Monitoring of Ozone Photochemical Precursors

Abstract

Accurate and continuous monitoring of atmospheric ozone precursors is crucial for effective air quality management. This study demonstrates the application of selected ion flow tube mass spectrometry (SIFT-MS) for real-time, high-frequency monitoring of volatile organic compounds (VOCs) and nitrogen dioxide (NO2) using SyftEnviro software. Unlike traditional methods that require complex sample preparation and extensive lab analysis, SIFT-MS provides near-instantaneous measurements, enabling rapid response to pollution events. With broad capabilities, a single SIFT-MS instrument has the potential to replace three separate units in a typical monitoring station by combining the ability to measure VOCs, carbonyls, and NO2. By seamlessly integrating weather station data SyftEnviro unlocks the speed, simplicity, and sensitivity of SIFT-MS measurements for improved air quality assessments and policy decisions.

Introduction

Photochemical smog has been a growing issue for large cities and industrial centers globally since the middle of the 20th century. Smog forms when sunlight drives chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) resulting in ground level ozone, which then oxidizes chemicals in the air, resulting in the formation of secondary organic aerosols (SOAs). While ozone provides a beneficial UV shield in the stratosphere, when present at ground level it has major harmful impacts on human health (Zhang et al., 2019). This is particularly problematic for large urban centers, as human activity is increasingly responsible for the elevation of atmospheric NOx and VOCs to dangerous levels. However, industrial processes and vehicle emissions are not the only causes of photochemical smog: VOCs are also emitted from biogenic sources – evidence for this can be seen in the ‘blue hazes’ that form over forested areas in summer such as the Blue Mountains (Australia) and Smoky Mountains (USA) (Wayne, 2000). This highlights the difficulty that regulatory bodies face when attempting to combat smog formation with sensible emissions policies.

As the emission sources of problematic compounds vary, measurement techniques must change to gain a full picture of the system. In response to the steadily growing risk that photochemical smog poses, regulatory bodies have instituted increasingly complex legislation to implement progressively more comprehensive monitoring programs for the chemical precursors to ozone. Preeminent among these is the United States Environmental Protection Agency’s 40CFR part 58, which has provided the framework for how air quality monitoring networks are designed and operated worldwide. The mechanism for ozone formation involves multiple steps and can be affected by various factors which mean that an in-depth understanding of the chemical composition of the atmosphere is necessary to implement effective mitigation strategies. These monitoring programs employ complicated and expensive methodologies to achieve full coverage of the compounds of concern, requiring countless workhours to ensure data quality. For example, the typical methodology for measuring carbonyls, a subset class of VOCs, involves a derivatization step, whereby the sample is loaded onto a DNPH impregnated cartridge, then eluted with organic solvent before analysis via HPLC (EPA, 2019). The status quo for other VOC measurements involves either cryogenic preconcentration (EPA, 1999), followed by dehumidification, or an 8-hour sampling time into an evacuated cylinder (EPA2023). These steps add complexity, cost, potential for error, and a measure of uncertainty. SIFT-MS eliminates all this, achieving the necessary sensitivity with direct sampling of whole air in real time.