Question

\(\mathbf{5}\) In Chinese village homes, heating is often by an open wood or coal fire. Where outdoor \(\mathrm{PAH}\) concentration is \(0.60 \mathrm{ng} \mathrm{m}^{-3}\), the rate of emission of \(\mathrm{PAH}\) from indoor combustion is \(3.5 \mathrm{ng} \mathrm{m}^{-3} \mathrm{~h}^{-1}\) and the air exchange rate is \(20 \mathrm{~min}^{-1}\), estimate the indoor air concentration of PAH compounds. Assume that the only loss mechanism is by air exchange.

Short answer

The estimated indoor PAH concentration is approximately 0.603 ng/m³.

Step-by-step solution

Understand the Problem

We need to estimate the indoor air concentration of PAH compounds given the rate of emission, the outdoor concentration, and the air exchange rate.

Define the Known Variables

We have the following values: outdoor PAH concentration = 0.60 ng/m³, indoor emission rate = 3.5 ng/m³/hr, and air exchange rate = 20 min⁻¹.

Convert Air Exchange Rate to Hourly Basis

Convert the air exchange rate from minutes to hours: \[20 \text{ min}^{-1} = 20 \times 60 = 1200 \text{ hr}^{-1}\]

Set Up the Mass Balance Equation

Using a steady-state balance for PAH indoors: \[I = E + C_{out} \cdot A\]where:- \(I\) is the indoor concentration,- \(E\) is the emission rate,- \(C_{out}\) is the outdoor concentration,- \(A\) is the air exchange rate.

Rearrange the Equation

Rearrange to solve for \(I\): \[I = \frac{E + C_{out} \cdot A}{A}\]

Substitute and Calculate

Substitute the known quantities into the equation:- \(E = 3.5 \text{ ng/m}^3\text{/hr}\)- \(C_{out} = 0.60 \text{ ng/m}^3\)- \(A = 1200 \text{ hr}^{-1}\)\[I = \frac{3.5 + 0.60 \times 1200}{1200}.\]

Perform the Calculation

Calculate the indoor PAH concentration:\[I = \frac{3.5 + 720}{1200} = \frac{723.5}{1200} \approx 0.603 \text{ ng/m}^3.\]

Key concepts

Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of organic compounds with multiple aromatic rings. These compounds are primarily formed through the incomplete combustion of carbon-containing materials like wood, coal, oil, and gas.
PAHs are found in both outdoor and indoor environments, and people are often exposed to them without even realizing it. Indoors, PAHs can be released from sources such as tobacco smoke, stoves, and fireplaces. The exercise highlighted above deals specifically with PAH emissions from indoor combustion sources.
PAHs in the air can adhere to surfaces such as dust, walls, and furniture, and can be inhaled or ingested, leading to potential health risks. These risks can be more pronounced when indoor air quality is poor, or when air exchange rates are low. Due to their chemical stability, PAHs can persist in the environment for long periods, making it important to understand and manage their levels, especially in enclosed spaces.

Air Pollution

Air pollution is the introduction of harmful substances into the atmosphere that can have negative effects on the environment and human health. It encompasses a variety of pollutants, including PAHs as discussed in this context.
When considering air pollution indoors, like in the Chinese village homes from the problem, emissions from coal and wood fires are common sources of indoor air contaminants. The outdoor environment also contributes to indoor air conditions, as there is typically a continuous exchange of air between the two spaces. Hence, the outdoor concentration of pollutants must be considered when estimating indoor air quality.
Polluted air can impair respiratory health, cause cardiovascular diseases, and exacerbate conditions like asthma. Managing indoor air pollution involves controlling sources of pollutants, increasing ventilation when feasible, and regularly assessing air quality to ensure safe levels of indoor contaminants.

Environmental Chemistry

Environmental chemistry is the study of chemical processes occurring in the environment which are impacted by human activities. It encompasses the study of natural and anthropogenic substances, their interactions, and their effects on the ecosystem.
In the context of this exercise, environmental chemistry helps us understand how PAHs behave in indoor settings, specifically focusing on their sources, emissions, and the techniques used to calculate their concentrations indoors.
Key concepts in environmental chemistry include understanding how different substances are transported, transformed, and degraded in the environment. This understanding is crucial for predicting pollutant dispersion and for designing strategies to mitigate their impact on the environment. Engineers and environmental scientists often develop models, like the mass balance equation used in the problem, to quantify pollutant levels and help manage and reduce air pollution effectively.