Optical carbon analysis on Teflon filters from the FRM network in New York

Highlights

• Optical carbon analysis was performed on Teflon filters across 19 sites in the New York FRM network.

• Mass absorption cross-sections ranging from 9.12 to 11.0 m²/g were determined.

• BC decreased by 30 % over the period 2015–2019 consistent with the replacement of older heavy duty diesel vehicles.

• A spatial variation of a factor of 2 and 5 was observed in mean BC at NYC and upstate locations, respectively.

• At the urban locations BC was highest in winter and summer while at the rural locations BC was highest in summer.

Abstract

Optical carbon analysis measurements were performed on Teflon filters from the Federal Reference Method (FRM) network in New York over the period 2015–2019. The optical measurements were performed using an inline Magee Scientific OT21 transmissometer coupled to the robotic weighing system (RWS) that measures PM_2.5 mass gravimetrically. Light attenuation from the Teflon filters was compared to elemental carbon (EC) measured on quartz-fiber filters across 8 locations from the Chemical Speciation Network (CSN). The coefficients of determination (R²) were highest at urban locations (where EC concentrations were highest) and lowest at the remote rural sites. The regression slopes provided mass absorption cross-sections (MAC) from 9.12 to 11.0 m²/g at 880 nm which are in agreement with literature values. Transmissometer measurements of black carbon (BC) were also compared to Aethalometer® BC as well as EC from a semi-continuous Sunset Laboratory Inc. OCEC carbon analyzer at two locations. BC estimated from the optical measurements decreased by approximately 30 % over the 2015–2019 period consistent with the replacement of older heavy duty diesel (HDD) vehicles. BC was highest across urban locations and lowest in the rural sites. At the urban locations BC was generally highest in winter and summer, whereas at the rural sites BC was highest in summer. Both locations showed the lowest BC was in spring. Weekday vs weekend analysis revealed BC was on average lower at the weekend by approximately 20 % in NYC and 9–16 % in upstate locations.

Introduction

Black carbon (BC) generally refers to the portion of ambient particles that absorbs light strongly in the visible and near infrared. Elemental carbon (EC) is operationally defined by thermal optical analysis methods whereby a particulate sample is oxidized, and the evolved gas quantified relative to a fixed methane concentration (Birch and Cary 1996; Chow et al., 2007). BC or EC are produced from incomplete fossil fuel combustion in, for example, motor vehicles including diesel engines, heating and biomass burning (Hildemann et al., 1991; Seinfeld and Pandis 1998 and references therein) and are considered indicators of primary combustion emissions. In New York State they represent a non-negligible fraction of the PM_2.5 (particles with an aerodynamic diameter less than 2.5 μm) mass, with higher proportions in urban environments (Rattigan et al., 2016).

BC impacts visibility (Watson, 2002), plays an important role in climate processes (Bond et al., 2007; Ramanathan and Carmichael, 2008) and may contribute adversely to human health (Pope and Dockery, 2006; Grahame et al., 2014). Residents living near major roadways and exposed to vehicle combustion particles including BC maybe at increased risk for respiratory symptoms and reduced lung function (Kim et al., 2004; Cornell et al., 2012).

BC is generally determined by optical techniques (Hansen et al., 1984; Chow et al., 2009) whereas EC is quantified by thermal optical methods following particle collection on quartz fiber filters (Birch and Cary, 1996; Chow et al., 2007). The United States Environmental Protection Agency (USEPA) Chemical Speciation Network (CSN) collects 24-h integrated filter samples for particle chemical analysis across the US. These 24-h integrated filter measurements are designed to monitor spatial and temporal patterns in PM_2.5 particle chemical components including EC (Schwab et al., 2004; Rattigan et al., 2016) useful for emission control implementation, and provide data in support of health-based research. The USEPA Federal Reference Method (FRM) network is a more extensive network and was established to determine compliance with National Ambient Air Quality Standards.

Recently, light attenuation (b_att) measurements performed on Teflon filters have been compared to EC measured on quartz fiber filters (Chow et al., 2010; Frank et al., 2010; White et al., 2016). Good correlations were observed suggesting that b_att measurements may be a convenient way to obtain EC measurements in cases where there are no quartz filter measurements available thus expanding the spatial scale of EC measurements. This may also prove to be cost effective if the measurements can be performed routinely on network filters (Presler-Jur et al., 2016, 2017).

In this study, we present measurements of BC from optical carbon analysis of FRM Teflon filters for samples collected across 19 sites in New York State from 2015 to 2019. We build upon earlier work that used a Magee OT21 transmissometer to evaluate mass absorption efficiency of samples previously collected as part of the CSN (Presler-Jur et al., 2017). We then compare BC from these measurements against BC data from absorption measurements made by the Aethalometer® in addition to EC measurements from the Sunset OCEC carbon analyzer at selected sites. Finally, we explore annual, spatial, seasonal and weekday vs weekend patterns in BC.