Aircraft rentals. The $12.0 million, or 18.9%, increase in aircraft rentals was primarily related to a shortened expected useful life on 21 leased CRJ aircraft that are in the process of being stored prior to the lease expiration in early 2024, which resulted in an acceleration of lease expenses on those aircraft.
Airport-related expenses. Airport-related expenses include airport-related customer service costs such as outsourced airport gate and ramp agent services, airport security fees, passenger interruption costs, deicing, landing fees and station rents. For clarity, our employee airport customer service labor costs are reflected in salaries, wages and benefits and customer service labor costs we outsource to third parties are included in airport-related expenses. The $33.1 million, or 31.7%, decrease in airport-related expenses for the year ended December 31, 2022, compared to the year ended December 31, 2021, was primarily due to a decrease in subcontracted airport services, landing fees and deicing events as a result of a decrease in the number of flights we operated under our prorate arrangements.
Special items. Special items for 2021 consisted of a non-cash impairment charge on SkyWest Airlines’ CRJ900 fleet of $84.6 million.
Payroll support grant. During 2021, we entered into two Payroll Support Program Agreements with U.S. Treasury that provided certain payroll support relief payments that were recognized as a reduction in labor expense over the periods the grants intended to compensate. We recognized $422.7 million in payroll support grant proceeds we received as a reduction to our operating expenses for the year ended December 31, 2021. We did not have a comparable payroll support grant during the year ended December 31, 2022.
Other operating expenses. Other operating expenses primarily consist of property taxes, hull and liability insurance, simulator costs, crew per diem, crew hotel costs and credit loss reserves. The $62.2 million, or 24.3%, increase in other operating expenses was primarily related to an asset held for sale write-down of $51.4 million during the year ended December 31, 2022, and an increase in training expenses and simulator costs related to an increase in pilot hiring during 2022.
Summary of interest expense, interest income, other income (expense) and provision for income taxes:
Interest Expense. The $4.0 million increase in interest expense was primarily related an increase in long-term debt subsequent to December 31, 2021, as well as a higher average effective interest rate in 2022 as compared to 2021. Our average effective interest rate for 2022 and 2021 was 4.0% and 3.8%, respectively.
Interest income. Interest income increased $16.5 million, from $1.1 million during the year ended December 31, 2021, to $17.6 million during the year ended December 31, 2022. The increase in interest income was primarily related to an increase in marketable securities from $602.0 million at December 31, 2021 to $944.2 million at December 31, 2022, as well as an increase in the average interest rates attributed to our marketable securities from 2021 to 2022.
Other income (expense), net. Other income (expense), net increased $24.1 million, compared to 2021. Other income (expense), net primarily consists of the unrealized gains on our investments in other companies, income related to our investment in a joint venture with a third party and gains or losses on the sale of assets. During 2022, we recorded gains from our investments in other companies of $11.4 million and gains, net of losses, of $7.9 million on the sale of assets.
Summary of provision for income taxes:
Provision for income taxes. For the years ended December 31, 2022, and December 31, 2021, our income tax provision rates were 21.2% and 25.7%, respectively, which include the statutory federal income tax rate of 21% and other reconciling income tax items, including state income taxes and the impact of non-deductible expenses. For the year ended December 31, 2022, the decrease in our effective tax rate primarily related to the lapse of a previously recorded uncertain tax position liability of $7.3 million that was recognized as a benefit in 2022, partially offset by a $1.7 million valuation allowance on state net operating losses anticipated to expire prior to utilization recorded in 2022. Our income tax provision rate may fluctuate each reporting period based on various factors including, but not limited to, the amount of our non-deductible operating expenses, relative to our income before income taxes.
Net Income. Primarily due to the factors described above, we generated net income of $73.0 million, or $1.44 per diluted share, for the year ended December 31, 2022, compared to net income of $111.9 million, or $2.20 per diluted share, for the year ended December 31, 2021.