Low-Cost Hourly Ambient Black Carbon Measurements at Multiple Cities in Africa

There is a notable lack of continuous monitoring of air pollutants in the Global South, especially for measuring chemical composition, due to the high cost of regulatory monitors. Using our previously developed low-cost method to quantify black carbon (BC) in fine particulate matter (PM2.5) by analyzing reflected red light from ambient particle deposits on glass fiber filters, we estimated hourly ambient BC concentrations with filter tapes from beta attenuation monitors (BAMs). BC measurements obtained through this method were validated against a reference aethalometer between August 2 and 23, 2023 in Addis Ababa, Ethiopia, demonstrating a very strong agreement (R2 = 0.95 and slope = 0.97). We present hourly BC for three cities in sub-Saharan Africa (SSA) and one in North America: Abidjan (Côte d’Ivoire), Accra (Ghana), Addis Ababa (Ethiopia), and Pittsburgh (USA). The average BC concentrations for the measurement period at the Abidjan, Accra, Addis Ababa Central summer, Addis Ababa Central winter, Addis Ababa Jacros winter, and Pittsburgh sites were 3.85 μg/m3, 5.33 μg/m3, 5.63 μg/m3, 3.89 μg/m3, 9.14 μg/m3, and 0.52 μg/m3, respectively. BC made up 14–20% of PM2.5 mass in the SSA cities compared to only 5.6% in Pittsburgh. The hourly BC data at all sites (SSA and North America) show a pronounced diurnal pattern with prominent peaks during the morning and evening rush hours on workdays. A comparison between our measurements and the Goddard Earth Observing System Composition Forecast (GEOS-CF) estimates shows that the model performs well in predicting PM2.5 for most sites but struggles to predict BC at an hourly resolution. Adding more ground measurements could help evaluate and improve the performance of chemical transport models. Our method can potentially use existing BAM networks, such as BAMs at U.S. Embassies around the globe, to measure hourly BC concentrations. The PM2.5 composition data, thus acquired, can be crucial in identifying emission sources and help in effective policymaking in SSA.


Addis Ababa Central, Ethiopia
Ethiopia is an East African country.The U.S. Embassy is only 85 m from, Algeria St, a major arterial road and ~1.6 km from a traffic circle connecting arterial roads.

Addis Ababa Jacros, Ethiopia
The BAM in the Jacros area of Addis Ababa is hosted by the warehouse facility of the U.S.
Embassy at Addis Ababa and is located merely at 50 m away from a major arterial road and ~800 m from a south of the A2 highway.

Pittsburgh, USA
The BAM is located at the Lawrenceville Allegheny County Health Department monitoring station and hosts PM2.5 speciation as a part of the Chemical Speciation Network (CSN) initiative by the U.S. EPA.The station is located in an urban residential area downwind of Central Business District.The air pollution at the station is dominated by vehicular emissions.

S4. Assigning date and time for BC from BAM spots
A BAM collects particles on a filter tape at a fixed flow rate every hour and measures PM2.5 by quantifying attenuation through the particle deposits.While there are other versions of BAMs available, we analyzed tapes from a BAM 1020 (Met One Instruments Inc.) used widely by the U.S. State Department to measure PM2.5 concentrations at U.S. Embassies and Consulates.
Therefore, BAM refers to BAM 1020 from Met One Instruments in our measurements.
The U.S. Embassy stores BAM tapes in their original boxes after use and labels each box with the date the corresponding was removed after use.Sometimes, there are torn parts of tapes in the boxes which makes it difficult to narrow down the exact date and time for BAM spots.Spots on BAM filter rolls exist as set of 24 equally spaced spots representing 24 hours of a day.Each set of 24 spots are separated from the next 24 spots by a relatively larger space.We discarded the broken tapes (accounting total number of spots, hence hours of BC data, discarded) and started the analysis for the first continuous 24 spots from the tape end.Additionally, we excluded the days with fewer than 24 spots during analysis to avoid estimation errors from sampling biases caused by unscheduled interruptions of BAM operations as well as erroneous date and time assignment for the spots.
We start by assuming the removal date as the last date for the measured BAM spots and correspondingly assign date and time for each BC estimation during the sampling period.Then, we download hourly PM2.5 measurements from AirNow website 1 for the SSA sites and from the Air Quality System (AQS) website 2 for the Lawrenceville site in Pittsburgh.We chose the coefficient of determination (R 2 ) between the hourly PM2.5 from AirNow and BCopt as the indicator, and recorded R 2 by moving up the last datetime for the measurement period.The datetime assignment with the highest R 2 value for that measurement period was finalized.

Figure S1 :
Figure S1: The map shows all the cities in SSA where U.S. Embassies measure PM2.5.The green

Figure S2 :
Figure S2: BAM tape spots are aligned against the reference card (shown above) one at a time and

Figure S3 :
Figure S3: Scatter plot between hourly BC estimated with BAM tapes and hourly BAM-based

Table S1 :
Sampling sites and their geographical informationAccra is another location in Western Africa among our measurement sites.The U.S. Embassy is in the Cantonment suburb, situated ~180 m away from a major traffic circle and sandwiched by two parallel arterial roads running at ~150 m on each side.The Embassy is ~1.5 km south of the Kotoka International Airport, and ~3.25 km north of the Gulf of Guinea.The city experiences high dust episodes from occasional Harmattan winds carrying Saharan dust between November to March.

Table S2 :
Sampling summary for the target cities in SSA.N indicates the number of sampling days.µBC, µPM2.5 and µratio represents mean hourly BC, PM2.5 and BC:PM2.5, respectively.R 2 particles values indicate correlation between hourly BC and PM2.5 for a location during the sampling duration.

Table S4 :
Mean weather3and pollution parameters and for Wet and Dry seasons in Abidjan.