentrations are proportional to traffic emissions, which allows inferring BC levels from traffic. However, the contribution of biomassburning activities may affect both the daily cycles of BC and the BC/NO2 ratios.
Among the methods apportionment techniques based on observation, the aethalometer model has been adopted in different studies [5], [6] to assess the contributions of fossil fuel and biomass combustion to equivalent black carbon (eBC) by analyzing light absorption at multiple wavelengths.
In this context, the goal of this research is to look the influence of biomass burning vs fossil fuel consumption on the Salé air pollution during the 2020 autumn season.

Sampling site
The Médersa of Mérinides (34°02'23.4"N 6°49'38.5"W) was built in 1341 JC/733h by the mérinide sultan Abou'l Hassan, and it represents the architectural masterpiece of the medina of Salé. It offers an unique decoration including an Andalusian excellence in architecture, spatial organization as shown in the

Data collection
The field campaign in Salé was performed from October 14 to November 27, 2020. Black carbon continuous measurements were carried out using a multiple spectrum carbon analyzer (The BC1054 by Met One Instruments, USA) on the terraced roof of Mderssa at the height of around 12 m-agl. The BC1054 analyzer automatically measures optical transmission at ten wavelengths ranging from 370 nm to 950 nm, through a filter on which particles have been deposited. The absorption at 880 nm in the near-infrared is used to compute the equivalent black carbon (eBC) content.
ENVEA Cairpol's low-cost electrochemical sensors were employed for continuous and high-resolution temporal NO2 monitoring. For each gas measurement, the uncertainty was on the order of +/-25 to 30%. Meteorological data, such as temperatures, wind speed, and direction were collected from the International Meteorology website (http://www.wunderground.com) and from (NOAA Air Resources Laboratory) with processing by R software [17]. Wind rose, and hourly change in temperature and humidity in October and November 2020, are illustrated in Figure 2 and Error! Reference source not found., respectively.

Characteristics of eBC and NO2 concentrations and sources
Numerous studies have shown that BC concentration is related to the site type (urban, suburban, rural, downtown) [7][8][9] and prevailing weather conditions [6][7]. The first method used in this study to apportion the sources of eBC is based on the correlations between eBC and traffic combustion tracer NO2. Figure 4 shows the hourly variation of BC and NO2 and Figure 5 their linear regression. Strong relationship between eBC and NO2 were observed during this study (R² ≈ 0.63) close to the values reported in Malaysian (R² = 0.71) [11] and English (R² = 0.88) [1] suburban areas as shown in Table 1. The ratio between eBC and NO2 is 0.17, which equivalent to a value of 0.14 according to the results obtained in Doha, Qatar [2] characterizing diesel fuel. The average eBC values recorded during this study was 1.9 ± 2.2 µ g/m3, which are comparable to the values recorded at an urban site in Milan, Italy (1.92 ± 0.88 µ g/m3) [10] and during periods of winter biomass burning (1.89 ± 0.5 µ g/m3) at residential urban areas in Kwadela, South Africa [12].  The eBC pollutant rose shown in Error! Reference source not found. indicate that the highest levels of eBC concentrations are recorded when the wind speed is low (not exceeding 3 m/s) and comes from the southeast. Daily and hourly pollutant rose (not shown) display that eBC concentrations peaked at midnight (~23:00h -00:00h) when the winds were from the south and southeast.

eBC source apportionment
Significant day-night variation in eBC levels was observed during the sampling period (mean night/day ratio = 1.4). This variability could be due to two processes: (a) the atmospheric boundary layer is shallower at night and retains pollutants in a smaller volume, and (b) the transport of emissions from fossil fuels produced at the local scale by the nighttime land breeze (Error! Reference source not found.). A third option is the higher emissions from fossil fuels during the night (possibly due to transportation engines). To discuss these hypotheses involved in the observed eBC changes and to assess eBC source apportionment, the exponent of the absorption Å ngström was used as a source-specific parameter to distinguish between wood combustion (BCwb) and fossil fuel (BCff) aerosols [8]. In this study, we used absorption Å ngström exponents of 1.0 and 2.0 for pure traffic (αff) and wood burning (αwb), respectively. Tracking the evolution of concentrations of the eBC, BCff, and BCwb by studying their hourly changes during the period October 14 to November 27, 2020. The diurnal variations of the mean values are presented in Figure . Significant diurnal cycles in eBC and BCwb and BCff concentration was recorded with two peaks, around 8 a.m and 11 p.m. For non-reactive pollutants like BC, the concentration increases at night when the boundary layer gets shallower than it is throughout the day [1]. Average eBC, BCff and BCwb values range from 1.9 ± 2.2, 1.6 ± 2.5, and 0.25 ± 0.72 μg/m3, respectively.
The primary contribution to BC concentrations in this study is fossil fuel use, with more than 86%, particularly in the morning and evening hours. Although our census campaign outlined 12 ovens and 8 traditional hammams within a radius of 400 meters of the sampling site, biomass burning makes a non-significant contribution of about 13% to autumn-time eBC.

Conclusion
This study demonstrated that the concentration of black carbon comes from a large percentage of vehicle fuel combustion. The contribution of biomass burning and hammams located around the Médersa site of Salé was evaluated by 13%. The average values of eBC, BCff, and BCwb ranged from 1.9 ± 2.2, 1.6 ± 2.5, and 0.25 ± 0.72 μg/m3. It was also determined that the increase in the concentration of eBC remains local and not related to wind gusts. There is a significant correlation between eBC concentration and NO2 with a value of 0.63 and an inverse asymmetric relationship with the atmospheric boundary layer.